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Pulling a Stop to Runway Overruns

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Pulling a Stop to Runway Overruns

Old 8th Jan 2006, 02:14
  #21 (permalink)  
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Re: Pulling a Stop to Runway Overruns

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

House rules are

(a) play the ball as hard as you wish

(b) don't play the player.

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

regards,

JT
john_tullamarine is offline  
Old 8th Jan 2006, 11:25
  #22 (permalink)  
 
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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?
Conan The Barber is offline  
Old 8th Jan 2006, 11:56
  #23 (permalink)  
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Re: Pulling a Stop to Runway Overruns

BASIL said
Ref heavy braking producing a nose-down pitching moment; isn't this thread about landing on more or less slippery runways where heavy braking would be impossible?
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To re-cover old ground:
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a. the CofG accords a basic nose-down weight distribution (ie. before any retardation cuts in)
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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)
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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.
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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.
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Old 8th Jan 2006, 16:17
  #24 (permalink)  
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In Response to Basil / Reply to Conan

BASIL
<<<disagree with the suggestion that up elevator has
any significant value. Very little of the aircraft weight is supported
by the nosewheel therefore up elevator will transfer little weight
before the nose lifts and, as IAS reduces, the elevator will, in any
case, become less effective.>>>
....That's why the
"progressive" backstick is being advocated. You can still fly the
sight-picture out the front window and temper the degree of backstick -
but I've never found "nosegear rising" to be an issue. It's an imaginary issue only. Surely you aren't claiming that
with spoilers up and under max reverse and braking, that there isn't a
strong cumulative nose-down pitching moment? That's why the term
"effective" weight-shift towards the nose is being used. The fact that a
747 (and other widebody aircraft) might have both body and wing-gear
doesn't change that "towards" directionality. Or maybe just think of it
as the elevator (and hoz stabilizer of course) levering the maingear into the ground by using the nose-gear as a fulcrum. You are "effectively" increasing the weight on the main-gear wheels courtesy of your up elevator input at speed (if used early enough you will stop the anti-skid cycling and help the autobrake achieve its programmed retardation schedule). Maybe it's just too difficult a concept for some to wrap their mind around.
.
<<<For those who have never turned e.g., a 747-400,
over wet piano keys, the sideskid even at low speed would come as a
surprising indication of how little weight is supported by the
nosewheels.>>>
.... This has not a lot to do with the
dynamics of landing. Any airplane's nosewheels will slip when turning on
wet piano keys.....whatever its C of G (and both pre-takeoff or
post-landing). It's less a function of weight and more to do with a loss
of traction due to a nosewheels' tread being hamfistedly cocked off from
the direction of a/c travel inertia, whilst on a slippery painted surface.
.
<<<On the landing roll I certainly do not wish to
remove weight from the nose wheel and reduce non-aerodynamic
directional control.>>>
You obviously don't subscribe to
the theory of wheelbarrowing then...but perhaps just consider it to be a
more equitable distribution of weight. Wheelbarrowing could be thought
of as "pushing a length of thread" or perhaps what happens when you
inadvertently use front wheel brakes (only) on a bicycle at speed (vice
both F & R hand-brakes). Those who advocate stick forward have obviously
not tried that. I wouldn't recommend it in a fighter, that's for sure. Be reassured that the progressive backstick technique considerably enhances directional control
.
<<<Auto spoiler deployment on landing can cause a
pronounced nose up pitching moment and a habit of applying nose up
elevator too soon after landing could cause a
tailscrape.>>>
Nobody is advocating any motor moronic
whipping into automatic "loadsa backstick". Please stop diminishing the
overall argument by escalating into gross handling nonsensicalities. The
advocated technique is to introduce progressive backstick as or AFTER the
reverse has cut in, spoilers are up and auto-braking (or toe-braking) is
underway. You can control what happens to the nose by observing through
the front window.
.
It's almost amusing, but actually quite tragic, that the
progressive backstick stopping technique has been allowed to fade into
obscurity. The plaintive cries of the now retired old and bold actually
seem to read as: "why wasn't I told?" Sorry about that.
Perhaps someone should start a thread on other "lost arts".
.
In Reply to Conan the Barber's Query
Effectiveness of the Technique?
In my secondary duty as a maint Test Pilot at an Advanced Jet Training
School I used to torment the Hell out of other instructors by planting
the jet on the numbers and turning off impossibly early and thereby
getting the really short taxi to parking - wet or dry runway and no
maxarets/anti-skid. I'd normally only do that on back-to-back functional
check-flights and be relatively heavy with fuel normally, because of
that. Additionally, but quite non-critical, I used to run the electric
elevator trim all the way forward once the main-gear was on, just to
have the trim-tab working for me as an elevator extension -rather than
against me => more power to the elbow. Those who tried to
emulate the feat, even at light weights at the end of an instructional
sortie, usually missed the turn-off (and had a long slowspeed taxi to
the next) and/or blew a tire. The Chief Flying Instructor eventually
blew up at a staff meeting, regaled everybody about all the blown tires
and demanded to know how I did it. In four engine aircraft I rarely used
it in anger, just for training and demo purposes.
My guess is that a figure of the order of 20% better than book figures
on dry runways and similar on wet runways, with maybe slightly less on
really nasty rubbery and slushy surfaces. But runway surfaces vary so much over their length, particularly because of the rubber buildups in that
critical last 2000 feet. Braking achieved early on, in the "clean"
second third of the runway, is what it's all about.
OVERTALK is offline  
Old 8th Jan 2006, 18:23
  #25 (permalink)  
 
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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.
Mad (Flt) Scientist is offline  
Old 9th Jan 2006, 07:30
  #26 (permalink)  
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In Response to a .....

Mad(Flt)Scientist
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"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.
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To achieve the claimed 20% improvement in braking effect would require increasing the mainwheel reaction force by the same amount, and having brakes capable of dealing with the increased torque.
Not sure whether this is sound logic (simple tech peasant that I am), however most a/c wheelbrakes that I've encountered in a lengthy career are [torquewise] quite capable of blowing tyres - unless it's very much later in the roll-out where brake-fade once or twice educated a fatigued and nonchalant me that I should have done the required braking much earlier (thank god for high-speed turn-offs).
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"Assuming that the plane has just touched down, the speed will be somewhere close to stall speed."
Not a totally safe assumption in the overrun cases that we are interested in stopping. Many overruns are plonked down fast and late by pilots with an incre"mental" ref speed buffer mindset.... and some just end up "hot" for a multiplicity of reasons. It's that sort of seemingly unavoidable "lap-of-the-gods" mishandling that we are here attempting to assist in uncrunching the outcome of...
"...so that the mainwheel reaction is essentially the aircraft weight. (ignoring the nosewheel reaction component of maybe 5% for now)"
5%?? Hmm. But the only factor in play for braking effectiveness is the wheel rotational torque being imparted by the runway/tyre-tread interface. Unfortunately, on contaminated surfaces, because of the tyre's hydrodynamically lifting bow-wave at speed, tyre rotation is inhibited for a significant period after touchdown and "effective" weight-on-wheels is much less than a/c weight (thanks to efficient wings, rigger's angle and flaps). That's the nub of the argument here. Backstick uses the nose-gear's pitch-down moment as a type 2 fulcrum point to leverage an artificial weight upon the brakeable main-gear (body or wing)... and from elevator to nose-gear is a considerable moment arm (and mechanical advantage). In another sense the backstick tactic becomes a logical utilization of the effective weight shift forward caused by spoilers, auto-brake and reverse thrust. That cumulative pitch-down force becomes a dynamic fulcrum with the nose as the pivot-point.
"If the tail is about 25% of the wing area and similarly efficient, that means I have to get to about 40% of the tail stall CL, which means I'm going to need substantial amounts of elevator to get there. "....and eventually it won't much matter where the elevator is."
. I won't quibble with the figures but yes, you do eventually end up with the stick right back in the gut (as speed dissipates, that's only logical). It's a progressive process. Just like braking itself, you do it "for effect". It's when you do it that counts. Obviously at less than 50 knots there's very little efffect available (ie. minuscule). Early on and sustained is good, whereas introduced very latterly as a panicky afterthought is pathetic. That's like deciding to save fuel after the "tanks low" lights illuminate.
What does that lot mean? It means that to achieve the kinds of braking efficiency improvements talked about would require SIGNIFICANT elevator movement because it requires significant redistribution of the wheel reactions.Agreed that it's a very unpansy-like commitment to stopping, but one applied as a handling technique and never motor-moronised by rote.
It also means, if you look at the impact on nosegear reaction, that in order to achieve 20% more mainwheel download, I need to offload the nosegear by 10% - maybe more depending on geometry. Since a typical nosegear load is of this magnitude this means I am definitely risking raising the nose with this technique.
I think I'll let others here assess the validity of your %'s argument and conclusion lest I arouse JT's ire.. The orders of magnitude in the pitch-down couples involved under reverse and heavy braking with spoilers up are such that inadvertently raising the nose is a very very minimal mishandling risk (in comparison with overrunning). The redistribution of weight towards the main-gear (in most cases a tricycle geometry) is a secondary effect of the technique that nicely enhances directional control at speed. As the AA1420 pilot found at Little Rock, light on wheels with a crosswind creates two compounding problems - stopping and directional control.
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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.
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Old 9th Jan 2006, 10:14
  #27 (permalink)  
 
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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.
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Old 9th Jan 2006, 11:28
  #28 (permalink)  
 
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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.
Mad (Flt) Scientist is offline  
Old 9th Jan 2006, 11:29
  #29 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

Originally Posted by alf5071h
OVERTALK your picture links suggest that you have a close association with IASA. If that is so, I suggest that you restrict the more extreme views on how to enhance flight safety, however well intention they might be, to the IASA web site; along with the comments from possible ‘associates’ participating in this thread.
For me, it seems just the opposite. While IASA is, ad pointed out, a private lobby group, it has a rather official-sounding name, compared to, say, IATA and ICAO. Therefore, it would be better to express extreme views in PPRuNe , where they can be and are violently critizised, rather than on IATA website where they are not critizised and might be acted upon...

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

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

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

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

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

What is the manufacturer-approved SOP pilot response (if any) to antiskid overruling the autobrake? Assuming that the nosegear remains, initially, supported by runway and has not (yet) slipped sideways?
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Old 9th Jan 2006, 12:25
  #30 (permalink)  
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Time for the Padlock?

"OVERTALK your picture links suggest that you have a close association with IASA. If that is so, I suggest that you restrict the more extreme views on how to enhance flight safety, however well intention (sic) they might be, to the IASA web site; along with the comments from possible ‘associates’ participating in this thread."
Guilty as charged. It's a widespread secret conspiracy designed to stop runway overruns and restore insurance co profitability, destroy the Brotherhood of Unemployed Overrunners and relieve pressure upon the owners and operators of the 305 marginal runways in the USA alone. You've seen through our shabby ploy. How perspicacious of you. But thanks for the giggle anyway. Keep taking that credibility medication and don't give up on that grammar and syntax primer. The only extreme views I hold are panoramic - from my penthouse suite.
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"your description of back stick "acting against the distant nose-gear fulcrum to lever the main-gear INTO the deck" requires the nose oleo to act as a solid structure whilst the main oleos compress as indicated in your diagram, the result of which will increase the AOA and wing lift, and thus defeats the objective."
I believe that the nose-gear was described as a dynamic fulcrum. It's not a rigid hinged pivot. In physics it is not unusual for one moment (the sum of the four pitchdown moments) to be counteracted by another moment (the up-elevator opposing moment). I think that it might in combination be called a "couple". The fact that the nose oleo's compression pressure might be eased somewhat by pilot's backstick doesn't mean that the AoA ogre will suddenly take-over and re-launch one luftwards. The reason that it's called a lever is that (for a 2nd class lever), the mid-span resultant is enjoying a significant mechanical advantage). The effectiveness should not be in question. It's basic physics old chap. Try and reconcile with the dynamics of the situation. The amount of backstick is tempered (modulated/introduced) against the degree of braking achieved..... much as in a taildragger.
Couple: A pair of forces acting in parallel but opposite directions, capable of causing rotation but not translation.
"Putting the rotational issues aside, the problem is actually about the transference of load on the gear. Aircraft designs are such that the majority of this is carried on the main gear; the load on the nose gear provides directional control, particularly as airspeed reduces. Thus, as MFS indicates, the % of load that can be transferred to the main gear is proportionally small and depends on many variables in the aircraft geometry and control characteristics. Thus, it is extremely difficult to judge any benefit to be gained, and if any, the effect diminishes rapidly with speed (V*2).
"Effective" weight transference is not about moving the aircraft's mass around like pumping fuel from tail-tank to wings. The backstick uses the ability of the elevator at speed on the runway, very shortly after touchdown, to generate a downwards horizontal stabilizer lift vector which lever-compresses the main-gear oleos and increases the footprint of all main-gear tyres (a very good thing for braking according to Boeing and Airbus - and incidentally also the measure that has most effect upon aquaplaning speeds). Driving the maingear into the pavement significantly increases the tyre surface in contact.... so wheel rotational torque increases, anti-skid becomes less active and braking is thus much more effective. Ipso (the means) facto (the indisputable outcome - real and not as you would have it: "imagined" or "simulatored").
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You should take the time to read this: (the technique would have helped that QF skipper avoid his hole-in-one jumbo).
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Thus, it is extremely difficult to judge any benefit to be gained, and if any, the effect diminishes rapidly with speed (V*2).
And so, I might add, doth runway remaining - if that speed isn't diminished by the most effective means possible. All the useful retardation will happen in that middle third of the runway. Backstick braking over that stretch will extend your lifespan as a professional pilot. Just sit back and think what it might feel like going off the end and through a blast fence at 50 knots with your toe-brakes fully depressed. You are advocating dismissal of a technique that might help avoid that trauma - just because you personally cannot cope with possible change/ come to terms with having been in ignorance for years? You can be a sceptic without being a heretic.
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"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.
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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.
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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.
OVERTALK is offline  
Old 9th Jan 2006, 23:37
  #31 (permalink)  
 
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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.
RatherBeFlying is offline  
Old 10th Jan 2006, 00:11
  #32 (permalink)  
 
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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.
Mad (Flt) Scientist is offline  
Old 10th Jan 2006, 01:40
  #33 (permalink)  
 
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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.
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Old 10th Jan 2006, 01:48
  #34 (permalink)  
 
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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.
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Old 10th Jan 2006, 02:13
  #35 (permalink)  
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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.
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Old 10th Jan 2006, 08:37
  #36 (permalink)  
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Not Losing Sight of the Aim.....

RatherbeFlying said:
The fulcrum on the ground is the maingear axle. Backstick produces down force at the tail which:
Adds to the total load on all gears
Produces a nose-up moment which relieves a portion of the load on the nose-gear such that the maingear supports a larger portion of the total weight plus any aerodynamic downforce.
Can't disagree with that. It's manifestly the distilled essence of the underlying theory. I might have said: "plus the aerodynamic downforce." Seeing it as the resultant of two moments or a type 2 lever acting upon the nose-gear as a dynamic fulcrum is also valid. But what is obvious (and missed by most) is that there is an "effective" weight shift towards the nose. i.e. We are getting side-tracked by static Center of Gravity computations saying that only about 5% of an aircraft's mass acts through the nosewheel. May be true but it's a red herring in this dynamic flight context. RBF's succinct statement also doesn't address (but obviously accepts) the reason why the nose stays down (a nose-down pitch is induced by reverse and whatever braking you are achieving courtesy of spoiler lift-dump when the progressive backstick input commences). As the backstick increases, wheel-braking becomes more effective, thus allowing more backstick etc etc. Eventually you do end up on the backstops, but by that time you will have killed that speed increment above the min aquaplaning speed -as well as any directional control or cross-wind induced problems that you may have otherwise had.
This may result in a change in AOA and lift....
The process is dynamic and what is being advocated is an early accelerated rate of deceleration. That dynamic process compares with the Air France, QF1 and SWA pregnant pause hiatus where there were interlock problems entering reverse and indecision/mind-changing. The latter process eats up landing real-estate at a great rate of knots and kills your options - whereas the backstick braking uses those knots to achieve very early efficient deceleration before the aircraft reaches that last 2000 odd feet of rubberised runway remaining. Chalk and cheese. Success and failure/disaster. I suspect that, in the fullness of time, Airbus (at least) will be automating the advocated process.
MFS said:
"assuming that there is NO aerodynamic download, which is an optimistic assumption for this method...."
MFS then goes on to draw invalid comparisons of parked a/c and the effect of significant winds, disregarding the nose-down pitching moments that result from reverse and spoiler-assisted early braking. His conclusions are invalid because we are talking about 2 completely different scenarios:
a. static parked weight distribution with nose-up stab trim combatting local wind effects and
b. a 1.1.Vs or greater handling technique with engines running and reversing, spoilers up and a pilot making appropriate control inputs).
.
It is difficult to clearly discuss the nuances of handling techniques with engineers (even flight engineers) - or so I've found. Handling is not like placing a switch at A, B or centrally OFF. Handling is about moderated input tempered by observed reaction. An OFF/ON selection of instant full backstick is not what's being advocated at all. However MFS does introduce (and remind us of) that all-important "square of the speed" factor - which is also a player when considering the effect of early backstick after touchdown. It is certainly what empowers that backstick-induced early breakthrough (to the bitumen) for the main-gear wheels. "EARLY" is also the point at which the effect of reverse thrust (and its pitchdown moment) is maximal (then gradually tapering off).
I'm as certain as I can be that you WILL lift the nose under those conditions. Well if it wasn't for the four additive factors producing the strong (and opposing) nose-down pitch during deceleration, I might concede that point. But MFS is single-mindedly stuck in his engineer's view of the airplane as a static entity. After all, that's how you achieve rotation, no? Well no, not really. You have to consider that there's a Thrust/Weight-Lift/Drag couple at work there during lift-off, ably assisted by leading- and trailing-edge devices (and minus spoilers, braking and reverse). If you fuel-chopped all four at the point of pre-rotate (V1) would it still rotate??
"We are scared of someone trying this out with pax in the back and either departing the side of the runway due to loss of NWS/directional control, or raising the nose and actually losing braking effectiveness."
A mite harum scarum -but that has been your [MFS] underlying theme. I'd suggest that it's only MFS with the trepidations. All the Xperimental TP's that I've known in the RAF, BAe and elsewhere would say: "well let's do some sums and then go find out. Anything that can stop this wasteful "off the end" bizzo is worthwhile exploring and exploiting. It may even be a great marketing tool if it was automated. Let's go talk to some aerodynamics people and perhaps get them to write a program for the iron bird. The simulator should tell us what's what."
RBF said:
"Static loads while parked and dynamic loads while braking are different cases with greater nose wheel loadings under braking."
Bravo for keeping an open mind and identifying the unsubtle difference.
MFS said:
1. "The problem is that the case where the nosewheel is most heavily loaded by the braking forces is the case where the brakes are ALREADY WORKING WELL!" MFS -Please see a+b+c+d below for your illogical sequitur
2. "Not enough to overcome full back stick; not even close for some types."
Five bucks for whoever can get MFS off his FULL backstick bandwagon.... and start entertaining the full vista of forces at play during the dynamics of landing deceleration. i.e.
,
a. REVERSE - greatest effect at the outset => strong nose-down pitch (NDP)
.
b. Braking - It is agreed that despite spoilers assisting, that braking is minimal at the outset on a slippery surface (but then again, that is the actual problem) but still =>(NDP) (initially only equivalent to only another +5% of a/c mass - according to MFS)
.
c. Center of Gravity - I won't venture whether this is MUCH more pro or con at landing (versus take-off) but I suspect that in the landing configuration it's in most cases helping our quest (for a counterbalance to introduced backstick). But no matter how far it's moved as fuel burns off, it will still be telling the nose firmly "to stay down" => (NDP)
.
d. Spoilers will be up and, per the T-W/L-D couple, should also be helping kill lift and admonishing the nose to stay down. i.e. =>(NDP)
.
a+b+c+d => a powerful counterbalancing (but varying) total nose-down force against which we can introduce our proposed progressive backstick
.
So throughout the early landing evolution, speed dissipates at "some rate" and braking will improve "somewhat". Once in reverse, early pilot introduction of progressive backstick will have the desired effect and will improve braking effectiveness (which will in turn enable more backstick, etc etc). Certainly "forward" stick has more than its fair share of perils (link) ("..it appeared that the captain’s technique for landing at Gibraltar differed from that at other destinations, with an evident, if unconscious, propensity to apply full nose-down elevator right after main landing gear touchdown. Immediately after the incident, Monarch modified its FDM software to include nose-down elevator as an "event."). G-MONC was out of service for over 2 months and required >$6M in repairs.
.
The "effective" weight-shift toward the nose will be that imparted by aircraft momentum and retardation forces acting around various axes (and thus summed in ft/lbs). Overall, the early "stickiness" of the nose very much depends upon the total magnitude of the nose-down couples mentioned above. MFS chooses to ignore the cumulative effect.... and why that should be is a mystery - but perhaps it just helps his argument. It is conceded that the diminishing effect of reverse as the a/c slows will be countered by the increased effectiveness of braking.... probably making it a zero-sum game. However that's of no real consequence because the pitch-up authority of the (by that time) full backstick will have faded to insignificant.
.
The theory disregards all the other factors acting for and against a successful stop within runway available as being irrelevant - however it should be noted that it will be particularly effective at combating high groundspeeds caused by landing with the wind (rather than against it). Sometimes it is expedient to accept a tailwind component, but always it is less safe to do so. At such times, the value of backstick braking will be to restore the balance in favour of the pilot not running out of career or runway. Not to be forgotten (also) is the stabilizing effect of getting the weight shift redistributed to the tricycle geometry that gives best directional control. Stick forward can be destructive (G-MONC) but the wheel-barrowing effect of any unopposed weight-shift forward can also cause great directional instability, particularly on a wet runway in a crosswind.
.
Ultimately the aim is to have the theory revalidated and assessed for effectiveness. It is nothing more than a handling technique that seeks to minimize the intervention of anti-skid on inhospitable surfaces by increasing the weight-on-wheels as soon as possible in the landing roll. If it is assessed as worthy, then everybody is better off....and at nil cost. It's a principle called regrets management....and it contrasts well with risk management (which lets us down more often than not).
Overtalk
Pilot Retard Advocate
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Old 10th Jan 2006, 09:12
  #37 (permalink)  
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Moderately Speaking....

JT
I recognize your MFS and alf5071h and apologize for any perceived brusqueness. However intolerant I might seem, I am interested in stopping the pointless overruns. The frustrations arise from "knowing how to" being just drowned (in typical PPRuNe fashion) by a sea of doubting thomases, many of whom need more than simple convincing. Argument for argument's sake normally sends me scarpering. However I've stuck with this one because I'd like to see somebody somewhere move convincingly on the overrun problem.
However I will also raise you a John Farley and a Tony Spence (ETPS grad) from amongst my familiars. Anthony Brown the well-known Canadian aerodynamicist was a one-time copilot of mine. We've corresponded on major accident causes.
I've previously fought this one out with the famed poisoned dwarf (Bo Plummer, the longest serving RAF A1 QFI) and "Farmer" Rod Brown (2nd longest) - and was not found wanting. Prof Peter Ladkin and I often agree as much as we disagree, but he knows who I am. Ray Hudson (MD-11 and F35 flt control designer) and I have often sparred - and always find middle ground. I've often corresponded with Graeme Braithwaite (Cranfield). I also write for ASW. I could go on.... but won't.
I base my theory on practice - and that's as close as anybody can ever hope to get to a working hypothesis.
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Old 10th Jan 2006, 11:26
  #38 (permalink)  
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Re: Pulling a Stop to Runway Overruns

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

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

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

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

And we are all on the same team .. and all have an equal interest in reducing the abort/landing risk scenario.
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Old 10th Jan 2006, 20:02
  #39 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

Gentlemen,

I bow to your obvious superior knowledge of aircraft braking effectiveness. However, I have landed an Electra on a dime with an "in ground effect flare maneuver." The aircraft came to a complete stop upon touchdown. I had to add power to taxi off the runway! This is just an extreme example of momentun control before touchdown, which is not utilized in todays flight operations, to the detriment of flight safety.
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Old 11th Jan 2006, 02:02
  #40 (permalink)  
 
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A Momentun of Your Time

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