Pulling a Stop to Runway Overruns
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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
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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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.
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..
.
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.
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.
Moderator
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
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
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.
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.
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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?
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?
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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.
Ref heavy braking producing a nose-down pitching moment; isn't this thread about landing on more or less slippery runways where heavy braking would be impossible?
To re-cover old ground:
.
a. the CofG accords a basic nose-down weight distribution (ie. before any retardation cuts in)
.
b. even minimally achieved braking will increase that (and reverse certainly will). At speed, spoilers will be helping greatly to get weight onto the wheels. However on contaminated runways a bow-wave of water/slush under the leading-edge of each tyre still tends to oppose wheel rotation (think of that as hydrodynamic lift)
.
c. However as RATHERbeFLYING pointed out, <<The autobrake demands whatever braking it takes to achieve a deceleration rate (and can do that as long as the wheels keep spinning) - otherwise anti-skid prevails (and the called-for rate is unachievable. That is why it is important to prevent the wheels from locking.>> So what we must try to do is to increase the weight-on-wheels to a maximum. In MANUAL braking, if you don't stop the wheels, you won't blow the tyres. Under AUTOBraking it's a matter of reducing the intervention of anti-skid - which stops you from blowing tyres but won't assist more effective braking. That can only be done by maximizing the rotational traction of the tyres. Progressive backstick (i.e. UP elevator) works well against the distant nose-gear fulcrum to lever the main-gear INTO the deck, increasing that weight on the main-gear wheels.
.
That it works like that is undeniable. Unfortunately the level of disbelief and denial seems to be built-in ("why, if this is so, is it only now being revealed?"). Well I used to teach and demonstrate it and wrote it into a few syllabi... but many a lost art has become irretrievably lost over time. Also firmly ensconced in the psyche is the belief that any minor variation from a strictly specified (but mostly imaginary) modus operandi means instant disaster. However if you read the two cited manufacturers' documents (one's a powerpoint and one's a pdf, referenced in earlier links), you will note that both Boeing and Airbus are emphatic that effective braking in the muck relies upon getting the weight off the wings and onto the wheels. It is indeed unfortunate that the handling technique of progressive backstick braking has become a lost art. If it hadn't, I'm sure that many, if not most, marginal (short) contaminated runway overruns might have been avoidable.
.
Of course it's something that needs to be introduced soon after spoilers are up and nosegear is down, as obviously the elevator authority will diminish with speed loss.
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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.
<<<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.>>>
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.>>>
"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.>>>
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.>>>
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.>>>
remove weight from the nose wheel and reduce non-aerodynamic
directional control.>>>
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.>>>
pronounced nose up pitching moment and a habit of applying nose up
elevator too soon after landing could cause a
tailscrape.>>>
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.
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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.
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??
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??
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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.
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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.
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"Assuming that the plane has just touched down, the speed will be somewhere close to stall speed."
"...so that the mainwheel reaction is essentially the aircraft weight. (ignoring the nosewheel reaction component of maybe 5% for now)"
"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."
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.
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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.
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.
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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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.
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.
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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.
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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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."
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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."
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).
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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).
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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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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.
NoseWheel not a Fulcrum
The fulcrum on the ground is the maingear axle. Backstick produces down force at the tail which:
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.
- 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.
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.
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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.
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.
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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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.
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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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.
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.