Pulling a Stop to Runway Overruns

[OVERTALK]

In at the Deep End and Off at the Bitter End
Braking Old Habit Patterns and Pulling a Stop to Runway Overruns
Hardly a day goes by without an airliner or bizjet sliding off the end of a slippery runway on landing (and now and again, off the side - probably after blowing a tire in a crosswind). In most cases the pilot will have made a simple error of judgment in arriving in disarray at his challenging runway environment. Rarely will the cause be attributable to an actual airplane unserviceability. Sometimes death and destruction will be the outcome, normally as a result of a marginal runway length, a contaminated surface and an obstacle-strewn or geologically inhospitable overrun area. Recent high profile cases have included the A340 at Toronto and the 737 at Chicago (Midway). Whatever the cause, the blame will eventually attach to both the pilot and the airline - so reputations are inevitably sullied. But why does it happen so repetitively? Is it just the "death of a thousand cuts" type of accident? Or is there something significant that is being overlooked? In ASW (08 Aug 05) we looked at the EMAS overrun bedding and in the 15 Aug 05 ASW, a landing performance monitor (LPM). In this discussion we will look at a largely overlooked pilot stopping technique that may just be a potent life-saver..
The Setup
If a pilot goes to his model's operating handbook, he will see what is expected of him for landing techniques - and his performance charts will tell him the landing distance required for wet and dry runways. However most operations into "contaminated" runways are just "not recommended" and it is there that pilots can enter unwillingly into a game of dungeons and dragons.
The "thousand cuts" aspect begins when an airfield's ATC decides that even though its duty runway now has a tailwind, it cannot be changed because that would affect instrument approaches and departures at a nearby major airfield. Perhaps it cannot change because into-wind approaches to the reciprocal runway would have a higher minima and wouldn't work in the prevailing ceiling and visibility conditions, leading to mass diversions. So a pilot gets thrown in at the deep end, placed in the invidious position of "having a go" and often ends up role-playing the villain. Unfortunately the data upon which he was working may have quietly become vaguely ill-defined. Runway friction measurements are notoriously variable, whether made by instrument or opinion. If the preceding pilot reports after landing that the braking was "fair", the next man down the slot has little to hang his hat upon were he to judiciously decide to divert anyway. Commitment and challenge is the name of this game. Unfortunately, one man's subjective "fair" may be based upon a slightly different wind, a lighter airplane weight or another type of mount altogether. Perhaps he's just enthused and infused by his own successful arrival and subconsciously laying down the gauntlet for the next man. It's invariably very precarious information upon which to make a potentially life-altering decision. As things now stand, land a little hot and a little too far in and you've just applied to join the rough-rider's Honor Roll of Infamy.
Down and Out (of Options)
Once a pilot is down, he must make an instant decision as to whether he can stop or should "go". That was the premise for advocating an LPM aid. Frequently a pilot will be quite unaware of how far in he's touched down and, early on, just how much runway remains (unless he's got sixth sense plus eyes in the back of his head). Tailwind, threshold crossing height, target threshold speed and runway downslope can make that "distance in" quite grossly variable. At night and in rain, pilots can also suffer from optical illusions and land long. But nevertheless, once he has moved the throttles into reverse, the pilot has made (and effectively announced) a conscious decision to "stay". It is this point, its aftermath and the ensuing degree of pilot control that we are interested in here. What are the factors in play here for a halting success?
*In both the SWA 737 accident and the AF A340 at Toronto there was an abnormal hiatus in achieving reverse. That can sometimes be a result of indecision and cross-cockpit last minute mind-changing or assumption of control by the captain (cf Qantas 747 overrun in Bangkok); yet it can also be a glitch by-product of a rushed premature grapple with the throttle interlocks that are there to prevent inflight reverse. The SWA 737 didn't achieve reverse until some 18 seconds after touchdown. Meanwhile that airplane's headlong rush ate up a lot of landing real-estate and over that period it was also too light upon its wheels for effective braking. An airplane's weight-on-wheels ground/air sensing circuitry must allow oleos to depress, radar altimeters to authorize and multiple microswitches to be "made". Once these pre-conditions are satisfied, hopefully the spoiler panels will spring up from the wings, reverser cowls can open and, once the mainwheels have spun up, the autobrake will quickly start "applying the anchors". It is the stuff of milliseconds. At this juncture is it all out of the hands of the pilot? - or is there something else determinative that he can do? - perhaps with his hands?
Slip-sliding Away
Think in terms of braking effectiveness; ruminate upon the criticality of "weight-on-wheels". Spoilers are designed to lift-dump a section of the wing and move the airplane's weight onto its wheels for better braking traction, whereas thrust reversers are intended to kill speed (and indirectly, lift). Until the airplane's weight is upon its mainwheels, the auto-braking is going to be initially restricted (for its effectiveness) by the anti-skid system. Anti-skid logic knows that if it allows a wheel-bogey's wheel-spin to even almost stop, the tires will be blown. Like an auto's ABS, its job is to detect any wheel's incipient wheel-skid and release the autobrake pressures sufficient to allow the wheels to maintain a threshold rotation rate. The end result of a hyper-active anti-skid is a lesser braking effectiveness overall. So we need to get MORE of the aircraft's weight off the wings and onto its mainwheels - and do that as early as possible. It will suppress the anti-skid's non-retardant interventionism. Is there a way to do this - i.e. get earlier weight-on-wheels?
Do (or Don't?) Hold Back?
There is a way - and it's one that has been largely overlooked by the civilian airline industry (although it is taught by the military).
*It is based upon the fact that reverse, braking and spoilers all serve to promote an effective weight-shift towards the nose. More precisely, all those stopping measures additively cause a strong nose-down pitching moment. They add greatly to the nose-oleo depressing moment that would be there even if spoilers stayed down and reverse and braking wasn't being used. But how can this cumulative weight-shift towards the nose help us stop? Quite simply, a pilot can confidently counter this nose-down pitch by introduction of progressive backstick once under reverse and braking. The pilot's up-elevator will oppose that nose-heavy pitch-down and push the main-wheels into the ground.... with no possibility of rotating the nose airborne again (a popular misconception and dread of some pilots). Now read through that again, just to make sure you understand.
So Non-Fatal a Traction
That up-elevator reaction will give greater rotational traction to the mainwheels and stop the anti-skid from interfering with the efficiency of the programmed auto-braking. It will significantly shorten the ground-roll. It will also assist greatly in avoiding aquaplaning by increasing each tire's foot-print. Aquaplaning is an oft-encountered wet runway condition where the tire is just sliding along (with little frictional incentive to rotate) upon a thin bow-wave of water, snow, ice or ultimately the thick wet rubber deposits at the runway departure end. The latter is called a reverted rubber skid (check Google). Braking just isn't happening under aquaplaning - yet blow-outs can. Are there any other bonuses besides that all-important reduction in landing roll-out? Well yes. Greater weight upon wheels will tend to cancel the "getting blown sideways" effect of a stonking crosswind component (a big factor in AA1420's accident at Little Rock). Is there a downside? Well according to all those who spent a whole career unaware of this technique, there just MUST be. Otherwise the God-like manufacturer's God-like test-pilots would've recommended the technique... well wouldn't they? Unfortunately these sceptical individuals cannot put a precise aerodynamic reason as to why the technique wouldn't work or (more ominously, rolling their eyes heavenwards) might indeed be dangerous ("you must only use the manufacturer's recommended techniques"). Some have ventured that the nose would rise once back-stick was introduced. Once they are challenged to delve into that mystic unaerodynamic development more deeply however, they tend to fall silent.
Others have ventured vaguely that directional control would suffer. Actually the up-elevator re-distributes the aircraft weight back towards the mainwheels, restoring a directionally stable tricycle geometry. Conversely the forward stick (advocated by some) creates a nose-heavy (and directionally unstable) wheel-barrowing effect. Others have waxed on (confusingly) about the inefficiencies of aerodynamic braking, where the nose is held off (via earlier backstick) and aerodynamic drag is allowed to slow the airplane. By contrast, the technique of progressive backstick is initiated after the nosewheel is ON and other retardation systems have kicked in. It is however nought to do with aerodynamic braking.... although the confusion is understandable.
Both Dextrous and Ambidextrous
So will Airbus now be displeased to know that there is still a role for pilot manual input, and a two-handed one at that, particularly when in extremis? In due course they may quietly automate this process. Perhaps they should. Perhaps it's something that Boeing could also do in the interests of Flight Safety. Until then, any pilot quickly running out of bitumen would do well to whip out a photo-copy of this article, have a quick refresh and then pull the stick back into his gut. Backstick is a great corporeal leverage point for meaningful toe-braking anyway. The alternatives, by then, are indisputably the stuff what nightmares are made of ..... so in for a penny, in for a pound (of backpressure). I might add that it works for all sizes. It's never the size of the dog in the fight, it's the size of the fight in the dog. At least it gives you something meaningful to do whilst motoring off the end. In the end, and particularly at the end, anything that you've neglected to do earlier just becomes simple wishful thinking. The status quo at present seems to be for the elevators to be left to seek their own equilibrium and the pilot to pay scant attention to yoke or sidestick pitch authority once the nosewheel is on.
There are many marginal length runways around. Prudence dictates that pilots should be apprised of, and experienced in, all valid stopping techniques. Not to do so is to push the envelope of commercial pressures to the scene of the next accident...and the next. Threat and error management dictates frequent reviews of operating procedures before your next accident. Qantas found that out the hard way and changed its policies after Bangkok. So, if you're still a non-believer, please put the weight upon the wheels in your organization to explain to you aerodynamically, in the words of Professor Julius Sumner Miller, "Please sir, why is it not so?"
from this link (http://www.iasa-intl.com/folders/belfast/off-the-bitter-end.htm)

[apruneuk]

Overtalk

I can't speak for a transport aircraft but, as a parachute pilot I regularly have to land on 550 m of wet grass with a 10-15kt cross-wind component. Full flap, 1.2 vso and a steeper than 3 degree approach followed by a fully stalled touchdown, flaps retracted and increasing back pressure (ie putting as much weight as possible on the mainwheels) while applying the brakes will usually have me stopped with 200m to spare. I have tried every technique under the sun and this one results in the shortest stopping distance every time.
I understand that my approach technique does not apply to a swept wing transport aircraft but I cannot see why it shouldn't work once the wheels are all on the ground. Interestingly, I took an airline pilot friend of mine to our dropzone last summer and he asked me why I used increasing back-pressure during the roll-out; as you say, he couldn't come up with any reason why I shouldn't, particularly as the wing was already fully stalled, but just seemed curious. My reply to him was that once on the roll-out my priority was to get as much weight as possible on the braked wheels and to alleviate stress on the nose wheel due to the uneven surface. Flap retraction and back pressure help to counter the inevitable pressure placed upon the noswheel during braking, particularly in the early stages of the landing roll. I should imagine that the aerodynamic braking effect of flap on a heavy transport aircraft with a relatively high vref outweighs the benefits of retraction for that type but increasing back-pressure can only help one the wheels are all on the deck, surely?

[Belgique]

With many pilots, light, medium or heavily mounted, one would just be preaching to the choir on this. However, looking at the two threads below, one can't help but be stunned that there are so many pilots out there who just aren't aware of this stopping enhancement technique.
The other day I was talking to an old mate about it and he freely said that he'd gone his whole thirty plus year career without ever thinking about it, let alone trying it. He started off on the Electra and then the 707. He's now flying a Challenger and he says that he can't see anything in the technique that goes against what both Boeing and Airbus are saying about the importance, for stopping, of getting the weight off the wings and onto the wheels.
I wonder how many have just blatted off the end in total ignorance? It would make a very interesting staistic. Now wait and see if the lawyers get hold of it....
http://www.pprune.org/forums/showthread.php?t=201568&page=13
http://www.pprune.org/forums/showthread.php?t=204112&page=2

[skianyn vannin]

Interesting comment about applying up elevator during the ground roll to increase braking effectiveness. We do this as a matter of course on the BAe146, not just during rejected T/Os but all landing rollouts. It is very effective. I'm surprised this isn't done on other aircraft.

But then, as I'm often told the BAE146 isn't a real airliner, and I'm not a real airline pilot!

[Hand Solo]

It is based upon the fact that reverse, braking and spoilers all serve to promote an effective weight-shift towards the nose

Errr, no they dont. Reverse and spoilers can act in opposite directions in terms of pitch moment. They certainly do on the A320 and B747. I don't know if the author has ever flown a high performance jet with an efficient wing but pulling back on the stick immediately after landing will put the nose back in the air, not to mention robbing you of all nose wheel steering.

The whole point of the article seems to be that it's better to get the weight on the wheels ASAP so the autobrakes can do their job. That is already the technique recommended by Airbus and Boeing. Once the autobrakes are active pulling back on the stick will make no difference to the braking distance as most autobrakes command a deceleration rate. If the aircraft isn't slowing they'll just put the brakes on harder. If you compare the performance of the brakes in the RTO setting compared to a normal landing setting you'll see theres a lot of spare left. I believe in the Air France case at Toronto even with full braking and reverse they wouldn't have stopped anyway because they'd landed about 2000m down the runway!

[skiesfull]

Then article on pulling the controls rather than pushing makes interesting reading and debate. However, nowhere in the article on over-runs does it mention the absolute need for the correct landing technique required for wet or contaminated runways and that is:- a firm touchdown on the centreline,on the touchdown zone, immediate deployment of spoilers with either full manual or max. autobrakes (dependent on type and flight manual recommendations) and full reverse used until a safe stop is ensured.
As for last years incidents, involving over-runs, am I being cynical when I ask if pilot error has now been replaced by "a break-down in CRM"?

[RatherBeFlying]

HS,
You are likely correct wrt the direct pitching moment from the spoilers on these 2 types, although there may be some subtraction as a result of lift dumping.
As far as the pitching moment from reverse is concerned, reverse from engines mounted below the wings will generate a nose-down pitching moment. The corollary is that forward thrust from engines in this position produce a nose-up pitching moment. Reverse from an engine mounted higher up can be a different matter.

[alf5071h]

The principles outlined in the IASA article and the posts in related threads advocating use of back stick are essentially correct. However, they do not consider the wide range of aircraft types, configuration, and variables that should be reviewed before a technique is recommended as a standard, let alone suggesting that crews could try it without such knowledge.

Braking effectiveness is improved by increasing weight on the main wheels, thus dumping lift with spoilers and lowering the nose to reduce AOA will provide major benefits. Similarly, so could raising the flap, but without comparing flap drag vs improved braking we cannot judge. Furthermore, the crew workload could be increased with change of technique, and based on the problems with selecting reverse cited in the article, that might not be a good course of action.
Further reduction in AOA (nose down stick) could increase the wheel force by producing ‘negative’ wing lift, but in those aircraft with high values of elevator power it could lighten the load on the main wheels or even worse break the air ground contact with detrimental effects.

Using back stick can generate increased wheel force with pitching moment, but this would be subject to the same variability of elevator power as forward stick i.e. it depends on the type. Other variables such as c of g, or engine / reverse configuration and their pitching moments also have to be considered.
A further contribution of back stick in increasing wheel load is to produce more ‘negative’ tail lift (part of the overall lift on the aircraft), but the amount generated also depends on aircraft type and configuration i.e. elevator vs all-flying tail. In proportion, the benefits of reduced tail lift may not be significant in comparison to correct spoiler operation and lowering the nose wheel. The tail forces will decrease as the aircraft slows down, thus this effect is also related to aircraft type and landing speeds, but also with crew workload immediately after touchdown where it is probably more important to confirm that spoilers have deployed, reverse is selected and max braking commenced.

One of the main problems with back stick is that the crew has no indication of the load on the wheels or the change that they are attempting to achieve; in some aircraft, the crew has no force feedback of elevator position. In other aircraft, it is possible to raise the nosewheel off the runway or reduce the load so that steering is ineffective. Noting that IASA suggested that crosswind performance would be improved, not that slipping sideways was eliminated, the article overlooked the yawing moment due to crosswind. Thus while the aircraft might not be as easily blown off the runway it could veer toward the runway edge and in some combinations the effects of yaw and side force are actually detrimental. (see the Airbus reference (www.wingfiles.com/files/safety/gettingtogripswithcoldweatheroperations.pdf ) 4.3mb re trading braking for steering). Rudder or nosewheel steering is required to counter yaw; the latter having reduced effectiveness and the former requiring additional crew vigilance and action, these are just some of the reasons why the crosswind limits are reduced (type dependant) on wet/contaminated runways.

The IASA article is potentially misleading by suggesting that crews “should be experienced in all valid stopping techniques”, implying that back stick is valid and approved. This is not the case in most aircraft types where the only approved technique is that published by the manufacturer.
The mental and physical effort in moving the stick rearwards might be much better used in checking that spoilers/reverse had deployed and that the feet are applying maximum brake pressure.

A safety organization such as IASA might have done better to take a wider view of the safety issue and seek to address some of the other causes. Whereas crew procedures and training are easy targets, (soft safety defenses), they are subject to human frailties, it is those harder safety boundaries that are usually the most effective.

The main problem in preventing overruns appears to originate from the contaminants on the runway. If the industry judges that the measurement of the contaminant and the relationship of those measurements with aircraft braking performance are unreliable, then IASA would be better advised to call for improvements in these areas or even to restrict operations to a clean runway operation only.

[Basil]

I now rarely contribute to PPRuNe but feel that the assertions contained in the article require a robust reply if only to prevent colleagues according undue weight to unattributed techniques.
I've read through the article and disagree with the suggestion that up elevator has any significant value.
Very little of the aircraft weight is supported by the nosewheel therefore up elevator will transfer little weight before the nose lifts and, as IAS reduces, the elevator will, in any case, become less effective. For those who have never turned e.g., a 747-400, over wet piano keys the sideskid even at low speed would come as a surprising indication of how little weight is supported by the nosewheels.
On the landing roll I certainly do not wish to remove weight from the nose wheel and reduce none-aerodynamic directional control.
Auto spoiler deployment on landing can cause a pronounced nose up pitching moment and a habit of applying nose up elevator too soon after landing could cause a tailscrape.
Up elevator will not PUSH the mainwheels into the runway; it will merely transfer the minimal load supported by the nosewheels (+ the elevator downforce; small at a large moment arm) to the mainwheels. After the nosewheel load reaches zero, the nose will rise.
Aquaplaning speed is proportional to sqr root of tyre inflation pressure. As weight increases the tyre footprint will increase BUT the footprint loading per unit square remains the same and therefore the aquaplaning speed will remain the same.
As runway friction decreases so the proportion of stopping effort becomes weighted toward aerodynamic and reverser drag to the point where only about 20% is due to braking and by what percentage that is influenced (UP OR DOWN) by elevator position I would not care to guess.
The wheel-barrowing effect is something which more commonly occurs at touchdown if flying too fast and consequently with the nose attitude too low.
<<Greater weight upon wheels will tend to cancel the "getting blown sideways" effect of a stonking crosswind component>> and, one has to say, increase the chance of weathercocking into wind.
<<Actually the up-elevator re-distributes the aircraft weight back towards the mainwheels, restoring a directionally stable tricycle geometry.>> I'm almost at a loss to understand what the writer means here. The aircraft CofG remains in the same place no matter what the pilot does. I think the writer is confusing light single piston ops or taildragger CofG position with big jets.
All of the foregoing applies to jet transport aircraft; not to military jets nor to rough grass strip operations.
So after 40 years in the business what do I think? Well I think that using the suggested technique will have you off the edge of the runway.
I'd prefer to fly the plane the manufacturers way without the latest DFO's little ways creeping in with the only changes being theatre dependant and other absolutely necessary exceptions.

[Bumblebee]

Basil, I couldn't agree more.

This is not a technique that would apply universally, and especially in the case of the 744, which has such effective control surfaces that you can effect a pitch or roll change at anything above almost 60-70kts.

[Dagger Dirk]

All posters since Skyanin Vannin here are using some really illogical arguments and assertions. Taking a contrary view really requires firstly, an understanding. Secondly you need honest conviction. Thirdly you need a cogent basis upon which to develop a coherent antithetical debate. It's point #3 that is largely lacking, despite those prepared to blandly assert that black is white.
I could easily follow OVERTALK's logic, but the counter-arguments from Bumblebee, Basil, Alf5071h, Skies Full and Hand Solo are just mind-blowingly and meaninglessly autocontrary.
Example:"Using back stick can generate increased wheel force with pitching moment,yes, OK but this would be subject to the same variability of elevator power as forward stick i.e. it depends on the type. Other variables such as c of g, or engine / reverse configuration and their pitching moments also have to be considered."What in Hell is that trying to say?. It sounds like: "Whereas I can accept that water whose temperature is reduced to below 32 degree Fahrenheit will freeze and become ice, surely whether or not it will remain ice will depend upon whether its temperature remains below freezing level" etc etc.In other words lots of repetitive meaningless gibberish, frequently discarding basic aeronautic principles and wandering off or trailing off. I could cite numerous examples in what I've read above. I hope none of you gents are instructors - but I would suspect that OVERTALK is. At least I can follow his theory.
Maybe some lucid thought and developed arguments gents? Eh? Sheer loquacity and self demeaning tech-drivel is neither impressive nor impactful. It certainly ain't convincing, particularly when presented by way of sweeping assertion and non-credible claims. That just lowers the tone of argument and will con only the gullible. Worse still, it will leave those who come here to the Tech Forum to be educated, just misinformed and hopelessly confused. Some of you are actually engineers with pilot quals. Unfortunately many engineers just do not have a great grip on the dynamics of the landing evolution, so I can accept that you are trying to argue honestly against what you believe to be patently false. Unfortunately you haven't succeeded.
I haven't heard anything that would dissuade me from accepting the basic aerodynamic truths of what OVERTALK is saying above. He is evidently advocating progressive introduction of backstick once other retardation devices are in use. Scare-mongering about what might happen when someone instantly hauls back after touchdown is therefore totally unwarranted.
No wonder the whole issue has remained cloudy for years. After reading what's been written above by the named individuals (admittedly some more than others), any eager young pilot would necessarily shy clear of the whole issue and just go away scratching his/her head. I can sense Alf5071h leaping in here and now saying: "that's exactly why we should never stray from the manufacturer's recommendations". Each to his own beliefs on that. However it shouldn't disable lucid argument based upon basic principles. Ominous sweeping statements, anecdotal or otherwise, are always indicative of a weak position and poorly disguised self-doubt.

[sky9]

Could I just make a plea; do what your aircraft operations manuals tells you, not what PPRuNe contributers or other aircraft's flight manuals suggest. That reduces the options when you are called to explain your actions with the FDR traceouts on your manager's desk.

[skiesfull]

I agree with Basil. I hope that any "eager young pilot" will shy clear of any Pprune suggestions, unless they are part of that pilots S.O.P.s

[Basil]

<<I now rarely contribute to PPRuNe >>
DD, thank you for justifying my decision - I rest my case.

For the benefit of less experienced pilots, please listen to those with real experience - Fly the aircraft according to the manual, not how some stranger of unknown provenance suggests; on PPRuNe or elsewhere.

[RatherBeFlying]

I see people advocating a procedure that potentially may improve braking, especially in contaminated conditions -- and I do agree with the underlying physics.
But it's a long way from basic physics to approved SOPs, and freelancing procedures in large a/c is much frowned upon for many good reasons.
Hopefully the manufacturers will develop the data and procedure and, if the procedure is found useful, start the ball rolling by adding it to their manuals.

[Hand Solo]

DD - have you ever heard the saying "Those who can do, those who can't teach"? OVERTALK may be an instructor, and I don't wish to malign instructors, but if you start using that Cessna 172 short field landing technique on a big jet you are going to get yourself into a world of trouble and no amount of big words are going to help when you rotate the nose of your 744 off the ground at 80 kts with insufficent rudder authority to steer and no nosewheel steering! You may be able to follow OTs theory but it doesn't mean its right.

Lets look at this statement:
He is evidently advocating progressive introduction of backstick once other retardation devices are in use.

I think that rather neatly sums up whats wrong with the whole idea. ONCE OTHER RETARDATION DEVICES ARE IN USE. If you've got the reversers out, the spoilers up and the autobrakes running then progressive introduction of backstick does ABSOLUTELY NOTHING to reduce your landing distance because the autobrakes are aiming for a deceleration rate. The only thing that is going to affect your distance from touchdown is your landing speed and the deceleration rate. If you pull back stick and put a fractional (and it will be fractional) increase in weight on the main gear the autobrakes will simply ease off to achieve the required decelaration.

Tell me, do you really, genuinely believe that in the last 30 years of big jets not one test pilot, engineer or researcher at Boeing, Airbus, Lockheed or Douglas has thought of that idea and considered its application in a jet transport?

[chornedsnorkack]

It is clear that the aircraft manufacturers and their test pilots must have given some thought and study to the possibilities of best stopping aircraft. And incorporated the choices they ended up preferring in the manufacturer-produced aircraft manuals, SOP-s and the design of avionics.

However, IASA is an official safety organization, too. And they accuse the manufacturers of having advised less than best way of stopping aircraft, and improperly rejecting a better technique.

So, can someone really figure out whether the manufacturers are doing the best, and IASA is mistaken, or whether the manufacturers have made an error and IASA is right?

[Basil]

<<IASA is an official safety organization>>
I'd think of them more as a private lobbying and accident news collation organisation. If their existence draws attention to safety omissions in our industry then that is clearly a good thing BUT, with respect, just because IASA says something most certainly doesn't make it correct procedure for a PROFESSIONAL JET TRANSPORT PILOT to follow.

[RatherBeFlying]

I earlier commented that reverse from engines under the wing would produce a nose-down pitching moment.

On further reflection, I realise that reverse can produce a nose-up moment about the maingear, even when the engines are mounted below the wing.
Because the distance is short, it won't be nearly as much as from fuselage mounted engines; also thrust angle from the pavement comes into play.

So with spoilers and reverse contributing nose-up moment about the maingear, you do need that much more nose-down moment from the braking -- but with poor braking action taking away from that nose-down moment, is there a safe cushion that can be counted upon before the nose begins coming back up:confused:

As the consultants love to conclude their reports, further study is required.

[TheShadow]

RatherbeFlying
1. Perhaps draw a 2D airplane in pencil and then get your eraser and rub out the runway.
Now imagine that you're flying along and manage to beat the G/A sensing and select symmetric reverse.
You're going to pitch nose-up? Really?
2. Now imagine that you're flying along and select just ground spoilers (and they come out). You're going to pitch nose-up are you? Really?
3. Don't bother doing the same exercise with the wheel-brakes. It's just going to wake up the stowaways in the wheel-wells. I always hate that when I'm travelling in steerage. It's like poking a stick into a bear in hibernation. It's neither polite nor politically correct. You're not a ticket inspector. Your sole reason for sitting there is to plan ahead on how you're going to stop that sonofabitch within the confines of a slippery runway.
And no, I'm not trying to get up your nose. Nose-upness isn't my style.
TS

[ant1]

Ratherbeflying,

You' re right. Reversers on the ground (the only time they should be used), since engines are above the wheels, will produce a pitch-up moment.

Edited ----------
Well, I guess we were wrong. Thrust reverser moment depends on wether the thrust line is above/below the acft CG.
----------------------
Besides asymetric thrust, pitch up moment is probably a reason not to use full reverse on highly mounted MD80's engines on contaminated runways since this tends to lighten the nose wheel.

Some poster said the nose wheel is light on 747's. It must be taken into account that when heavy braking is applied, the load on the nose wheel increases substantially. If you are able to rotate the airplane at 80kts during the take off roll I am not so sure you could during a heavy breaking deceleration. Has this been "flight" tested in the first place?

About the better tricycle directional stability, the pull coming from the tail's aerodynamic braking tends to make the acft more directionally stable.

The autobrake puts whatever braking it takes to achieve a decelaration rate as long as the wheels keep spinning otherwise anti-skid prevails. That is why it is important to prevent the wheels from locking.

I agree with one of the posters that no smart theory can help you when you are in the cockpit, so maybe some test should be made and results discussed.

Just my 2 cents

:ok:

[Dagger Dirk]

Besides asymetric thrust, pitch up moment is probably a reason not to use full reverse on highly mounted MD80's engines on contaminated runways since this tends to lighten the nose wheel. Actually more than 80% reverse thrust will blank the rudder and exacerbate or introduce directional control problems. Spoiler and autobrake will easily overcome that diminutive pitch-up effect.
The autobrake puts whatever braking it takes to achieve a decelaration rate as long as the wheels keep spinning otherwise anti-skid prevails. That is why it is important to prevent the wheels from locking.Congrats on a clear exposition of this fact. Yes, that's why it is important to maximize weight on wheels on wet, greasy and slippery runways.
I agree with one of the posters that no smart theory can help you when you are in the cockpit, so maybe some test should be made and results discussed..It's just a handling technique (proof of the pudding.... and all that
.
Now we shall watch, semi-amused, and see which of the manufacturing behemoths first decide to surreptitiously slip this advice into their pilot's handling notes. "Semi-amused" only, because we realise that this little trick may have precluded scores if not hundreds of overrun accidents in the past decade - and saved quite a few reputations (and possibly a few lives / lots of bent metal).
.

"Semi-amused" because there are still people around who believe that the appropriate stick position under braking is forward of neutral. And of course there are also the vast multitudes who've just never thought about it (nor thought it out at all).

[RatherBeFlying]

Shadow,

Quite correct -- rubbing out the runway (aka. taking off) changes the fulcrum of pitch moments from the maingear to the CG and can reverse the direction of moments from spoilers and reverse.

This thread has caught a number of us discussing pitch moments wrt the wrong fulcrum.

[john_tullamarine]

Good to see a spirited debate .. however, the tone of some posts is getting a little too much towards playing the player.

House rules are

(a) play the ball as hard as you wish

(b) don't play the player.

We pride ourselves in this forum for a civilised approach to life .. please follow that philosophy.

regards,

JT

[Basil]

Spoiler deployment can induce a pitch up moment under a couple of circumstances:
1. Inboard spoilers slow to deploy cf outboards (for the straight wing contributers, remember this is a swept wing).
2. Landing with flap/spoiler defects.
I've had it happen in practice so no amount of armchair theory can deny that it takes place.

Ref heavy braking producing a nose-down pitching moment; isn't this thread about landing on more or less slippery runways where heavy braking would be impossible?

Changing a successful technique for no really good reason is the road to disaster. That applies equally to all the little personal safeguards that we build in over a lifetime of aviating.

[Conan The Barber]

Do the advocates of this technique have any numbers to support their claims? By how much is the brake efficiency increased? Is it 10%, 20% or 0%? Or is it more a feeling that it works?

If there is any data that supports this technique, I would be very interested to see it.

On my own type the reference speed for the anti-skid system is coming from the nosewheels. I would very much like to know what effect reducing the weight on the nosewheels would have on the effectiveness of the anti-skid, before applying such a technique.

Perhaps Dagger Dirk or OVERTALK could comment?

[OVERTALK]

BASIL said Ref heavy braking producing a nose-down pitching moment; isn't this thread about landing on more or less slippery runways where heavy braking would be impossible?
.
To re-cover old ground:
.
a. the CofG accords a basic nose-down weight distribution (ie. before any retardation cuts in)
.
b. even minimally achieved braking will increase that (and reverse certainly will). At speed, spoilers will be helping greatly to get weight onto the wheels. However on contaminated runways a bow-wave of water/slush under the leading-edge of each tyre still tends to oppose wheel rotation (think of that as hydrodynamic lift)
.
c. However as RATHERbeFLYING pointed out, <<The autobrake demands whatever braking it takes to achieve a deceleration rate (and can do that as long as the wheels keep spinning) - otherwise anti-skid prevails (and the called-for rate is unachievable. That is why it is important to prevent the wheels from locking.>> So what we must try to do is to increase the weight-on-wheels to a maximum. In MANUAL braking, if you don't stop the wheels, you won't blow the tyres. Under AUTOBraking it's a matter of reducing the intervention of anti-skid - which stops you from blowing tyres but won't assist more effective braking. That can only be done by maximizing the rotational traction of the tyres. Progressive backstick (i.e. UP elevator) works well against the distant nose-gear fulcrum to lever the main-gear INTO the deck, increasing that weight on the main-gear wheels.
.
That it works like that is undeniable. Unfortunately the level of disbelief and denial seems to be built-in ("why, if this is so, is it only now being revealed?"). Well I used to teach and demonstrate it and wrote it into a few syllabi... but many a lost art has become irretrievably lost over time. Also firmly ensconced in the psyche is the belief that any minor variation from a strictly specified (but mostly imaginary) modus operandi means instant disaster. However if you read the two cited manufacturers' documents (one's a powerpoint and one's a pdf, referenced in earlier links), you will note that both Boeing and Airbus are emphatic that effective braking in the muck relies upon getting the weight off the wings and onto the wheels. It is indeed unfortunate that the handling technique of progressive backstick braking has become a lost art. If it hadn't, I'm sure that many, if not most, marginal (short) contaminated runway overruns might have been avoidable.
.
Of course it's something that needs to be introduced soon after spoilers are up and nosegear is down, as obviously the elevator authority will diminish with speed loss.

[OVERTALK]

BASIL
<<<disagree with the suggestion that up elevator has
any significant value. Very little of the aircraft weight is supported
by the nosewheel therefore up elevator will transfer little weight
before the nose lifts and, as IAS reduces, the elevator will, in any
case, become less effective.>>> ....That's why the
"progressive" backstick is being advocated. You can still fly the
sight-picture out the front window and temper the degree of backstick -
but I've never found "nosegear rising" to be an issue. It's an imaginary issue only. Surely you aren't claiming that
with spoilers up and under max reverse and braking, that there isn't a
strong cumulative nose-down pitching moment? That's why the term
"effective" weight-shift towards the nose is being used. The fact that a
747 (and other widebody aircraft) might have both body and wing-gear
doesn't change that "towards" directionality. Or maybe just think of it
as the elevator (and hoz stabilizer of course) levering the maingear into the ground by using the nose-gear as a fulcrum. You are "effectively" increasing the weight on the main-gear wheels courtesy of your up elevator input at speed (if used early enough you will stop the anti-skid cycling and help the autobrake achieve its programmed retardation schedule). Maybe it's just too difficult a concept for some to wrap their mind around.
.
<<<For those who have never turned e.g., a 747-400,
over wet piano keys, the sideskid even at low speed would come as a
surprising indication of how little weight is supported by the
nosewheels.>>>.... This has not a lot to do with the
dynamics of landing. Any airplane's nosewheels will slip when turning on
wet piano keys.....whatever its C of G (and both pre-takeoff or
post-landing). It's less a function of weight and more to do with a loss
of traction due to a nosewheels' tread being hamfistedly cocked off from
the direction of a/c travel inertia, whilst on a slippery painted surface.
.
<<<On the landing roll I certainly do not wish to
remove weight from the nose wheel and reduce non-aerodynamic
directional control.>>> You obviously don't subscribe to
the theory of wheelbarrowing then...but perhaps just consider it to be a
more equitable distribution of weight. Wheelbarrowing could be thought
of as "pushing a length of thread" or perhaps what happens when you
inadvertently use front wheel brakes (only) on a bicycle at speed (vice
both F & R hand-brakes). Those who advocate stick forward have obviously
not tried that. I wouldn't recommend it in a fighter, that's for sure. Be reassured that the progressive backstick technique considerably enhances directional control
.
<<<Auto spoiler deployment on landing can cause a
pronounced nose up pitching moment and a habit of applying nose up
elevator too soon after landing could cause a
tailscrape.>>> Nobody is advocating any motor moronic
whipping into automatic "loadsa backstick". Please stop diminishing the
overall argument by escalating into gross handling nonsensicalities. The
advocated technique is to introduce progressive backstick as or AFTER the
reverse has cut in, spoilers are up and auto-braking (or toe-braking) is
underway. You can control what happens to the nose by observing through
the front window.
.
It's almost amusing, but actually quite tragic, that the
progressive backstick stopping technique has been allowed to fade into
obscurity. The plaintive cries of the now retired old and bold actually
seem to read as: "why wasn't I told?" Sorry about that.
Perhaps someone should start a thread on other "lost arts".
.
In Reply to Conan the Barber's Query
Effectiveness of the Technique?
In my secondary duty as a maint Test Pilot at an Advanced Jet Training
School I used to torment the Hell out of other instructors by planting
the jet on the numbers and turning off impossibly early and thereby
getting the really short taxi to parking - wet or dry runway and no
maxarets/anti-skid. I'd normally only do that on back-to-back functional
check-flights and be relatively heavy with fuel normally, because of
that. Additionally, but quite non-critical, I used to run the electric
elevator trim all the way forward once the main-gear was on, just to
have the trim-tab working for me as an elevator extension -rather than
against me => more power to the elbow. Those who tried to
emulate the feat, even at light weights at the end of an instructional
sortie, usually missed the turn-off (and had a long slowspeed taxi to
the next) and/or blew a tire. The Chief Flying Instructor eventually
blew up at a staff meeting, regaled everybody about all the blown tires
and demanded to know how I did it. In four engine aircraft I rarely used
it in anger, just for training and demo purposes.
My guess is that a figure of the order of 20% better than book figures
on dry runways and similar on wet runways, with maybe slightly less on
really nasty rubbery and slushy surfaces. But runway surfaces vary so much over their length, particularly because of the rubber buildups in that
critical last 2000 feet. Braking achieved early on, in the "clean"
second third of the runway, is what it's all about.

[Mad (Flt) Scientist]

To achieve the claimed 20% improvement in braking effect would require increasing the mainwheel reaction force by the same amount, and having brakes capable to dealing with the increased torque. Assuming the latter, lets look at the 20% increase in download.

Assuming that the plane has just touched down, the speed will be somewhere close to stall speed. Also, let's assume that without the suggested elevator technique, there is zero aerodynamic lift or downforce, so that the mainwheel reaction is essentially the aircraft weight. (ignoring the nosewheel reaction component of maybe 5% for now)

Now, if I want to increase the mainwheel reaction by 20%, that means, taking moments about the nose gear and assuming the tail is as far behing the mains as the nose in front, I need a download of approx 10% of aircraft weight at the tail to generate that 20% download. If the tail is about 25% of the wing area and similarly efficient, that means I have to get to about 40% of the tail stall CL, which means I'm going to need substantial amounts of elevator to get there.

Now, the plane continues to slow until we get to about 70%Vs. I still need to generate 10% of the aircraft weight at the tail, but that now is something like 80-90% of the tail stall capability. Clearly, at much lower speeds I'm not going to be able to generate enough download to get my 20% number, and eventually it won't much matter where the elevator is.

What does that lot mean?

It means that to achieve the kinds of braking efficiency improvements talked about requires SIGNIFICANT elevator movements because it requires significant redistribution of the wheel reactions.

It also means, if you look at the impact on nosegear reaction, that in order to achieve 20% more mainwheel download, I need to offload the nosegear by 10% - maybe more depending on geometry. Since a typical nosegear load is of this magnitude this means I am definitely risking raising the nose with this technique.

[Basil]

Overtalk, with respect, may we have an indication of your aviation background? Are you an ETPS graduate?
Don't forget that we are discussing large jet transport multicrew operations here. SOPs are too important to monitoring to have a situation where one captain pushes and the next pulls. Poor old FO will wonder what's going on.
I think we need an input from a manufacturer. Any takers??

[OVERTALK]

Mad(Flt)Scientist
.
"20% better than book figures" relates to my ballpark guesstimate of what I could achieve in terms of stopping distance by concerted manual braking using the advocated technique but (as a qualifier) always being focused and directed at a specified turn-off. In other words, if the book said the jet needed 2800ft at the weight, I'd be taking about 0.8 times that ("around" 2240ft) with no concern at all that I'd possibly blow a tyre (never ever have BTW). Bettering that figure would obviously be possible with autobraking/antiskid. In all other respects, without supporting mensuration, the actual achievement is somewhat subjective. However I can recall that the same turn-off was also achievable with standing water on the ungrooved and heavily rubberised concrete runway during heavy rain. It wasn't just an egotistical exercise. We'd had a rash of braking and undercarriage and tyre failures being experienced dual and solo - so I'd had the hard-braking requirement written into the test schedule for myself and the other part-time UTP's to "have to" functionally check brake serviceability on every checkflight, so that solo students wouldn't be unduly hazarded by the vagaries of Dowty Rotol and BFGoodrich's very variable products. That included a check of the emergency brake for symmetrical braking on acceptance flights post major servicings (and I really hated it when engineering officer pilots in the front seat would subconsciously twist the handle and set its other function - as park brake. That was the nearest I've come to blowing a tyre, so thank goodness for reverted rubber skidding and a front-seater who understood the principle of "first up undo what you just did". That works for split flap events without flap-brakes I'd also found).
Because I didn't do ETPS I'm not sure that a 20% reduction in roll-out equates to your "claimed 20% improvement in braking effect". But no point in quibbling. There is a distinct, discernible and tangible improvement in braking effect on all airplanes that I've flown - particularly on nasty wet and snow-covered or slushy runways. I've even used it to wind up a 4 eng air display routine and once for a high-speed heavy-weight abort. Because the braking is heavy but of a more limited duration, the brake-heating seems to be no worse than usual. I've never had the thermal plugs take down a tyre afterwards for instance.
.
To achieve the claimed 20% improvement in braking effect would require increasing the mainwheel reaction force by the same amount, and having brakes capable of dealing with the increased torque. Not sure whether this is sound logic (simple tech peasant that I am), however most a/c wheelbrakes that I've encountered in a lengthy career are [torquewise] quite capable of blowing tyres - unless it's very much later in the roll-out where brake-fade once or twice educated a fatigued and nonchalant me that I should have done the required braking much earlier (thank god for high-speed turn-offs).
.
"Assuming that the plane has just touched down, the speed will be somewhere close to stall speed." Not a totally safe assumption in the overrun cases that we are interested in stopping. Many overruns are plonked down fast and late by pilots with an incre"mental" ref speed buffer mindset.... and some just end up "hot" for a multiplicity of reasons. It's that sort of seemingly unavoidable "lap-of-the-gods" mishandling that we are here attempting to assist in uncrunching the outcome of...
"...so that the mainwheel reaction is essentially the aircraft weight. (ignoring the nosewheel reaction component of maybe 5% for now)" 5%?? Hmm. But the only factor in play for braking effectiveness is the wheel rotational torque being imparted by the runway/tyre-tread interface. Unfortunately, on contaminated surfaces, because of the tyre's hydrodynamically lifting bow-wave at speed, tyre rotation is inhibited for a significant period after touchdown and "effective" weight-on-wheels is much less than a/c weight (thanks to efficient wings, rigger's angle and flaps). That's the nub of the argument here. Backstick uses the nose-gear's pitch-down moment as a type 2 fulcrum point to leverage an artificial weight upon the brakeable main-gear (body or wing)... and from elevator to nose-gear is a considerable moment arm (and mechanical advantage). In another sense the backstick tactic becomes a logical utilization of the effective weight shift forward caused by spoilers, auto-brake and reverse thrust. That cumulative pitch-down force becomes a dynamic fulcrum with the nose as the pivot-point.
"If the tail is about 25% of the wing area and similarly efficient, that means I have to get to about 40% of the tail stall CL, which means I'm going to need substantial amounts of elevator to get there. "....and eventually it won't much matter where the elevator is.". I won't quibble with the figures but yes, you do eventually end up with the stick right back in the gut (as speed dissipates, that's only logical). It's a progressive process. Just like braking itself, you do it "for effect". It's when you do it that counts. Obviously at less than 50 knots there's very little efffect available (ie. minuscule). Early on and sustained is good, whereas introduced very latterly as a panicky afterthought is pathetic. That's like deciding to save fuel after the "tanks low" lights illuminate.
What does that lot mean? It means that to achieve the kinds of braking efficiency improvements talked about would require SIGNIFICANT elevator movement because it requires significant redistribution of the wheel reactions.Agreed that it's a very unpansy-like commitment to stopping, but one applied as a handling technique and never motor-moronised by rote.
It also means, if you look at the impact on nosegear reaction, that in order to achieve 20% more mainwheel download, I need to offload the nosegear by 10% - maybe more depending on geometry. Since a typical nosegear load is of this magnitude this means I am definitely risking raising the nose with this technique. I think I'll let others here assess the validity of your %'s argument and conclusion lest I arouse JT's ire.. The orders of magnitude in the pitch-down couples involved under reverse and heavy braking with spoilers up are such that inadvertently raising the nose is a very very minimal mishandling risk (in comparison with overrunning). The redistribution of weight towards the main-gear (in most cases a tricycle geometry) is a secondary effect of the technique that nicely enhances directional control at speed. As the AA1420 pilot found at Little Rock, light on wheels with a crosswind creates two compounding problems - stopping and directional control.
.
Basil
Background ATPL >14.5K hrs, >half on 4eng, <half on jet trainer instruction, a lengthy helo tour in SVN, instructional (inc 4eng) since 72
Meaningful input or feedback from a manufacturer would be good. But it's rarely the equipment that gets blamed don't forget; so are they really likely to be interested? The NTSB is too busy kicking tin and are not really into the business of telling operators what to do... just what not to do. The FAA isn't really in the pilot education business either. If you are perturbed by all the unnecessary overruns (as am I) then you may perceive it more as a pilot education and training problem. That's best dealt with by trainers who belonga operators (who always have most to lose). If insurers had a more realistic interface with the coal-face they might be interested. However it's far easier just to get the actuaries to adjust the premium tables after each statistical entry event.
http://www.iasa-intl.com/folders/belfast/off-the-bitter-end_files/second_class_lever-1.jpghttp://www.iasa-intl.com/folders/belfast/off-the-bitter-end_files/lever-1a.jpg

[alf5071h]

OVERTALK, your description of back stick “acting against the distant nose-gear fulcrum to lever the main-gear INTO the deck” requires the nose oleo to act as a solid structure whilst the main oleos compress as indicated in your diagram, the result of which will increase the AOA and wing lift, and thus defeats the objective.

A better example would be to consider the compromise between a compressible and a rigid oleo for both the nose and main gear. At some point, the fulcrum moves to the main gear – a first order lever, and the aircraft will rotate about the ‘rigid’ main oleo. This is the opposite of de-rotating after touchdown, where the aircraft rotates about the main gear as the amount of up-elevator is reduced, transferring some load to the nose gear..
Putting the rotational issues aside, the problem is actually about the transference of load on the gear. Aircraft designs are such that the majority of this is carried on the main gear; the load on the nose gear provides directional control, particularly as airspeed reduces.

Thus, as MFS indicates the % of load that can be transferred to the main gear is proportionally small and depends on many variables in the aircraft geometry and control characteristics. Thus, it is extremely difficult to judge any benefit to be gained, and if any, the effect diminishes rapidly with speed (V*2).

What many pilots perceive is that up-elevator feels as though it helps, but we should remember that the forces being sensed during decelerations can be misleading, particularly so in a simulator.

The simple overview is that the majority of the theory is correct, but in practice, there are too many variables for crews to judge.Thus they should not deviate from the manufacturer’s advice.

OVERTALK your picture links suggest that you have a close association with IASA. If that is so, I suggest that you restrict the more extreme views on how to enhance flight safety, however well intention they might be, to the IASA web site; along with the comments from possible ‘associates’ participating in this thread.

[Mad (Flt) Scientist]

Two points.

One simple. Since distance can be found from (V^2-u^2)/2a assuming constant decel, a 20% change in distance requires a 20% change in accel (decel) and, all other things being equal a 20% change in force. Assuming only wheel brakes are causing any decel that means 20% more braking force, and 20% more mainwheel reaction.

Secondly, there seems to be an implicit assumption that the nosewheel is NEVER going to rise. This is not the case; on our types the 'stick forward' SOP is designed to prevent precisely that, based on experience in flight test. That simple fact is what leads me to plead that at least pilots of our types NOT try this at home.

[chornedsnorkack]

OVERTALK your picture links suggest that you have a close association with IASA. If that is so, I suggest that you restrict the more extreme views on how to enhance flight safety, however well intention they might be, to the IASA web site; along with the comments from possible ‘associates’ participating in this thread.

For me, it seems just the opposite. While IASA is, ad pointed out, a private lobby group, it has a rather official-sounding name, compared to, say, IATA and ICAO. Therefore, it would be better to express extreme views in Pprune , where they can be and are violently critizised, rather than on IATA website where they are not critizised and might be acted upon...

As for the matter:
So, what exactly is supposed to happen when the wheel friction decreases to the extent that the anti-skid overrules autobrake?

As the plane decelerates, the airspeed, and therefore the aerodynamic forces and momenta, decrease in one direction, continuously and smoothly.

Whereas the friction forces can change in any direction, suddenly, as the wheels encounter portions of runway where different conditions prevail.

When the friction under the main wheels changes above the point where the autobrake works at the set deceleration rate, the acceleration loads are unchanged and therefore the load distribution between main gear and nosegear are not subject to sudden changes (they change slowly, with airspeed)

The instant that the friction decreases so far the antiskid takes over, the deceleration rate falls and the weight distribution changes. The weight supported by nosegear decreases (and it was relatively small to begin with). As the nosegear loading falls, the maximum frictional force available to resist sideslip falls (and the friction coefficient may be falling at the same time).

What is the manufacturer-approved SOP pilot response (if any) to antiskid overruling the autobrake? Assuming that the nosegear remains, initially, supported by runway and has not (yet) slipped sideways?

[OVERTALK]

"OVERTALK your picture links suggest that you have a close association with IASA. If that is so, I suggest that you restrict the more extreme views on how to enhance flight safety, however well intention (sic) they might be, to the IASA web site; along with the comments from possible ‘associates’ participating in this thread." Guilty as charged. It's a widespread secret conspiracy designed to stop runway overruns and restore insurance co profitability, destroy the Brotherhood of Unemployed Overrunners and relieve pressure upon the owners and operators of the 305 marginal runways in the USA alone. You've seen through our shabby ploy. How perspicacious of you. But thanks for the giggle anyway. Keep taking that credibility medication and don't give up on that grammar and syntax primer. The only extreme views I hold are panoramic - from my penthouse suite.
.
"your description of back stick "acting against the distant nose-gear fulcrum to lever the main-gear INTO the deck" requires the nose oleo to act as a solid structure whilst the main oleos compress as indicated in your diagram, the result of which will increase the AOA and wing lift, and thus defeats the objective." I believe that the nose-gear was described as a dynamic fulcrum. It's not a rigid hinged pivot. In physics it is not unusual for one moment (the sum of the four pitchdown moments) to be counteracted by another moment (the up-elevator opposing moment). I think that it might in combination be called a "couple". The fact that the nose oleo's compression pressure might be eased somewhat by pilot's backstick doesn't mean that the AoA ogre will suddenly take-over and re-launch one luftwards. The reason that it's called a lever is that (for a 2nd class lever), the mid-span resultant is enjoying a significant mechanical advantage). The effectiveness should not be in question. It's basic physics old chap. Try and reconcile with the dynamics of the situation. The amount of backstick is tempered (modulated/introduced) against the degree of braking achieved..... much as in a taildragger.
Couple: A pair of forces acting in parallel but opposite directions, capable of causing rotation but not translation.
"Putting the rotational issues aside, the problem is actually about the transference of load on the gear. Aircraft designs are such that the majority of this is carried on the main gear; the load on the nose gear provides directional control, particularly as airspeed reduces. Thus, as MFS indicates, the % of load that can be transferred to the main gear is proportionally small and depends on many variables in the aircraft geometry and control characteristics. Thus, it is extremely difficult to judge any benefit to be gained, and if any, the effect diminishes rapidly with speed (V*2). "Effective" weight transference is not about moving the aircraft's mass around like pumping fuel from tail-tank to wings. The backstick uses the ability of the elevator at speed on the runway, very shortly after touchdown, to generate a downwards horizontal stabilizer lift vector which lever-compresses the main-gear oleos and increases the footprint of all main-gear tyres (a very good thing for braking according to Boeing and Airbus - and incidentally also the measure that has most effect upon aquaplaning speeds). Driving the maingear into the pavement significantly increases the tyre surface in contact.... so wheel rotational torque increases, anti-skid becomes less active and braking is thus much more effective. Ipso (the means) facto (the indisputable outcome - real and not as you would have it: "imagined" or "simulatored").
.
You should take the time to read this: (http://www.geocities.com/profemery/aviation/qf1_bangkok.html) (the technique would have helped that QF skipper avoid his hole-in-one jumbo).
.
Thus, it is extremely difficult to judge any benefit to be gained, and if any, the effect diminishes rapidly with speed (V*2). And so, I might add, doth runway remaining - if that speed isn't diminished by the most effective means possible. All the useful retardation will happen in that middle third of the runway. Backstick braking over that stretch will extend your lifespan as a professional pilot. Just sit back and think what it might feel like going off the end and through a blast fence at 50 knots with your toe-brakes fully depressed. You are advocating dismissal of a technique that might help avoid that trauma - just because you personally cannot cope with possible change/ come to terms with having been in ignorance for years? You can be a sceptic (http://www.answers.com/topic/doubting-thomas-2?gwp=19) without being a heretic.
.
"The simple overview is that the majority of the theory is correct, but in practice, there are too many variables for crews to judge. Thus they should not deviate from the manufacturer’s advice.
.
I believe this to be what they call "the neo-Luddite Approach (http://en.wikipedia.org/wiki/Neo-luddism)" You say you agree, but its novelty and other portents (re-introduction of a "new" technique) just strains your brain..... so far better to have the PNF just maintain a religiously neutral stick position after the nose is on - lest the great unknown should up and smite thee for your heresy.
In the fullness of time the industry may re-discover this technique of avoiding overruns. However I must admit that I have little personal faith that this may happen. There is great opposition to change of any sort within the airline industry. But who knows? In five to ten years time, Airbus might have automated the process. Please revisit this thread not before then.
.
CHORNEDSNORKACK
Sorry, but I don't understand technical Esperanto. You will have to re-post that in pellucid English prose. If another Ppruner understood what you were postulating and it made sense to him, I'm sure that he will assist by re-positing the theory. I fear that this thread is being intentionally gibberished (a form of obfuscation) in order to denature it, so I'd ask the mods to please now lock it up.

[RatherBeFlying]

The fulcrum on the ground is the maingear axle. Backstick produces down force at the tail which: Adds to the total load on all gears Produces a nose up moment which relieves a portion of the load on the nose gear such that the maingear supports a larger portion of the total weight plus any aerodynamic downforce.This may result in a change in AOA and lift, but as certain pilots have found this technique useful on certain types, it stands to reason that there remains a substantial net benefit on those types.
The military, certainly in Overtalk's case, often has a more nimble organisational culture that allows researching and promulgating new techniques at a faster pace than do the airlines and the regulators. Improvisation when you're the only one in the a/c is a quite a different matter than when there's a few hundred people in the back. I do hope the manufacturers, regulators and training departments pick up the ball on investigating what appears a promising technique.

[Mad (Flt) Scientist]

Since this point appears to be being overlooked, I'll make it again.

Typical nosegear reactions statically are of the order of 5-10% of aircraft weight; the other 90-95% being borne by mainwheels.

To achieve even a 10% improvement in braking decel - assuming that there is NO aerodynamic download, which is an optimistic assumption for this method - requires a tail download of approx 5% weight and a corresponding nosewheel unload of similar magnitude. That is perilously close to completely balancing the static force on the nosegear, which would mean totally uncompressing the nose oleo, and loss of all NWS effectiveness plus increasing AoA and so reducing downforce on the mains.

For those who doubt that this can happen: one of our test aircraft was offsite conducting Xwind tests in very windy conditions - 30kts plus with gusts to 40kts. While the aircraft was parked into wind, the crew noticed that the effect of nose-up stab trim was such that they almost unloaded the oleos on the nose. AT FORTY KNOTS. At eighty knots - the kind of speeds this technique is being advocated for - the effect would be 4 times as powerful. I'm as certain as I can be that you WILL lift the nose under those conditions.

The idea that significant amounts of nose-up elevator or stab don't risk raising the nose is not well founded. After all, that's how you achieve rotation, no?

I don't believe my OEM is "scared of a new technique". We are scared of someone trying this out with pax in the back and either departing the side of the runway due to loss of NWS/directional control, or raising the nose and actually losing braking effectiveness. I've seen our aircraft at low speeds (typical of landing rollouts) with the nose in the air and have no desirte to see that repeated by line pilots.

[RatherBeFlying]

Static loads while parked and dynamic loads while braking are different cases with greater nose wheel loadings under braking.
Having scuffed my shoes on snow covered runway surfaces, I can testify that there can be substantial difference in runway friction coefficient if the contamination is crunched through.
If that 5/10/20% maingear loading differential yields some 50% increase in friction coefficient, the stopping distance goes down substantially.
So maybe somebody will pull an old jet transport airframe out of the boneyard, take it where there's snow and a long runway and produce some real numbers.

[Mad (Flt) Scientist]

I knew someone would say that ....

The problem is that the case where the nosewheel is most heavily loaded by the braking forces is the case where the brakes are ALREADY WORKING WELL!

For the case where you are most at risk of an overrun - poor braking action - the nosewheel reaction due to braking is small.

Consider the fulcrums and levers everyone here is so keen on. The cg might be, say 5ft above the mains (for a smallish jet). The nosewheel may be 25ft ahead of the mains 9and perhaps more, depending how 'stretched' it is.

Let's assume decent braking action, equivalemnt to 0.25'g' decel. That means that, taking moments about the mains, there's an inertia force of 0.25*aircraft mass acting 5ft above the mains. If that is ALL 'thrown forward' onto the nosegear, the reaction there will be 0.25 * aircraft mass * 5ft / 25ft = 0.05 * aircraft mass.

In other words, even with pretty reasonable braking effectiveness, theres only another 5% mass onto the nose. If braking is poor - say 0.10'g' decel - the effective reaction increase at the nose is only about 2% of aircraft weight. Not enough to overcome full back stick; not even close for some types.

[john_tullamarine]

A thread which I am following with some interest ...

Overtalk,

I don't know your background but it is obvious that you are passionate about your topic and that is a good thing.

However, it may be useful to draw your attention to one point in particular ..

PPRuNe is very fortunate to have a wide range of folk participating in the sandpit and we include some VERY experienced, competent, and knowledgeable technical people. We have, for example, engineering folk who hold university Chairs (one of whom posts with reasonable frequency), quite a number of folk with PhDs, folk who have long track records in engineering design, certification, etc., test pilots and flight test engineers who are eminent in their disciplines, and so the list goes on ....

(a) MFS, for instance, is one such engineer who works for a major aircraft manufacturer. One dismisses his technical comments out of hand at one's peril.

(b) alf5071h, for instance, demonstrates great knowledge and is well credentialed in the human factors arena, in particular. Again, one dismisses his comments out of hand at one's peril.

I only mention these particular posters as I don't know the others ...


Are you, by any chance, falling into the trap of viewing the nosewheel to ground interface as being more of a tied-together mechanism than a tenous conjoining of two surfaces (alf5071h raises this point in his post) ?

I ask this only because you appear uncomfortable with the idea of a low proportion of aircraft weight being on the nosewheel. While this might vary a little, I have to agree with MFS on the 5-10 percent ... and I have done probably more loading statics calculations than all the posters in this thread put together several times over.

Yes, of course the CG moment about the mains under braking muddies the water and one has to look at the overall moment balance .. but, given enough tail force (however much that may be) ... the nose must break ground.

[OVERTALK]

RatherbeFlying said:
The fulcrum on the ground is the maingear axle. Backstick produces down force at the tail which:
Adds to the total load on all gears
Produces a nose-up moment which relieves a portion of the load on the nose-gear such that the maingear supports a larger portion of the total weight plus any aerodynamic downforce.
Can't disagree with that. It's manifestly the distilled essence of the underlying theory. I might have said: "plus the aerodynamic downforce." Seeing it as the resultant of two moments or a type 2 lever acting upon the nose-gear as a dynamic fulcrum is also valid. But what is obvious (and missed by most) is that there is an "effective" weight shift towards the nose. i.e. We are getting side-tracked by static Center of Gravity computations saying that only about 5% of an aircraft's mass acts through the nosewheel. May be true but it's a red herring in this dynamic flight context. RBF's succinct statement also doesn't address (but obviously accepts) the reason why the nose stays down (a nose-down pitch is induced by reverse and whatever braking you are achieving courtesy of spoiler lift-dump when the progressive backstick input commences). As the backstick increases, wheel-braking becomes more effective, thus allowing more backstick etc etc. Eventually you do end up on the backstops, but by that time you will have killed that speed increment above the min aquaplaning speed -as well as any directional control or cross-wind induced problems that you may have otherwise had.
This may result in a change in AOA and lift.... The process is dynamic and what is being advocated is an early accelerated rate of deceleration. That dynamic process compares with the Air France, QF1 and SWA pregnant pause hiatus where there were interlock problems entering reverse and indecision/mind-changing. The latter process eats up landing real-estate at a great rate of knots and kills your options - whereas the backstick braking uses those knots to achieve very early efficient deceleration before the aircraft reaches that last 2000 odd feet of rubberised runway remaining. Chalk and cheese. Success and failure/disaster. I suspect that, in the fullness of time, Airbus (at least) will be automating the advocated process.
MFS said:
"assuming that there is NO aerodynamic download, which is an optimistic assumption for this method...." MFS then goes on to draw invalid comparisons of parked a/c and the effect of significant winds, disregarding the nose-down pitching moments that result from reverse and spoiler-assisted early braking. His conclusions are invalid because we are talking about 2 completely different scenarios:
a. static parked weight distribution with nose-up stab trim combatting local wind effects and
b. a 1.1.Vs or greater handling technique with engines running and reversing, spoilers up and a pilot making appropriate control inputs).
.
It is difficult to clearly discuss the nuances of handling techniques with engineers (even flight engineers) - or so I've found. Handling is not like placing a switch at A, B or centrally OFF. Handling is about moderated input tempered by observed reaction. An OFF/ON selection of instant full backstick is not what's being advocated at all. However MFS does introduce (and remind us of) that all-important "square of the speed" factor - which is also a player when considering the effect of early backstick after touchdown. It is certainly what empowers that backstick-induced early breakthrough (to the bitumen) for the main-gear wheels. "EARLY" is also the point at which the effect of reverse thrust (and its pitchdown moment) is maximal (then gradually tapering off).
I'm as certain as I can be that you WILL lift the nose under those conditions. Well if it wasn't for the four additive factors producing the strong (and opposing) nose-down pitch during deceleration, I might concede that point. But MFS is single-mindedly stuck in his engineer's view of the airplane as a static entity. After all, that's how you achieve rotation, no? Well no, not really. You have to consider that there's a Thrust/Weight-Lift/Drag couple at work there during lift-off, ably assisted by leading- and trailing-edge devices (and minus spoilers, braking and reverse). If you fuel-chopped all four at the point of pre-rotate (V1) would it still rotate??
"We are scared of someone trying this out with pax in the back and either departing the side of the runway due to loss of NWS/directional control, or raising the nose and actually losing braking effectiveness." A mite harum scarum -but that has been your [MFS] underlying theme. I'd suggest that it's only MFS with the trepidations. All the Xperimental TP's that I've known in the RAF, BAe and elsewhere would say: "well let's do some sums and then go find out. Anything that can stop this wasteful "off the end" bizzo is worthwhile exploring and exploiting. It may even be a great marketing tool if it was automated. Let's go talk to some aerodynamics people and perhaps get them to write a program for the iron bird. The simulator should tell us what's what."
RBF said:
"Static loads while parked and dynamic loads while braking are different cases with greater nose wheel loadings under braking." Bravo for keeping an open mind and identifying the unsubtle difference.
MFS said:
1. "The problem is that the case where the nosewheel is most heavily loaded by the braking forces is the case where the brakes are ALREADY WORKING WELL!" MFS -Please see a+b+c+d below for your illogical sequitur
2. "Not enough to overcome full back stick; not even close for some types." Five bucks for whoever can get MFS off his FULL backstick bandwagon.... and start entertaining the full vista of forces at play during the dynamics of landing deceleration. i.e.
,
a. REVERSE - greatest effect at the outset => strong nose-down pitch (NDP)
.
b. Braking - It is agreed that despite spoilers assisting, that braking is minimal at the outset on a slippery surface (but then again, that is the actual problem) but still =>(NDP) (initially only equivalent to only another +5% of a/c mass - according to MFS)
.
c. Center of Gravity - I won't venture whether this is MUCH more pro or con at landing (versus take-off) but I suspect that in the landing configuration it's in most cases helping our quest (for a counterbalance to introduced backstick). But no matter how far it's moved as fuel burns off, it will still be telling the nose firmly "to stay down" => (NDP)
.
d. Spoilers will be up and, per the T-W/L-D couple, should also be helping kill lift and admonishing the nose to stay down. i.e. =>(NDP)
.
a+b+c+d => a powerful counterbalancing (but varying) total nose-down force against which we can introduce our proposed progressive backstick
.
So throughout the early landing evolution, speed dissipates at "some rate" and braking will improve "somewhat". Once in reverse, early pilot introduction of progressive backstick will have the desired effect and will improve braking effectiveness (which will in turn enable more backstick, etc etc). Certainly "forward" stick has more than its fair share of perils (link) (http://www.aviationtoday.com/sia/20030401.htm) ("..it appeared that the captain’s technique for landing at Gibraltar differed from that at other destinations, with an evident, if unconscious, propensity to apply full nose-down elevator right after main landing gear touchdown. Immediately after the incident, Monarch modified its FDM software to include nose-down elevator as an "event."). G-MONC was out of service for over 2 months and required >$6M in repairs.
.
The "effective" weight-shift toward the nose will be that imparted by aircraft momentum and retardation forces acting around various axes (and thus summed in ft/lbs). Overall, the early "stickiness" of the nose very much depends upon the total magnitude of the nose-down couples mentioned above. MFS chooses to ignore the cumulative effect.... and why that should be is a mystery - but perhaps it just helps his argument. It is conceded that the diminishing effect of reverse as the a/c slows will be countered by the increased effectiveness of braking.... probably making it a zero-sum game. However that's of no real consequence because the pitch-up authority of the (by that time) full backstick will have faded to insignificant.
.
The theory disregards all the other factors acting for and against a successful stop within runway available as being irrelevant - however it should be noted that it will be particularly effective at combating high groundspeeds caused by landing with the wind (rather than against it). Sometimes it is expedient to accept a tailwind component, but always it is less safe to do so. At such times, the value of backstick braking will be to restore the balance in favour of the pilot not running out of career or runway. Not to be forgotten (also) is the stabilizing effect of getting the weight shift redistributed to the tricycle geometry that gives best directional control. Stick forward can be destructive (G-MONC) but the wheel-barrowing effect of any unopposed weight-shift forward can also cause great directional instability, particularly on a wet runway in a crosswind.
.
Ultimately the aim is to have the theory revalidated and assessed for effectiveness. It is nothing more than a handling technique that seeks to minimize the intervention of anti-skid on inhospitable surfaces by increasing the weight-on-wheels as soon as possible in the landing roll. If it is assessed as worthy, then everybody is better off....and at nil cost. It's a principle called regrets management....and it contrasts well with risk management (which lets us down more often than not).
Overtalk
Pilot Retard Advocate

[OVERTALK]

JT
I recognize your MFS and alf5071h and apologize for any perceived brusqueness. However intolerant I might seem, I am interested in stopping the pointless overruns. The frustrations arise from "knowing how to" being just drowned (in typical Pprune fashion) by a sea of doubting thomases, many of whom need more than simple convincing. Argument for argument's sake normally sends me scarpering. However I've stuck with this one because I'd like to see somebody somewhere move convincingly on the overrun problem.
However I will also raise you a John Farley and a Tony Spence (ETPS grad) from amongst my familiars. Anthony Brown the well-known Canadian aerodynamicist was a one-time copilot of mine. We've corresponded on major accident causes.
I've previously fought this one out with the famed poisoned dwarf (Bo Plummer, the longest serving RAF A1 QFI) and "Farmer" Rod Brown (2nd longest) - and was not found wanting. Prof Peter Ladkin and I often agree as much as we disagree, but he knows who I am. Ray Hudson (MD-11 and F35 flt control designer) and I have often sparred - and always find middle ground. I've often corresponded with Graeme Braithwaite (Cranfield). I also write for ASW. I could go on.... but won't.
I base my theory on practice - and that's as close as anybody can ever hope to get to a working hypothesis.

[john_tullamarine]

My friend ... no-one is saying that your position is wrong, only that the analysis may be more complex than you suggest.

I would suggest that the others are NOT purporting that the braking loads case is equivalent to the stationary aircraft .. indeed, to do so would make us consider them in a rather strange light.

As you would be aware, no doubt, JF is a regular in these halls and I imagine that he has been following the thread and may well choose to wade in at an appropriate time. The venerable Milt, likewise, probably is finding it hard to restrain himself.

So long as we observe the normal civil niceties, go your hardest with the argument .. this sort of thread is the single most valuable aspect of this forum.

And we are all on the same team .. and all have an equal interest in reducing the abort/landing risk scenario.

[wsherif1]

Gentlemen,

I bow to your obvious superior knowledge of aircraft braking effectiveness. However, I have landed an Electra on a dime with an "in ground effect flare maneuver." The aircraft came to a complete stop upon touchdown. I had to add power to taxi off the runway! This is just an extreme example of momentun control before touchdown, which is not utilized in todays flight operations, to the detriment of flight safety.

[UNCTUOUS]

Wsherif said: "This is just an extreme example of momentun control before touchdown, which is not utilized in todays flight operations, to the detriment of flight safety."
.
Perhaps you should elaborate on your point.
.
As I understand it, OVERTALK's suggestion (by contrast) is to propose a straight-forward means of stopping contaminated runway overruns (ie. accidents) by enhancing post-touchdown braking effectiveness. That might indeed be an example of something:
<<<which is not utilized in todays flight operations, to the detriment of flight safety.>>>
.
Would you not agree? Or are you disputing that backstick braking works? i.e. as a means of achieving effective braking on slick runways. That doesn't appear to be the case. But perhaps you didn't review the whole thread.
.

[Mad (Flt) Scientist]

Simple question.

Is the nose landing gear oleo FULLY COMPRESSED under braking?

If not then ANY reduction in nosegear download WILL raise the nose, by an amount equal to the reduction in download times the effective spring stiffness of the oleo. I would suggest that there are very few types which completely compress the nose oleos, if for no other reason than to completely compress on an oleo risks mechanical damage and significantly affects nose gear damping behaviour.

If that is a significant distance - and on some types it will be, you may rest assured of that, especially at more aft cgs and lighter weights - then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.

Just to be clear: I recognise that OVERTALK and others are suggesting a progressive increase in tail download. The reasons I will CONTINUE to harp on about "full back stick" are:

(1) a procedure which requires modulation of input will ALWAYS carry the risk of someone pulling full back on the "if a little is good, a lot must be better" reasoning

(2) in order to obtain a significant improvement in braking a LOT of tail download is required - to the extent that full back stick will be used at some point

There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load.

[OVERTALK]

MFS said:
Is the nose landing gear oleo FULLY COMPRESSED under braking?..............................................
....then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift. I think that we have lost the physics bubble here. The four retardation scenario factors discussed (a-b-c-d above) are producing a strong nose-down pitching moment. By opposing that moment with progressive backstick soon after spoilers are up and reverse kicks in, we must end up with an effective weight increase on the main-gear - which helps the "rubber more intimately meet the road" so to speak - instead of glissading over the wet and slippery surface. It makes early braking much more effective by minimizing the intervention of anti-skid; and it's even got the improved directional control bonus. If you multiply that tyre/bitumen area interface increase on one tyre by all the tyres on a plane's main-gear, you can appreciate that the overall effect will be worthwhile.
<<<"then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.>>> This is a little confusing but I imagine that you meant to say that up elevator would cause the nose oleo to extend and consequently the wing's AoA to increase, thereby acting in an opposite sense to what is being claimed (i.e. reducing the weight upon the main-gear). That that is a fallacy can be exposed by a simple illustration. Put a soft pea (or jelly-bean say) under the midpoint of a 12" rigid ruler and press down on both ends of the ruler. Do it with roughly equal force, simulating a pilot tempering his backstick input to just below what "could" (not saying "would") possibly cause the nose to rise. In other words, braking "for effect" by flying the attitude out the front window. The pea gets squashed of course. That simple experiment replicates the 4 retardation factors producing a nose-down moment at one end of the ruler and, at the other end, the opposing nose-up (i.e. tail-down) moment induced by the backstick. It adequately demonstrates that the resultant will be a down-force upon the maingear (the pea). Now do it with a new "soft pea" yet without the simulated pilot's up-elevator input. The nose drops, tail rises and the pea is unsquashed. That simulates the lack of anything other than the ruler's weight acting upon the "maingear" pea. It emphasizes the need for early introduced backstick braking when the chips are down (i.e. you suspect that you may have landed too far in on a slippery runway and urgently need max effective braking ASAP).
If I was in the RHseat and the reverse had cut in and I knew that we were "down, come what may", yet running short of bitumen - well I'd be urgently calling for backstick and double-checking that the spoilers were up. I have a mind's eye image of both SWA 737 pilots urgently tromping their toe-brakes at Midway recently, the end looming large - yet salvation having been only a pole-grip away. That's the needless futility of not understanding the logic behind backstick braking.
Just to be clear: I recognise that OVERTALK and others are suggesting a progressive increase in tail download. The reasons I will CONTINUE to harp on about "full back stick" are:
(1) a procedure which requires modulation of input will ALWAYS carry the risk of someone pulling full back on the "if a little is good, a lot must be better" reasoning
(2) in order to obtain a significant improvement in braking a LOT of tail download is required - to the extent that full back stick will be used at some point
Disagree with your point (1) because I've been there and done that. Pilots that I have trained are told to introduce toe-braking first and then (but almost simultaneously) progressively feed in the backstick, ensuring that the nose stays down. Unless they soon ease up significantly on the brakes while maintaining "the pull", the nose won't rise - in part because, by that time, you will have slowed significantly. It is the hard braking that keeps the nose down (as well as the airspeed loss). The danger of teaching it in a jet without maxarets or anti-skid on a wet runway is perversely that BLOGGs will overcook on the toe-brakes without having adequate backstick in. It's the backstick that keeps the wheels loaded up and the tyres rotating. If you don't stop the wheel, you don't blow the tyre. I can recall writing that across the top of whiteboards in my briefing cubicle for many early type-conversion handling briefs. With autobrakes and anti-skid, the whole exercise is just too easy (unless you're ignorant of the technique and/or don't mind motoring mindlessly off the end). I reiterate that pilots are taught to use two controls at once (in fact directional rudder inputs plus toe-braking, aileron into wind and steering with cocked throttles to compensate for wind is a normal pilot activity that has four unique and discrete inputs). And if challenged, most could probably make an R/T call at the same time. You seem to have a picture of pilots as motor morons incapable of coping with a simple manipulative task. Landing a heavy-weight asymmetric airplane on a wet runway with 50 kts across used to make me salivate with anticipation..... not dread.
(2) <<<....to the extent that full back stick will be used at some point. Agreed (that full backstick may be used), but if that occurs more latterly in the piece, where's the harm? I reiterate that once heavy braking is underway, with or without reverse, that nose-rise just won't happen.
There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load. Unfortunately that 67% is roll-out, if I'm not mistaken. Few pilots manage to get to grips with the first few feet of runway. Indeed many PAPI's and glidepaths would have you landing over 1500ft in. When pilots misjudge and make a name for themselves, it's usually because they've grossly misjudged. That can happen as a result of an optical illusion or down-sloping runway or a tail-wind. It's relatively easy to discard 2 or 3 thousand feet of bitumen, yet be unaware of it. Why? Because we haven't got eyes in the backs of our head and a touchdown too far in is not readily apparent. Distance-to-run marker boards are usually only found at military airfields. Landing 1/3rd into a runway at night off a low ceiling in poor vis? That's easily done. When the runway is additionally greasy and ATC has arranged a tailwind for you, don't start any wishful thinking after you've touched down and ripped it into reverse. At that point the writing's already on the wall at the far end of your marginal runway.

[john_tullamarine]

(a) the landing distance factor is applied to the demonstrated flight test/aerodynamic model data as accepted by the Regulator and applies to the total demonstrated distance from 50ft, not rollout distance.

(b) may I suggest that, if the approach is flown accurately to a predetermined aiming point, few pilots with any experience to speak of will fail to detect that they have floated significantly beyond the planned touchdown zone ...

[UNCTUOUS]

<<<total demonstrated distance from 50ft,>>> by test pilots, probably in good weather in a pristine jet.

<<<<if the approach is flown accurately to a predetermined aiming point, few pilots with any experience to speak of will fail to detect that they have floated significantly beyond the planned touchdown zone ...>>>>

Must be another explanation for all the overruns then, although most that I have read about or heard about seem to have landed well down the runway and frequently hot. Usually there's a weather component or tailwind. The Air France A340 in Toronto touched down around 2000m in.

Rejecting a proposed technique on the basis or with the assertion that pilots wouldn't have problems if they did everything correctly is to deny that fatigue, illusions, CRM breakdowns and mishandling regularly occurs.

That ruler and pea analogy above (in OVERTALK's post) works for me. What further proof does anybody need (beyond flight test)?

[wsherif1]

.
Perhaps you should elaborate on your point.
.
As I understand it, OVERTALK's suggestion (by contrast) is to propose a straight-forward means of stopping contaminated runway overruns (ie. accidents) by enhancing post-touchdown braking effectiveness. That might indeed be an example of something:
<<<which is not utilized in todays flight operations, to the detriment of flight safety.>>>
.
Would you not agree? Or are you disputing that backstick braking works? i.e. as a means of achieving effective braking on slick runways. That doesn't appear to be the case. But perhaps you didn't review the whole thread.
.

Wsherif1 I do agree.

[alf5071h]

There is general agreement about the theory of how progressive back stick during landing should increase main wheel load and assist braking, although there are some interesting and debatable explanations of the physics, actually mechanics, of how this is achieved.
There have been several parallel inputs about the overall effectiveness of the technique when applying it practically to a wide range of aircraft and operations, and many issues have been identified which could negate any improvement or even be detrimental to safety, especially in crosswinds.
Given these inputs, it is difficult to understand how individuals can persist with the view that their specific experiences on one or a few aircraft will apply universally to all aircraft and operations. There is an overwhelming need to remind operators to heed manufacture’s advice to ensure continued safe operations; the doubters should read the thread on the Emirates A340 incident.

I had hoped that from the posts of 10th Jan alternative aspects of the ‘interest in stopping pointless overruns’ would be explored.
There have been several threads on the hazards of contaminated runways (where 67% margin may not be available), and then there are the issues of landing long and / or fast, for which there are approved solutions with proven safety benefits.
In recent years, there has been more focus on human factors, particularly the frailties in awareness and decision making. Therefore, perhaps the safety focus that we require is ensuring pilots’ use of their ‘superior judgment’ before landing, rather than the superior skill on the runway, which some would claim.

[Blip]

Mad (Flt) Scientist said:

There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load.


My understanding of the margins built in to the Landing Distance Required (for the B737 at least) are:

DRY RUNWAY: Actual demonstrated landing distance from 50 ft to complete stop multiplied by 1.67.

WET RUNWAY: (no more than 3 mm of standing water): Actual demonstrated landing distance (on a dry runway) multiplied by 1.67, multiplied by 1.15.

CONTAMINATED RUNWAY: (more than 3 mm but no more than 13 mm of standing water, compacted snow, ice): "Calculated" landing distance for either Good/Fair, or Poor reported braking action multiplied by 1.15. So the margin for error and variables is only 15%, not 67%.

Interestingly, our manual states that contaminated runway performance data is only "Guidance Information".

So with the contaminated runway scenario in mind, any technique that improves the actual landing distance by just 1.5% increases the safety margin by a whopping 10% (1/10th of 15%)!!

It's been a very interesting thread so far. Thanks everyone.

[Dagger Dirk]

Re BLIP's Post
Book figures for landing are good for planning and inflight review (except for contaminated runway performance data).
Unfortunately when flight crews get it wrong, they seem to do it big-time, with or without CRM. Or maybe it's just that many of the variables are actually indeterminants. As OVERTALK said in his first sentence at post #1 on this thread, landing overruns are happening all the time. It's only when they have a nasty outcome that goes beyond embarrassment, lost jobs and muddy, scrubbed or flattened tires, that we get to hear much about it. The fact that the industry is (or was) unaware of the value of progressive backstick braking on nasty surfaces is unsurprising. Offhand I can think of numerous things that the industry was apparently unaware of:
.
a. the dangers of certain types of wiring and neglecting wiring husbandry (EAPAS NPRM comments close 03 Feb 06)
b. the dangers of Concorde tire failures (court-case soon to begin)
c. the perils built into faux redundancy (two computers each empowered to take over from the other when it is assessed that the primary computer has failed) i.e. G-VATL's fuel transfer double-flameout.
d. the dangers inherent in forgetting to ARM spoilers (AA1420)
e. the lethal unpredictability of SLD icing (freezing rain)
f. the flammability of heated tank ullage
g. the dangers inherent in resetting CB's (because ensuing arc-tracking faults won't re-trip them)
h. the hazards of designing identical interchangeable fuel gauges for different models (ATR42/ATR72)
i. the hazards of sandpaper textured rime ice on supercritical wing sections. (CL-600)
j. the pitch-up illusion (GulfAir A320)
k. the flammability of metallized mylar thermal-acoustic blankets
l. the idiocy of having a take-off configuration warning horn identical to a pressn warning horn. One oft heard, the other rarely heard - and every chance that a hypoxic crew wouldn't make the right choice. Inaccessible hypoxic pilots behind an impenetrable reinforced door.
m. Software that can allow a fatigued crew to leave previous take-off calc figures in place (and operable) for their next departure (HFX 747)
n. A closed runway with lethal WIP that can be mistaken for the duty runway due to insufficient markings (no active alerting/just passive cues) SQ006
o. A bogus FMC fuel usage consumption that would suck in a crew transiting with the gear down (Hapag A310) or an FMC that allows a crew to enter nonsense take-off parameters (SIA 744 Auckland).
p. Fuel leakage scenarios and checklists that can sucker a crew into believing it's fuel imbalance
q. the ability to down a modern airliner by just bugging the pitot or taping over the static ports
r. A CRJ engine that requires a massive sustained speed increase to achieve sufficient fan rotation for an inflight relight
s. A rudder handling and RTL design flaw that allows a vertical fin to be torn off in the blink of two eyes. A rudder that can disintegrate in flight with little more than a shudder/shake (and no pilot input - Air Transat ex Cuba)
t. The design of a runway incursion system that is useless in rain and doesn't warn pilots directly (AMASS)
u. A Beech 1900 maint manual error that was ancient but would sucker engineers into fatally misrigging an elevator (Colgan and Air Midwest)
v. Standby horizons that are panel central nowhere near the scan of PF or PNF (aka "twinning, where any anomaly between roll or pitch-rate of an immediately adjacent STBY instrument and primary attitude reference is immediately apparent - KAL747F Stansted, Air India 747 Bombay and many others).
w. Using an a/c NAV configuration that is open to auto-resetting to a VOR operating on TEST (A320 North Africa)
x. Jeppesen databases that could dial up a distant NDB in error (AA 757 Cali) long enough to CFIT the crew
y. A speedbrake that wouldn't auto-retract in response to a full power GPWS response. (AA 757 Cali)
z. bits falling off shuttles and NASA conveniently assuming that high-speed light-weight foam was harmless (despite ample evidence over many flights that tile damage was occurring).
.
etc etc etc. Yes we're in great shape. Unawareness is blissful ignorance. But this thread has adequately demonstrated also how some people will argue on quite specious grounds for their right to not know or be told.
The only sure thing is that nobody will be surprised at the next jaw-dropping fatal revelation.

[john_tullamarine]

Dagger Dirk indirectly highlights a very important consideration ... SOPs seek to address the fact that the guy on the line doesn't have all the information or answers and, to a significant degree, can insulate himself from embarrassment by sticking with the published words of wisdom .. not a guarantee, just an example of sensible risk management/minimisation.

So far as landing is concerned, the principal hazards which the pilot can influence include

(a) approach profile control, minimising float .. ie land shortly after the aiming point ... ALL the time, even if the runway has 15000 ft to play with.

(b) getting the configuration/speed correct - autobrake, autospoiler, flap setting, speed additives, manual brake, reverse and spoiler use, and so on.

At the end of the day, the pilot is paid to think and be aware of what is going on around him .. the enquiry will never be complimentary if the aircraft was hot, incorrectly configured, floated way in (how about a miss ?), boards/autobrake/reverse failed to operate .. but the pilot just sat there with a dazed expression on his face.

Some of us wonder whether this will become an increasing philosophical problem as/if the depth of training and knowledge is watered down in the pursuit of profit ?

[wsherif1]

.
Perhaps you should elaborate on your point.
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As I understand it, OVERTALK's suggestion (by contrast) is to propose a straight-forward means of stopping contaminated runway overruns (ie. accidents) by enhancing post-touchdown braking effectiveness. That might indeed be an example of something:
<<<which is not utilized in todays flight operations, to the detriment of flight safety.>>>
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Would you not agree? Or are you disputing that backstick braking works? i.e. as a means of achieving effective braking on slick runways. That doesn't appear to be the case. But perhaps you didn't review the whole thread.
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I do agree. wsherif1

[Mad (Flt) Scientist]

I think that we have lost the physics bubble here. The four retardation scenario factors discussed (a-b-c-d above) are producing a strong nose-down pitching moment.
Not so, at least on our types.
a. Reverse thrust on our aft-mounted types causes a strong tendency for the nose to rise; for that very reason our advice to pilots is moderate forward stick force to prevent the nose from rising.
b. Braking. As I noted above, the best you can hope for is 5% or so additional mass on the nose - under relatively good braking action. With poor braking action - say 0.10'g' or lower decel - you'll be lucky to see more than 1 or 2% mass 'transfer' occur.
c. c.g. - not sure exactly what you allude to here. Are you assuming that the landing cg is further forward than at takeoff? It hardly matters, for most practical aircraft configurations this is basically the static nosewheel load - usually, as discussed above, of the order of 5-10% of aircraft weight.
d. spoilers. Depends on their location on the wing, degree of sweep etc; entirely possible to have a nose UP moment due to spoilers, since they are AFT of the cg and above or aft of the main gear.

The best you could hope for is the 5-10% basic, static, nose loading, plus an additional 5% due to decel 'weight transfer' under good braking. There are other factors acting to raise the nose, on our types at least, so that represents an OPTIMISTIC assessment of the force on the nose gear.

If you multiply that tyre/bitumen area interface increase on one tyre by all the tyres on a plane's main-gear, you can appreciate that the overall effect will be worthwhile.

Actually the contact area itself is of negligible impact on braking effectiveness; it might be counter-intuitive, but all that matters is contact download, not the area of tyre/ground contact.

This is a little confusing but I imagine that you meant to say that up elevator would cause the nose oleo to extend and consequently the wing's AoA to increase, thereby acting in an opposite sense to what is being claimed (i.e. reducing the weight upon the main-gear).
Absolutely; any unloading of the nose gear will allow some extension of the gear, raising the nose and increasing AoA with the noted unloading of the mains. Unless your nose is basically burying itself in the ground, with oleo fully compressed, before you apply the back stick then the nose MUST lift. The question is, how much.

That that is a fallacy can be exposed by a simple illustration. Put a soft pea (or jelly-bean say) under the midpoint of a 12" rigid ruler and press down on both ends of the ruler. Do it with roughly equal force, simulating a pilot tempering his backstick input to just below what "could" (not saying "would") possibly cause the nose to rise. In other words, braking "for effect" by flying the attitude out the front window. The pea gets squashed of course. That simple experiment replicates the 4 retardation factors producing a nose-down moment at one end of the ruler and, at the other end, the opposing nose-up (i.e. tail-down) moment induced by the backstick. It adequately demonstrates that the resultant will be a down-force upon the maingear (the pea). Now do it with a new "soft pea" yet without the simulated pilot's up-elevator input. The nose drops, tail rises and the pea is unsquashed. That simulates the lack of anything other than the ruler's weight acting upon the "maingear" pea. It emphasizes the need for early introduced backstick braking when the chips are down (i.e. you suspect that you may have landed too far in on a slippery runway and urgently need max effective braking ASAP).

OK, let's stick with the 12" ruler, and make it more like the actual aircraft conditions.

Put the "mainwheel jelly baby" in the middle, and stick something fairly heavy on top of the ruler, pretty much right above it. Say a coffee mug or something. That squashed the baby a bit - representing the bulk of the aircraft weight on the mainwheels.

Now put a smaller jelly baby - just a head, perhaps? - near the "front" of the ruler, to represent the nose gear, and then slide the mug forward a little until there's a little bit of load on the "nosegear jelly baby" too. That represents the aircraft under normal static conditions.

Now take a small glass - say "shooter glass" sized and place it above the nose gear. That's our "weight transferece" due to various effects. Fill it with water (it might tip off, so don't risk alcohol, no sense in wastefulness) to simulate the effect of increased braking/increased nose weight transfer. What should happen is that the "nosegear jelly baby" gets more squished due to this increased load.

Now, apply back stick by pushing down at the back of the ruler with your finger. As you do so you will both squish the "mainwheel jelly baby" AND "unsquish" the "nosewheel jelly baby". The only circumstances under which you will NOT unsquish the "nosewheel jelly baby" is if the load was so high that it totally flattened it first. In which case you'd have some ability to apply tail download before affecting the angle of the ruler.

And THAT is why I mentioned complete nosegear oleo compression.

You seem to have a picture of pilots as motor morons incapable of coping with a simple manipulative task. Landing a heavy-weight asymmetric airplane on a wet runway with 50 kts across used to make me salivate with anticipation..... not dread.

That's all very well for a skilled pilot; procedures are written, have to be written, so that a pilot of minimal skill can apply them reliably. I'm afraid I've seen enough cases where everyone is sat round the table - our own training and test pilots included, I might add - looking at each other, wonder what the HELL was pilot X thinking when he did whatever the FDR is showing.

And, incidentally, let's assume that I'm wrong, and that a reliable and simple technique can be demonstrated to provide a, say, 10% reduction in landing distance. Then what would happen next is that we (the OEMs) would take credit for the technique in our published distances which would result in every landing becoming potentially more marginal. The technique wouldn't be something in your back pocket for a bad day when you need it; it'd be something you'd have to use every day, and would be assumed to use.

[John Farley]

This thread has become rather personalised – but that so often happens here.

It started with an excellent proposal – pull the stick back to improve braking in the landing run.

This simple idea is in danger of being lost so (if I may be allowed) I will start the thread all over again.

A suggestion to reduce the chances of an overrun accident

On a tricycle gear aircraft pulling the stick back increases the effective ‘weight’ on the main wheels and in many circumstances this will increase the retardation available from the brakes. It will never make the brakes less effective. The military use this technique widely so does anybody know why the civil manufacturers apparently ignore it?

My guess is that they ignore it because the precise effect would be too difficult to quantify due to the many variables that would be involved between different landings. So therefore they say if you can’t actually quantify the benefit it has no place in the manual.

Anybody know?

Any civil testers out there care to comment?

[Genghis the Engineer]

Civil practice is generally straightforward enough.

You define the practice used to achieve the scheduled field performance in the manual, and describe that in the POH.

It's actually common to have aircraft where it's known that the POH scheduled performance could be improved upon - but a conscious decision was made to use a more conservative (and thus predictable) set of handling actions to safeguard the company from liability in the event that the pilot failed to take the actions exactly as the company TP had.

Incidentally, the last light aircraft I test flew (a new French model) had all three wheels braked - it was like hitting a brick wall!

G

[john_tullamarine]

.. as with the nosewheel brake mod to the 727 ... gets the pilot's attention smartly if the pedals are pushed harder than intended in normal operation.

[RatherBeFlying]

Have finally been driven to pull out my '72 edition of Kermode's Mechanics of Flight. Appendix 2 presents acceleration formulae in various units.

Taking 4b. F = (W/g)a:
a 744 at 644,000 lbs with autobrakes producing a 10 ft/s/s deceleration and g of 32.2 ft/s/s produces a braking force of:

F = (644,000/32.2)10 = 200,000 pounds

Wildly assuming a CG 15 ft above the ground for this tall a/c (pending a better figure from the more knowledgeable) produces a nose-down moment of 3,000,000 ft-lb.

Assuming a mlg to nosewheel distance of 100 ft. produces a corresponding nosewheel loading of 30,000 (3,000,000/100) lbs from braking.
Assuming a static nosewheel loading of some 5% produces some 30,000 lbs of loading from the CG before the mains.

So braking near autobrake max potentially doubles nosewheel loading.

Poor braking action can drastically reduce the extra loading on the nosewheel, but there remains a potential that as deceleration increases as a result of some backstick, greater amounts can be progressively applied.

The maximum case is nosewheel loading reduced to zero by back stick with good braking action. The mlg loading will in this case be increased by: the static and dynamic loading previously on the nose wheel the tailplane downforce (approximately equal to the nosewheel loading)which comes to some 120,000 pounds, i.e. some 20% increase in mlg loading.

I would welcome the application of precise numbers for any type.

[alf5071h]

My guess is that they ignore it because the precise effect would be too difficult to quantify due to the many variables that would be involved between different landings. So therefore they say if you can’t actually quantify the benefit it has no place in the manual.
Anybody know?
Any civil testers out there care to comment?

John, the answer is partly in your guess. It is very difficult to quantify; manufacturers may even have different or even opposing techniques amongst their aircraft types. It is not just about improving the published stopping distance, which in normal (regulatory approved) operations should have sufficient safety margin, but also about many other aspects of control on the runway, which may not have such generous margins, e.g. crosswind.
Civil testers? Well at least two have commented.

I have sat in many meetings similar to those related by MFS; in addition to manufacturers asking ‘why did they do that’, chief pilots should consider ‘could their pilots do that?’. Many people cannot contemplate the range of situations they could encounter or put themselves in, or the behaviors that pilots might (or in fact do) exhibit. Even when individuals consider ‘could it ever happen to me?’ a realistic answer may only be available in hindsight through analysis and understanding, only then is the error provoking situation or the personal behavior seen to be hazardous.

Part of this discussion is about discipline, the need to follow procedures and avoiding hazardous attitudes such as ‘I can do’. Similarly, there is the need to resist peer pressure, which is not aided by well intentioned suggestions that are promoted beyond responsible boundaries.
Discipline is the foundation of airmanship; this thread also relates to other qualities such as skill, knowledge, situation awareness, and judgment.
In seeking an end to runway overruns, pilots’ need thinking skills in addition to those of flying, they need greater knowledge of the regulatory assumptions to identify those situations where the margins in landing distance are significantly degraded, or the unreliability of braking coefficient values that could lead to misunderstanding the situation. These aspects have been discussed in related threads.

An overview of accident reports identifies two fundamental causal themes, either the crew did not understand the situation, of if they did, then they chose the wrong course of action. If pilots have to consider alternative techniques on the runway then they have failed in the first instance, misjudging the situation, the need for a go around, as well as not applying their skills to land at the correct speed or position; if they had been successful in these aspects then there should be no doubt about stopping. However, having made such mistakes and arrived on the runway, a back stick technique is not guaranteed to recoup the hazardous situation and may make it worse.

A quick and un-scientific assessment of recent accidents (before the facts are known) suggest that a small increase in braking effectiveness at high speed would be unlikely to have prevented the result; it might have alleviated some of the consequences, but equally it could have resulted a lateral deviation into far greater hazards (N.B. Toronto wind shift and off-runway hazards).
Of the 5 or so overrun accident investigations that I was associated with, none would have been prevented by a small increase in braking effectiveness. The majority had root causes involving human behavior in the air, the others, human error on the ground, which involved perception and incorrect use of retardation devices.
Thus, we already have large variability in human behavior before anyone adds more from aircraft control techniques.
_______________________________________________
Unless specifically authorized everything else is forbidden.

[John Farley]

Thanks Alf

The aim of my post was to simplify things and get back to the basics of the notion.

It failed!

JF

[skiesfull]

I would suggest an answer to why this maneouvre is not used by the civil aviation world, is quite simple. Most line pilots would be incapable of landing safely on a contaminated limiting runway using a technique that involves usage of rudder,ailerons and pitch (other than lowering the nose) and handling reverse levers at the same time. The proof is in the safety statistics which show that basic piloting abilities do not seem to have improved much over the years, but our training is full of CRM and situational awareness courses and the addition of various gadgets and gizmos mean that airline boards insist on their usage, to the detriment of manual handling improvement.

[OVERTALK]

MFS said:
Not so, at least on our types.
a. Reverse thrust on our aft-mounted types causes a strong tendency for the nose to rise; for that very reason our advice to pilots is moderate forward stick force to prevent the nose from rising.
Can't dispute that authoritatively except to say that there are quite a few limitations on high aft-mounted engines as far as reverse goes (i.e. the restriction to 80% available reverse due to rudder blanking on MD80/717 types would also help limit any pitch-up due to reverse). So McDD put a forward stick recommendation in the Pilots' Handling Notes eh? I wasn't aware of that. Only goes to show that it's always horses for courses and no one handling technique can be seen to be a universal panacea. I'd have thought that spoiler and braking would've easily overcome any such pitch-up tendency - with there being not much of a lever arm between those rear engines and the main-gear. The demise of the 717 may help write finis to this consideration in the fullness of time. But many newer design rear-engined smaller jets such as the CRJ seem to have similarly mounted engines....so perhaps someone can comment on whether the CRJ has a similar caution written into the Pilots' Notes. link (http://www.tsb.gc.ca/en/reports/air/1996/a96a0050/a96a0050.asp)
Then what would happen next is that we (the OEMs) would take credit for the technique in our published distances which would result in every landing becoming potentially more marginal. The technique wouldn't be something in your back pocket for a bad day when you need it; it'd be something you'd have to use every day, and would be assumed to use. At present the FAA is very very circumspect in defining techniques and configurations for establishing "book" distances - so I'd see that view as being unnecessarily alarmist. e.g. <<The standard curves (i.e., equations) of braking coefficient versus speed prescribed in §25.109(c)(1) are based on a tire tread depth of 2 mm. Since the tread depth of new tires is usually 10-12 mm, 2 mm represents no more than 20 percent of the original tread depth. FAA Advisory Circular 121.195(d)-1A, which provides guidance for determining operational landing distances on wet runways, specifies that the tires used in flight tests to determine wet runway landing distances should be worn to a point where no more than 20 percent of the original tread depth remains...etc>>. So the backstick braking technique would be no different to (say) not instinctively trying to pick up a dropped wing with aileron near the stall (another non-intuitive pilot-developed skill). I'm not sure that ALPA or IFALPA would agree with your continued portrayal of professional pilots as having to conform to your depiction of their abilities, skills and challenges necessarily being limited (and conforming) to a lowest common denominator.
any unloading of the nose gear will allow some extension of the gear, raising the nose and increasing AoA with the noted unloading of the mains. Unless your nose is basically burying itself in the ground, with oleo fully compressed, before you apply the back stick then the nose MUST lift. The question is, how much.Not sure that this is a productive piece of imagery. It's one reason why my first inclination was to first use the analogy of the second-class lever (with the nose as fulcrum). Using that logic the backstick just lowers the tail and loads up the main-gear, with the nose-gear acting as a pivot-point. John Farley has endorsed the ability of backstick to "load up" the main-gear for more effective braking on mush - so perhaps we can leave this nicety to be thrashed out by the aerodynamicists (who will see it as the resultant of couples (nosedown pitching moment and taildown pitching moment). I've personally never been able to raise the nose whilst under backstick braking. As a pilot it's obviously something that would be quite attention-getting. Because the ergonomics are optimal, you find that the more you haul back, the greater the braking that you can apply without wheel-skid. The dynamism of backstick braking is such that any minor degree of "nose-rise" induced mainplane AoA would be more than offset in the lift equation by the rapidly diminishing IAS.....However if anyone is into cheap thrills, try removing the backstick whilst under concerted backstick braking. I guarantee that you'll only try that once, if at speed on a wet or slushy runway.
Actually the contact area itself is of negligible impact on braking effectiveness; it might be counter-intuitive, but all that matters is contact download, not the area of tyre/ground contact. The NASA study on a tyre tread's effect on hydroplaning and braking friction coefficients places great emphasis upon the grooved circumferential treads being in contact with the runway, and that being a function of weight-on-wheels. For more groovy contact I'd imagine that you'd need a larger "footprint". However we are all agreed (I think) that weight-on-braked-mainwheels is the name of this game.
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HYDROPLANING is a condition that can develop whenever a tire is moving on a wet surface. The tire squeezes water from under the tread generating water pressures which can lift portions of the tire off the runway and reduce the amount of friction the tire can develop. On a runway contaminated by rain or wet snow, it can be impossible for an airplane to accelerate to take-off speed and then to stop on the remaining runway in an aborted take-off. During landing, deceleration and stopping an airplane can be similarly compromised.
Just to be clear (due to some PM's Rx'd). There are three types of hydroplaning. We're interested in #2 below and R-R hydroplaning to a minor extent (because of heavy rubber deposits).
Viscous hydroplaning occurs when there is a thin film of water and relatively low tire speeds. The water lubricates the surface and decreases traction. A water film of only a tiny fraction of a centimeter will drastically reduce the friction between the tire arid the pavement and double the stopping distance.
Dynamic hydroplaning requires deeper water and results in complete loss of tire contact with the pavement. The tire lifts off the runway and rides on a wedge of water.
Reverted-rubber hydroplaning can occur when a locked tire skids on a wet or icy runway. Frictional heating raises the tire temperature causing rubber particles to shred off the tread. These particles accumulate behind the tire forming a dam that blocks the escape of water. The trapped water heats and turns to steam. The steam pressure lifts the tire from the surface.
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Your counter-analogy to the "ruler and pea" is totally and transparently specious. One nose-heavy moment versus a tail-heavy moment and the main-gear as pivot-point..... that is adequately illustrative of the forces involved.
worth reading (http://www.aviationnow.com/avnow/news/channel_awst_story.jsp?id=news/NTSB12205.xml)
also worth a look (http://oea.larc.nasa.gov/trailblazer/SP-4216/chapter8/ch8-3.html)

[Dagger Dirk]

Alf5071h said:
<<<John, the answer is partly in your guess. It is very difficult to quantify; manufacturers may even have different or even opposing techniques amongst their aircraft types. It is not just about improving the published stopping distance, which in normal (regulatory approved) operations should have sufficient safety margin, but also about many other aspects of control on the runway, which may not have such generous margins, e.g. crosswind.>>>If margins were sufficient, there wouldn't be so many overruns. Runway side excursions are probably more about crosswind technique, gusting crosswinds and/or momentary loss of visibility (e.g. link (www.tsb.gc.ca/en/reports/air/2003/A03A0012/A03A0012.asp)). Off-the-end overruns tend to be more about landing long and hot where there's inadequate braking because of the slippery surface. That's where backstick braking comes in as a means of improving upon the situation that a pilot is stuck with - once in reverse.
.
<<<I have sat in many meetings similar to those related by MFS; in addition to manufacturers asking ‘why did they do that’, chief pilots should consider ‘could their pilots do that?’.>>> In challenging conditions the PF should be the captain. If a captain cannot embrace a simple handling technique, then that organization should look closely at its own upgrade standards. It's not as if it's not replicable in a simulator. <<<Many people cannot contemplate the range of situations they could encounter or put themselves in, or the behaviors that pilots might (or in fact do) exhibit. Even when individuals consider ‘could it ever happen to me?’ a realistic answer may only be available in hindsight through analysis and understanding, only then is the error provoking situation or the personal behavior seen to be hazardous.>>> Hmm, take a simple handling nuance and surround it with portentous fear and ominous loathing. A little OTT I feel.
<<<Part of this discussion is about discipline, the need to follow procedures and avoiding hazardous attitudes such as ‘I can do’. Similarly, there is the need to resist peer pressure, which is not aided by well intentioned suggestions that are promoted beyond responsible boundaries.>>> Authoritarian suggestions of latent irresponsibility deteriorate in the following sentence to good old placatory and advocatorial homilies. Yet it does nothing to add to the discussion and is quite devoid of useful relevant argument.
Discipline is the foundation of airmanship; this thread also relates to other qualities such as skill, knowledge, situation awareness, and judgment.
.
<<<In seeking an end to runway overruns, pilots’ need thinking skills in addition to those of flying, they need greater knowledge of the regulatory assumptions to identify those situations where the margins in landing distance are significantly degraded, or the unreliability of braking coefficient values that could lead to misunderstanding the situation. These aspects have been discussed in related threads.>>>Strictly by-the-book stuff. ALF, methinks that you are ignorant of (or forgetting) that arriving on "perhaps contaminated" marginal runways is a totally inexact science. In fact it is a "no mans land" full of hidden mines. That aspect and all the operational pressures, subtle and unsubtle, is being neatly side-stepped by you ALF. Pilots also need to be well insulated against fatigue and errors of judgment when the adrenaline suddenly cuts in and they're down and in reverse and truly have nowhere to go but off the end - "insulated" by having a fall-back position. So if somebody in the RHS (or LHS) is aware of this backstick braking technique and later honestly attributes not overrunning to backstick braking, are YOU going to issue plaudits or censures? Good risk management is all about providing accessible fallback positions..... not arbitrarily curtailing them by decree.
.
<<<An overview of accident reports identifies two fundamental causal themes, either the crew did not understand the situation, of if they did, then they chose the wrong course of action.>>>Dishonestly simplistic. <<<If pilots have to consider alternative techniques on the runway then they have failed in the first instance, misjudging the situation, the need for a go around, as well as not applying their skills to land at the correct speed or position;>>>and therefore deserve to have an accident but only in the approved manner. <<<if they had been successful in these aspects then there should be no doubt about stopping. However, having made such mistakes and arrived on the runway, a back stick technique is not guaranteed to recoup the hazardous situation and may make it worse.>>>....so these pilots and their pax should just take their lumps in the approved manner by actively discarding such potentially disastrous and unapproved last-ditch measures such as backstick braking. By "may make it worse" ALF is suggesting that the accelerating qualities of backstick braking are as yet unknown.... but suspected to be potentially lethal. If not, then what is being suggested?
.
<<<A quick and un-scientific assessment of recent accidents (before the facts are known) suggest that a small increase in braking effectiveness at high speed would be unlikely to have prevented the result; it might have alleviated some of the consequences, but equally it could have resulted a lateral deviation into far greater hazards (N.B. Toronto wind shift and off-runway hazards).>>>But notwithstanding that it's "before the facts are known" (which they generally are BTW), ALF is suggesting that unapproved and untested potential solutions by known heretics should be discarded as potentially even more dangerous (than a total hull loss and a very near loss of 300 souls). Really? That's really forward thinking and progressive stuff. No wonder we are seemingly stuck in the overrun rut. Reality check required here methinks. Grandiose verbiage but quite lacking in incisive thought processes.
.
<<<Of the 5 or so overrun accident investigations that I was associated with, none would have been prevented by a small increase in braking effectiveness.>>>When all else fails and you've lost the argument, concede that "in the bigger picture" the advocated effect could only ever be "small". Well in fact it's about 20% at least - and that's not "small" in overrun terms. It can make a 5000ft runway in effect a 6000ft runway (or a 6000ft runway a 7200ft one). Nothing small about that in overrun terms. <<<The majority had root causes involving human behavior in the air, the others, human error on the ground, which involved perception and incorrect use of retardation devices.
Thus, we already have large variability in human behavior before anyone adds more from aircraft control techniques.>>>And perish the thought that we should ever offer a straw to a drowning man that's made a "human error on the ground", complete with the sudden realization of just having built his own funeral pyre. Yes, perish that thought. Reversed mixed metaphor: burning men normally jump into water and drown - but it makes the point that once on the ground, there's no "going back", only the possible redemption offered by backstick braking. QF1 in BKK created its own marginality by constructive indecision about "whether to go or try to stop?". Maybe having a fallback position of backstick braking would remove that now familiar "sudden uncertainty". Think about that aspect. Pilots under duress do need an "equalizer". It shouldn't be denied them by puffball prohibitionists in exalted positions.
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They're certainly turning out a thicker more durable and obdurate brand of closed-mind Luddite nowadays. Oh for a cogent argument with some real substance, pith and apple-vinegar.
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And JF. Not a failure in any sense. Your input is always valued (by operators anyway). Your last posting's brevity was perhaps more impactful than my assuredly futile counter-points above.
.
And one final point. Backstick braking is by no means new. Search the web and you'll find some few references to it. It's just that it was perhaps never really understood, despite being taught to "the old school" military. In the age of autobrake and anti-skid it has now however "come of age" again. I would be surprised if one (or both) of the two major manufacturers didn't someday automate it.

[alf5071h]

DD, or are you the Oz side of the IASA web site and OVERTALK; still attempting to further your worthy, but often poorly founded objectives through Pprune?
To be brief;
“It is not the critic who counts, not the man who points out how the strong man stumbles, or where the doer of deeds could have done them better. The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood. He who strives valiantly; who errs and comes short again and again; because there is not effort without error and shortcomings; but who does actually strive to do the deed; who knows the great enthusiasm, the great devotion, who spends himself in a worthy cause, who at the best knows in the end the triumph of high achievement and who at the worst, if he fails, at least he fails while daring greatly. So that his place shall never be with those cold and timid souls who know neither victory nor defeat.” - 'The Man in the Arena', Theodore Roosevelt, 1910

[Dagger Dirk]

Only know OVERTALK via PM's on this subject. IASA is not a private lobby group but a non-profit educational group working actively for aviation safety and funded by an aircrash "survivor" who lost her husband on Swissair 111. Details are on the web-site at www.iasa-intl.com. IASA authors contribute to a number of air safety periodicals. IASA has been around since Swissair's MD11 went down.
.
"I care not what others think of what I do, but I care very much and come up short again and again, who at best knows the high achievement of triumph, who at worst, fails while daring greatly for he knows his place shall never be with those cold and timid souls who know neither victory no defeat."
--Theodore Roosevelt (from link (http://home.att.net/~quotesexchange/theodoreroosevelt.html)) (fifth quote down)
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Alf, better get those meds reviewed. I'm now waiting for your quotes from the venerable bard (surely the last refuge of tech dilettantes with nothing further to offer).
DD

[Mad (Flt) Scientist]

But many newer design rear-engined smaller jets such as the CRJ seem to have similarly mounted engines....so perhaps someone can comment on whether the CRJ has a similar caution written into the Pilots' Notes.
Actually, I can, because your assumption that the manufacturer I work for is based south of the "49th Parallel" is misplaced. So while I don't know what McDD (why does it censor the 6 letter version?) may have recommended in the past for their product line, Bombardier (in the form of Canadair, specifically) does have such a recommendation.

At present the FAA is very very circumspect in defining techniques and configurations for establishing "book" distances - so I'd see that view as being unnecessarily alarmist.
If we were allowed to take credit for all available braking devices, rather than having to leave one 'in reserve' as it were, I can assure you that simple commercial pressure would REQUIRE us to do so, or lose performance and hence sales to our competitors. So if there were a reliable technique for increasing braking performance that we could take credit for, there would be very power motivation to use it.

I'm not sure that ALPA or IFALPA would agree with your continued portrayal of professional pilots as having to conform to your depiction of their abilities, skills and challenges necessarily being limited (and conforming) to a lowest common denominator.

I recall being at a light aircraft design conference in the UK where one of the keynote speakers - who was I believe from the 'pilot side of the fence' was urging us, as designers, to design aircraft that even an idiot couldn't kill themselves in. I'm not the only person thinking in terms of lowest common denominators.

Not sure that this is a productive piece of imagery. It's one reason why my first inclination was to first use the analogy of the second-class lever (with the nose as fulcrum). Using that logic the backstick just lowers the tail and loads up the main-gear, with the nose-gear acting as a pivot-point.

No, no, a million times NO. The nose gear is NOT rpt NOT a fixed pivot. It will extend or compress the nose oleo in response to load, as indeed will the mains. Any increased download on the tail MUST both increase mainwheel download (compressing the main oleos more) and decrease nosewheel load (necessarily extending the nose oleo). EITHER of these two effects will cause the pitch attitude to increase; both together certainly will.

The question remains whether the amount that the nose rises is either
(a) a risk of actually raising the wheel out of ground contact - something to be avoided at almost any cost, and a DEFINITE risk on our types. Any aircraft where that is a concern shouldn't be using this advocated technique.
(b) a small increase in pitch attitude and hence AoA and a MILD unloading of the nose. The former will act to increase wing lift and counter the downloading you're seeking to add; the latter may have directional control implications.

On those types not subject to the "pitch up" risk the technique MAY help; and it may not. It's by no means certain to. But until demonstrated by the OEMs and endorsed by appropriate handling advice, you are essentially assuming that you know better than our test pilots did when they developed the advice we provide. I'd really rather people didn't try that, certainly in our product line.

Oh, and ...

John Farley has endorsed the ability of backstick to "load up" the main-gear for more effective braking on mush - so perhaps we can leave this nicety to be thrashed out by the aerodynamicists (who will see it as the resultant of couples (nosedown pitching moment and taildown pitching moment).

That aerodynamicist would, in fact, be me, I'm afraid. At least for my company (in company with my colleagues, of course; no one works alone these days). So I'm quite familiar with the nature of on-ground modelling for aerodynamic behaviour; I think I've been doing it, off and on, for about 16 years now.

[Sick Squid]

My company's flying manuals mention this technique, but discount it in a risk-management formula against the increased risk of tailstrike.

Interestingly, the contaminated braking co-efficients acceptable for the 320 have just been clarified... down. Essentially we can now land on "slippery" runways, but not take-off.

Don't know if I'm brave enough for that, might be time for a little Commander's discretion to be applied there.

Squid

[LEM]

After reading the whole thread, I'm surprised the what I thought was the major reason for pushing the stick while braking has been almost totally overlooked: to decrease the residual lift from the wings, thus getting MORE weight on the main gear.

In simple words: while it's true that UP elevator will tend to increase somewhat the apparent weight for reasons discussed at lenght before, it will also increase the incidence of the wing, while elevator full down will reduce the attitude by maybe a couple of degrees.

My point is: the spoilers DON'T kill ALL lift.

A portion of the wing is not influenced by them, and even in areas with spoilers extended above the wings, some lift is still generated on the lower surface, by pure angle of incidence.

So the question is: by pushing the stick (almost) fully forward, as I've done since now, what do get out of the balance?

More lift from the tail, or more-reduced lift from the wings?

See what I mean?

My feeling has always been that the net result from this equation is in favor of PUSHING, not of PULLING on the yoke.

This lift reduction on the wings by a pitch decrease is the key to this issue, and I'm really surprised nobody has given this the due attention.

Or have I missed something?

Try to visualize, Gentlemen, that we are talking about VERY POOR braking coefficient, so very little pitch moment generated by normal braking, very little compression of the nose oleo and so on...

Imagine the airplane slipping on pure ice, and trying to brake.
Pushing or pulling can change the pitch by how many degrees?

What's the difference in lift generated?

Here we need the math people!

But only after we have the right picture in mind!

LEM

[Centaurus]

One of the contributory causes of landing over-runs is excessive threshold speed. Manufacturers recommend specific additives to the basic Vref. Boeing, for example, recommend adding half the steady headwind component and all of the gust with a total of not more than 20 knots above Vref. Boeing also say that the headwind component additive should be bled off approaching touch-down while maintaining the gust additive. These additives are taken into account in landing distance calculations.

What is often observed are HW additives applied as recommended, but with no real attempt to bleed off the HW additive before arrival over the fence. This may result in a long float. And if the gust additive is maintained, a still longer float or a high touch down speed may occur.

With full gust additive applied right into to the flare, Murphy's Law dictates that the expected lull accompanying the gust will fail to eventuate and you are left with lots of excess speed. When the runway is slippery and a float is allowed to occur to obtain a smooth touch-down, chances are you are risking an over-run. With a gust factor at 90 degrees and the same additive applied, the extra speed is not always dissipated.

While a speed-deficient arrival over the fence is undesirable for several reasons it can be sometimes countered with judicious use of thrust but it takes a deft touch. On the other hand unwanted speed over the fence has been proved to lead to over-runs.

From personal observations on line and in simulators, it is rare to see the half-the-HW component deliberately bled off - mostly people argue it happens naturally at the flare. Maybe so, but not too often. In general, most crews don't worry about it and simply plant the aircraft or go for a smooth landing via the float. Fine if the runway is long and not wet.

I would like to see aircraft manufacturer's have another look at their recommendations on HW and gust additives when conducting manual thrust landings - instead of auto-throttle engaged landings where the Boeing recommendation is to add five knots only to Vref for all landings.

Defining the term "approaching touch-down" where it refers to bleeding off the HW additive, would be helpful. In theory, the free-stream wind gradient starts around 2000 ft and upwards so it is logical to start bleeding off the HW component then -and not leaving it to the flare. Is "approaching touch-down" a specified distance from the runway or a specified height above the aerodrome on final approach?

Accepting some over-runs could be prevented if airspeed control was more precise, manufacturers could consider fine-tuning their advice on the subject of airspeed additives. In short, their strict application may in some cases cause more problems than they are meant to fix.

[OVERTALK]

I thought what was the major reason for pushing the stick while braking has been almost totally overlooked: to decrease the residual lift from the wings, thus getting MORE weight on the main gear.
In simple words: while it's true that UP elevator will tend to increase somewhat the apparent weight for reasons discussed at length here, it will also increase the incidence of the wing, while elevator full down will reduce the attitude by maybe a couple of degrees.
.
Elevator full down (i.e. stick full forward) will only effectively weight transfer onto the nosewheel, leading to "wheel-barrowing" (a condition of directional instability). It won't, by any useful measure, decrease the wing's AoA. Why? Think of a depressed oleo as only being able to soak up shock i.e. weight-bearing oleos are effectively incompressible to flight-control inputs - therefore fully-down elevator will just load up the nosewheel. It won't depress the nose oleo appreciably more, nor take any angle-of-attack off the wings. By contrast backstick will load up the mainwheels, increasing traction, enhancing directional stability and you will achieve effective braking much earlier - particularly on a wet or contaminated runway. Why wouldn't backstick raise the nose? Don't forget the combined pitchdown effect on nose touchdown of engine reverse, braking and spoilers. During the important period for effective braking this pitchdown couple enables the backstick's effect of loading up the mainwheels - by stopping the progressive up elevator from raising the nose. Why progressive? Well obviously a pilot is not going to immediately put his yoke to the backstops after nosewheel on. But as speed decreases, inevitably, if going for the maximum backstick braking effect, the yoke will end up fully back. Obviously differently configured airplanes will have slightly different characteristics.
.
...by pushing the stick (almost) fully forward, as I've done since now, what do we get out of the balance?
More lift from the tail, or more-reduced lift from the wings? You will note that internationally well-known Experimental Test-pilot John Farley came out in support of the backstick braking technique but was rudely rebuffed, essentially in mid-post, by one of those who cannot accept the practical facts..... and who instead waxed on with ever-confusing hypotheticals. Backstick braking is a proven effective stopping technique. Unfortunately it's not yet been automated and, because it is a dynamic process, it is easily misunderstood. Those who normally oppose change have been well represented on this thread and have used quite illogical reasoning in an attempt to deny its effectiveness and conjure up fanciful possible dangers.
.
This lift reduction on the wings by a pitch decrease is the key to this issue, and I'm really surprised nobody has given this the due attention.
Or have I missed something? You most certainly have missed the big points essential for understanding how the technique works.

[LEM]

Elevator full down (i.e. stick full forward) will only effectively weight transfer onto the nosewheel, leading to "wheel-barrowing" (a condition of directional instability). It won't, by any useful measure, decrease the wing's AoA. Why? Think of a depressed oleo as only being able to soak up shock i.e. weight-bearing oleos are effectively incompressible to flight-control inputs - therefore fully-down elevator will just load up the nosewheel. It won't depress the nose oleo appreciably more, nor take any angle-of-attack off the wings.

OVERTALK, that's where we disagree. I believe a full forward stick can change your AoA by say a couple of degrees, depending on type, of course.

Remember, we're having a very hard time braking on this slippery surface, so the nose oleo isn't compressed by a braking momentum.

And another point where we disagree is that reverse thrust does produce a pitch down effect.

Actually, the pivot being the main wheels, reverse thrust produces a pitch up effect, even on low mounted engines like a B737, unlike in flight where it would produce a pitchdown effect.

I can't believe it's so hard to understand...:rolleyes:


Btw, maybe pushing the scenario to the limit will help get a clearer picture.
Imagine the worst case, an airplane with no spoilers.
Or simply the pilot forgetting to arm them.
Or simply a model without any spoiler!
Upon touch down, the wing is "still flying", and as speed decreses, lift gradually decreases.
Of course after touchdown one can get the impression the wing is fully stalled.
Not so, especially after you've lowered the nose.
The Aoa will not be beyond the stall limit at all, and the wing is still producing lift.
What would you do? Push or pull?

I suggets you push, my friend, to reduce the attitude (yes, the nose oleo IS compressible!), and decrease the residual lift as much as possible.

It's as natural as that. I know the tail will produce some lift, but that's almost nothing if compared to the lift loss on the wings.

Now, if this scenario is right without spoilers, probably it's still right with spoilers extended.

My assumption is that spoilers kill only a fraction of the lift.

If this is NOT true, then you're right.

MFS should have the numbers about the exact percentage, at least on his type.

LEM

[L337]

One of the contributory causes of landing over-runs is excessive threshold speed

As an ex DHC-6 pilot, and a current 747-400 Captain, my experience on limiting runways, is that you have to, absolutely have to, land at the beginng of the runway. Do not float, stick it down and get on with the business of braking. A few extra knots is not an issue. The issue is that if you land half way down, you are going off the end.

L337

[Mad (Flt) Scientist]

Elevator full down (i.e. stick full forward) will only effectively weight transfer onto the nosewheel, leading to "wheel-barrowing" (a condition of directional instability). It won't, by any useful measure, decrease the wing's AoA. Why? Think of a depressed oleo as only being able to soak up shock i.e. weight-bearing oleos are effectively incompressible to flight-control inputs - therefore fully-down elevator will just load up the nosewheel. It won't depress the nose oleo appreciably more, nor take any angle-of-attack off the wings. ....

Sorry, but this is nonsense. ANY increase in the load on an oleo will cause it to further compress, unless it is ALREADY fully compressed. For very good reasons relating to the risk of internal damage and loss of shock-absorbing capacity, designers will include sufficient margin that under any kind of foreseeable operation the oleos - all of them - are NOT fully compressed.

Therefore, back stick WILL raise the nose (by compressing the mains and unloading the nose) and forward stick WILL lower the nose (by compressing the nosegear and unloading the mains). It's simple physics, no magic about it.

The QUESTION which is type dependent is whether the directly created mainwheel download by back-stick will or will not outweigh the unloading caused by increased AoA. It's type dependent and CG dependent and technique dependent and...and...and...
it certainly is not a given that backstick is the preferred option.

You will note that internationally well-known Experimental Test-pilot John Farley came out in support of the backstick braking technique but was rudely rebuffed, essentially in mid-post, by one of those who cannot accept the practical facts..... and who instead waxed on with ever-confusing hypotheticals. Backstick braking is a proven effective stopping technique. Unfortunately it's not yet been automated and, because it is a dynamic process, it is easily misunderstood. Those who normally oppose change have been well represented on this thread and have used quite illogical reasoning in an attempt to deny its effectiveness and conjure up fanciful possible dangers.
I for one prefer a soundly based theory to hero-worship - with all due regard to JF, he cannot possibly know the characteristics of every type and I will repeat once again for those who missed it:
THE SPECIFIC ADVICE TO CREWS FOR MY COMPANY'S MAIN PASSENGER AIRCRAFT IS PROGRESSIVE FORWARD STICK DURING BRAKING. This is based upon flight test experience of the aircraft, not upon a theory. I would hate one of our crews to decide to go against the advice of Test Pilots who flew our certification testing and try out some technique of their own.

And it doesn't matter how 'automated' the process is: ANY transference of load from nose to main MUST raise the nose. The less load you transfer, the less the nose will raise and the less use also will be the backstick method....

[UNCTUOUS]

LEM says: And another point where we disagree is that reverse thrust does not produce a pitch down effect.
Actually, the pivot being the main wheels, reverse thrust produces a pitch up effect, even on low mounted engines like a B737, unlike in flight where it would produce a pitchdown effect.
.
Beg to differ here (and I did notice that MFS decided to gloss over this basic point).
But there is so much fundamental confusion about cause and effect in this thread that it wouldn't be worthwhile embarking upon any indepth explanation.
.
MFS said: ANY increase in the load on an oleo will cause it to further compress, unless it is ALREADY fully compressed. For very good reasons relating to the risk of internal damage and loss of shock-absorbing capacity, designers will include sufficient margin that under any kind of foreseeable operation the oleos - all of them - are NOT fully compressed.
.
There's a significant difference between a compression able to be induced by flight control loads and the additional shock absorption capabilities of an oleo.
.
MFS says: THE SPECIFIC ADVICE TO CREWS FOR MY COMPANY'S MAIN PASSENGER AIRCRAFT IS PROGRESSIVE FORWARD STICK DURING BRAKING. This is based upon flight test experience of the aircraft, not upon a theory. I would hate one of our crews to decide to go against the advice of Test Pilots who flew our certification testing and try out some technique of their own.
.
It has been conceded in ths thread that there is a difference between your tail-mounted engines types (now a dying configurational breed) and others. BAe146/RJ85/RJ100 pilots have said that they use backstick braking because it works and therefore it's an endorsed handling technique. At some time in the future we will probably see the practise automated (in Airbus A340, A380, A330, and 737, 767, 787 etc). However by that time there will be but a few aft-mount beasties around (Embraers and Bombardiers). They will be trapped in the technology of their era.
.
MFS said: ANY transference of load from nose to main MUST raise the nose. The less load you transfer, the less the nose will raise and the less use also will be the backstick method....
.
Another confusing injection. The up elevator's aerodynamics is exerting a download on the maingear, not off-loading the nose-gear by any "transference". It's a relative change (nose to mains loadings) only. As speed dissipates, that up elevator ability will be progressively diminished - however it is at the higher speed that we would like (and benefit from) maximized maingear wheel traction, so that's the beauty of "backstick braking".
.

[Mad (Flt) Scientist]

There is, and MUST be, transference of load from nose to mains.

Consider the simple levers that keep getting quoted herein. Three points of action/reaction. The nosegear, the main gear, and the tail. Mains are equidistant from nose and tail, to keep it simple.

Suppose the aircraft to have, under whatever braking, reverse thrust, whatever condition, to have zero tail load, and 10 tonnes of load on the nose gear, and 90 tonnes on the main gear (total). In equilibrium, in pitch.

Apply X tonnes of down force at the tail. Taking moments about the nose first: downforce must increase at main gear by 2X tonnes (since nose-to-tail is twice nose-to-mains).

Therefore, since we only added X tonnes of down force to the system, but appear to be pushing down more on the ground by 2X at the mains, we must be removing X from the nose also. Calculating the moments about either main gear or tail shows this, as does consideration of the vertical force balance.

Therefore in addition to adding our X tonnes of aerodynamic downforce from the tail to the mains, we are also effectively TRANSFERRING X from nose to mains.

If the ratio of nose-to-main and main-to-tail is not unity, the exact numbers change, but the principle doesn't; the only way a lever can act to INCREASE the download at the main gear relative to the applied tail load is to decrease a force somewhere else.

Now, for any given type you can calculate the ratio between added mainwheel download and nosewheel unload, and make a specific calculation for a specific tail load.

You can then calculate the amount by which the aircraft WILL pitch as a result, as the main and nose oleos re-adjust to account for the change in load.

You can then calculate what change in wing lift will account for the resulting change in AoA and compare that to the direct load increase.

If the transferred tail load effect outweighs the wing lift effect, you have NET increased mainwheel download. If it doesn't, you've actually unloaded the mains.

Whether a given aircraft is particularly nose up or down in tendency doesn't matter to that; all that matters is whether the oleos are fully compressed. Except in exceptional circumstances, they are not. Therefore increased download at the tail must pitch the aircraft.

The factors which will determine the effectiveness or otherwise of this method are the relative nose-main-tail geometry, the stiffnesses of the oleos and the sensitivity of the aerodynamic lift to pitch changes. It's not a one-way bet, and depends on how those interact. As the oleos approach infinite stiffness, the backstick method becomes more viable; as the wing effectiveness improves, the backstick method becomes less useful.

[LEM]

Hi UNCTUOUS.
You are right about reverse pitch moment.
Geometrically the pivot are the main wheels, but being a dinamic scenario, the cg of the whole thing still remains above low mounted engines.
Point taken.

Now, back to main topic: is it true or not that spoilers kill only a fraction of the residual lift?
Is it true or not that we can change our pitch by yoke input during rollout?

Mad (Flt) Scientist,
It would be great if you have the (approximate) figures: what percentage of total lift do the spoilers kill after touchdown?
How much residual lift is killed by a pitch reduction of, say, two degrees?
Is the net result outweighing the lift created by down elevator?

[UNCTUOUS]

MFS said (unQUOTEd) <<Far too much in his confusing post above to bear repeating so...>>(/UNderQUOTEd)
Imagine you are at the speedway. Note the racing cars with the huge slanting spoilers on top. They weigh 1300kgs each. The secret of their success is that, courtesy of the spoilers, their effective weight (nil weight transference) is increased by their air-speed (i.e. the square of their IAS). That gives them more traction for cornering, braking and holding onto the racetrack. If you were to mount them on a weigh-trolley at speed you would note that their weight effectively increases by 100's of kgs as their airspeed increases. Their mass and its distribution remains essentially the same.
.
As they now go around a 180 degree turn and enjoy a 15kt tailwind (instead of a 15kt headwind) but remain at the same groundspeed, their effective weight will be changed (i.e. significantly reduced) by the difference in the squares of the two airspeeds and the coefficient of negative lift that comes courtesy of the spoilers.
.
Backstick braking works on the same principle. There is no weight-shift between the nose-gear and main-gear. The benefit comes from the up-elevator (the equivalent of the racing car's spoiler). It is what pushes the maingear into the ground, increasing traction and enhancing braking at speed.
.
I hope that you will be able to see that to be a fact......
But I have absolutely nil faith that you will......

[UNCTUOUS]

LEM queried:(1) is it true or not that spoilers kill only a fraction of the residual lift?
(2) Is it true or not that we can change our pitch by yoke input during rollout?
answers:
1. A sufficiently large enough fraction that, on an uncontaminated runway, braking coefficients are more than adequate. However it is the very wet and contaminated runways that we are addressing in this thread.
.
2. We can (and would) by backstick braking aft yoke input - if it wasn't for the nose-down pitch effect of spoilers, reverse and natural C of G distribution plus autobrake. Those four pitchdown elements permit a proportional (and increasing) aft yoke (i.e. up elevator) pilot input as the braking effect increases - which in turn increases the weight-on-wheels and the nose-down pitch due to braking.... which allows eventually up to max backstick.
.
As you may have read earlier in this thread, some airliner's SOP's do recommend it and military schools do teach it (backstick braking). It's an example of the widening conceptual gulf between civil and military as less and less of our pilots are now ex-military. Inevitably as the huge tally of overruns continues, manufacturers will be forced to look into ways and means of reducing the toll. Bigger reversers, larger capacity brakes, larger spoilers? Or maybe just automating backstick braking?

[LEM]

Absolutely right about the racing car.

However, an airplane differs from the racing car by having a huge inverted spoiler forward or the aft spoilers, which is creating lift!

If a wing is capable of creating a 50 tons lift, after spoilers are extended and pitch is zero when on the runway, it will still generate, for example, 10 tons of lift.

Now, the tailplane is designed to create a constant negative lift.
By pushing the yoke fully forward, I think you can reduce this negative lift, or even create a positive one - it doesn't matter - by, say, 3 tons (imaginary figures).

Doing so your pitch will decrease by a couple of degrees.

This may reduce the residual positive 10 tons on the wing to 5 tons.

You see from all this simple math that the net result is in favor of pushing, because the apparent weight of the airplane has increased by 2 tons (-3 on the tail, +5 on the wings).

Thus more weight on the wheels.

Of course all this is type and conditions dependant.
If conditions permit a further compression of the nose oleo, for example, it's better to push, otherwise it's better to pull.
If the imaginary math above is in favor of pulling on a certain type, so be it, pull.
And the list is long.

That's why it's almost impossible to establish an official procedure.



Ps: an instructor once criticised me for pushing too much after touchdown.
He said Boeing's official technique was to neither push or pull.
Just to leave the yoke neutral.

I can't recall such a statement in any book. Anybody has got the official reference?

LEM

[Mad (Flt) Scientist]

MFS - I'll Try and Simplify it for you

I hope that you will be able to see that to be a fact......
But I have absolutely nil faith that you will......

That's very 'generous' of you.

Please actually READ what I wrote, and work out the moment balance about main, nose and tail with and without download. It's not rocket science, it's not even 'flight' science.

You will find that ANY download at the tail MUST produce a LARGER downforce at the maingear and a corresponding, proportionate, DECREASE at the nose. That is weight transference.

to address the racing car analogy

You may have noticed that F1 and similar cars, which do use aerodynamic downforce, have both FORWARD and REAR spoilers, all of which are intended to produce downforce. The ideal is to achieve an aerodynamic downforce distribution, courtesy of this feature AND of the car body design, which is proportionate to the existing weight distribution, as that means that the handling is more consistent. Very few aircraft have the luxury of a 'front tail' analogous to the F1 front spoilers.

Also, those vehicles with large rear spoilers alone are often using them to counteract the large aerodynamic lift being exerted on the back of the vehicle (in a typical 'saloon car' configuration) rather than to generate 'extra' downforce; they're just trying to keep the back wheels on the ground.

It is highly amusing that the 'backstick lobby' bandies around accusations of closed-mindedness, yet refuses to even do any even mildly dissenting view the courtesy of considering its points.

[Conan The Barber]

Yes LEM, you are quite correct in believing that reverse thrust does not give a pitch-down moment. Neither do spoilers for that matter. But I suspect we have moved onto the point of religion. And you know what happens then.

Anyhoo, some airlines might have this 'technique' in their books, some might not. I suspect those who do, have so because those wielding the pen are ex-warriors. Is the military the sole proprietor of the gospel? I could have sworn I've seen the camouflaged ones off the runway too.

No matter, I think the 'pro-lobby' here is not a large as appearences might suggest.

[RatherBeFlying]

The front and rear spoilers of F1 cars are mounted very close to the two sets of gear. I suspect that the downforce vectors are located between the axles, in which case there is no unloading of an axle. In the case that spoiler(s) are located beyond an axle: the moment arm is enormously less that that from a tailplane so that the result is very little unloading of the opposite axle -- or the car goes off the road:uhoh:
Any unloading from one spoiler is more than counterbalanced by the action of its opposite number.In the case of a/c we are dealing with moment arms of a hundred feet or so as opposed to much less in F1.

[Fropilot]

The most important things in my humble is opinion are:

1. Is the runway safe for landing LAW, X-wind Tailwind etc etc? I do not care what the last landing pilot said. If the information about runway status is not clear then why would one want to try if not sure. This is not a game where you try but must make solid judgements and decisions.
2. Make a firm landing at the correct touch down point and speed. Do not try to impress the passengers your copilot or new flight attendant you are daydreaming about, by hunting for a smooth landing and floating.
3. Apply recommended stopping technique; auto brakes, spoilers reverse as recommended.

Lastly I have flown with pilots who apply so much brute force on the reverse levers so hard, that once the conditions are met for reverse they cannot get reverse because they are applying such a high force that the solenoid or whatever prevents reverse on (your aeroplane) they cannot get anything beyong reverse idle. Only after this white knuckle stage are they able to get reverse which is not as effective at low speed. (this may not apply to all aeroplane types but applies to all I have flown.)

[TheShadow]

MFS
In your 23 March 03:51 post you completely and purposely neglected to factor in (to the couples at work or "in play") the nose-down cumulative effects of spoilers, reverse and braking (as well as the c of g position that's tending to keep the nose on the ground).
.
If you go back to the beginning of this thread, the original (and very reasonable) postulate was that
sufficient backstick could be progressively fed in so that the nose would stay down and the main-gear benefit
from the aerodynamic downforce of the horizontal stabilizer under backstick - thus gaining considerable
braking traction upon contaminated runways at higher speeds (i.e. not long after nosewheel on).
.
Using convoluted math that excludes the most significant factors or (like LEM) hypothesizing with completely
imaginary figures and drawing bogus conclusions from them, is to cobble together an "argument" without any useful
substance.
.
There may well be weight transference that's cumulatively in favor of the technique, but there is also (quite unquestionably) an aerodynamic downforce (on the maingear) courtesy of the backstick.
.
I suggest that you should now reflect upon the fact that backstick braking is a proven methodology - and then
drive yourself even maddder trying to figure out WHY.http://www.speedway.net.au/photos/drivers/3824_car.jpg
http://www.extremephotos.com.au/2006ClassicJan28/images/Amato2627.jpg

[Mad (Flt) Scientist]

MFS
In your 23 March 03:51 post you completely and purposely neglected to factor in (to the couples at work or "in play") the nose-down cumulative effects of spoilers, reverse and braking (as well as the c of g position that's tending to keep the nose on the ground).

No, I did not, because I am dealing with the delta arising from the use of the technique versus the non-use of the technique.

Whether the effect of reverse thrust is nose-up or nose down is irrelevant. The question is, what is the incremental effect of the technique: does it cause more load on the main gear or not? Whether there is an offset in overall pitching moment or not is utterly irrelevant, provided we do not have to consider the catastrophic case of causing the nose to rise such that we actually lift off again.

As long as the characteristics remain linear - which is why I keep pointing out that oleos are NEVER fully compressed, please note - the consideration must be between the DIRECT effect of increased download on the main gear, arising from the pure lever action of the tail download, and the INDIRECT effect of increased pitch attitude, increased AoA and hence increased overall aerodynamic lift. If the former is greater, there's a net benefit; if the latter, an net degradation is decel capability.

As with many things in life, there is no single right solution; on some types, depening on the characteristics, it will work. On others, it won't. On certain types it may be dependent on other factors - like weight or cg - as to what is the better option. To pretend that there is some undiscovered holy grail of truth, if only the stupid OEMs would recognise it, is to be entirely misguided. As I mentioend a long time ago, if it was that simple it'd be in the AFM data AND we'd be taking credit for it already.