Pulling a Stop to Runway Overruns
 

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

Re: Pulling a Stop to Runway Overruns
 

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?

Preaching to the Choir
 

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/showthr...201568&page=13
http://www.pprune.org/forums/showthr...=204112&page=2

Re: Pulling a Stop to Runway Overruns
 

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!

Re: Pulling a Stop to Runway Overruns
 

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!

Re: Pulling a Stop to Runway Overruns
 

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"?

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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 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.

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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.

Talk about a mess of corrupted Conniption.......
 

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.

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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

Re: Pulling a Stop to Runway Overruns
 

<<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.

A long way to go from Basic Physics to Approved SOPs
 

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.

Re: Pulling a Stop to Runway Overruns
 

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:
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?

Re: Pulling a Stop to Runway Overruns
 

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?

Re: Pulling a Stop to Runway Overruns
 

<<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.

Correction
 

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.

Underwing Mounted Engines Produce a Pitch-up? Really?
 

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

Re: Pulling a Stop to Runway Overruns
 

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:

Re: Pulling a Stop to Runway Overruns
 

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.
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.
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).

Fulcrum? -- Which Fulcrum?
 

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.

Re: Pulling a Stop to Runway Overruns
 

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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?

Re: Pulling a Stop to Runway Overruns
 

BASIL said .
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.

In Response to Basil / Reply to Conan
 

BASIL
....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.
.
.... 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.
.
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
.
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.

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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??

In Response to a .....
 

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.
.
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).
.
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...
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.
. 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.
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/bel...ss_lever-1.jpghttp://www.iasa-intl.com/folders/bel...s/lever-1a.jpg

Re: Pulling a Stop to Runway Overruns
 

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.

Re: In Response to a .....
 

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.

Re: Pulling a Stop to Runway Overruns
 

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?

Time for the Padlock?
 

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.
.
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.
"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: (the technique would have helped that QF skipper avoid his hole-in-one jumbo).
.
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 without being a heretic.
.
I believe this to be what they call "the neo-Luddite Approach" 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.

NoseWheel not a Fulcrum
 

The fulcrum on the ground is the maingear axle. Backstick produces down force at the tail which:

1. Adds to the total load on all gears
2. 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.

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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.

Re: Pulling a Stop to Runway Overruns
 

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.