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Pulling a Stop to Runway Overruns

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Pulling a Stop to Runway Overruns

Old 5th Jan 2006, 06:04
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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
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Old 5th Jan 2006, 08:39
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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?
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Old 5th Jan 2006, 09:16
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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
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Old 5th Jan 2006, 09:43
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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!
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Old 5th Jan 2006, 10:44
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Re: Pulling a Stop to Runway Overruns

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!
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Old 5th Jan 2006, 12:08
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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"?
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Old 5th Jan 2006, 12:19
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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.
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Old 5th Jan 2006, 13:32
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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.
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Old 5th Jan 2006, 14:26
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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.
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Old 5th Jan 2006, 15:27
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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.
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Old 5th Jan 2006, 16:00
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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.
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Old 5th Jan 2006, 16:11
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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
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Old 5th Jan 2006, 16:40
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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.
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Old 5th Jan 2006, 18:12
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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:
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?
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Old 6th Jan 2006, 12:09
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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?
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Old 7th Jan 2006, 16:45
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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

As the consultants love to conclude their reports, further study is required.
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Old 7th Jan 2006, 17:23
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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
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Old 7th Jan 2006, 17:41
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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


Last edited by ant1; 7th Jan 2006 at 22:51.
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Old 8th Jan 2006, 00:54
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Re: Pulling a Stop to Runway Overruns

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
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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).
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"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).
Dagger Dirk is offline  
Old 8th Jan 2006, 01:35
  #20 (permalink)  
 
Join Date: Sep 2001
Location: Toronto
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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.
RatherBeFlying is offline  

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