Pulling a Stop to Runway Overruns

[alf5071h]

Re: Pulling a Stop to Runway Overruns

Originally Posted by John Farley

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

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

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

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

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

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

[John Farley]

Re: Pulling a Stop to Runway Overruns

Thanks Alf

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

It failed!

JF

[skiesfull]

Re: Pulling a Stop to Runway Overruns

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

[OVERTALK]

In Reply to Some More MFS Vacillations

MFS said:

Not so, at least on our types.
a. Reverse thrust on our aft-mounted types causes a strong tendency for the nose to rise; for that very reason our advice to pilots is moderate forward stick force to prevent the nose from rising.

Can't dispute that authoritatively except to say that there are quite a few limitations on high aft-mounted engines as far as reverse goes (i.e. the restriction to 80% available reverse due to rudder blanking on MD80/717 types would also help limit any pitch-up due to reverse). So McDD put a forward stick recommendation in the Pilots' Handling Notes eh? I wasn't aware of that. Only goes to show that it's always horses for courses and no one handling technique can be seen to be a universal panacea. I'd have thought that spoiler and braking would've easily overcome any such pitch-up tendency - with there being not much of a lever arm between those rear engines and the main-gear. The demise of the 717 may help write finis to this consideration in the fullness of time. But many newer design rear-engined smaller jets such as the CRJ seem to have similarly mounted engines....so perhaps someone can comment on whether the CRJ has a similar caution written into the Pilots' Notes. link

Then what would happen next is that we (the OEMs) would take credit for the technique in our published distances which would result in every landing becoming potentially more marginal. The technique wouldn't be something in your back pocket for a bad day when you need it; it'd be something you'd have to use every day, and would be assumed to use.

At present the FAA is very very circumspect in defining techniques and configurations for establishing "book" distances - so I'd see that view as being unnecessarily alarmist. e.g. <<The standard curves (i.e., equations) of braking coefficient versus speed prescribed in §25.109(c)(1) are based on a tire tread depth of 2 mm. Since the tread depth of new tires is usually 10-12 mm, 2 mm represents no more than 20 percent of the original tread depth. FAA Advisory Circular 121.195(d)-1A, which provides guidance for determining operational landing distances on wet runways, specifies that the tires used in flight tests to determine wet runway landing distances should be worn to a point where no more than 20 percent of the original tread depth remains...etc>>. So the backstick braking technique would be no different to (say) not instinctively trying to pick up a dropped wing with aileron near the stall (another non-intuitive pilot-developed skill). I'm not sure that ALPA or IFALPA would agree with your continued portrayal of professional pilots as having to conform to your depiction of their abilities, skills and challenges necessarily being limited (and conforming) to a lowest common denominator.

any unloading of the nose gear will allow some extension of the gear, raising the nose and increasing AoA with the noted unloading of the mains. Unless your nose is basically burying itself in the ground, with oleo fully compressed, before you apply the back stick then the nose MUST lift. The question is, how much.

Not sure that this is a productive piece of imagery. It's one reason why my first inclination was to first use the analogy of the second-class lever (with the nose as fulcrum). Using that logic the backstick just lowers the tail and loads up the main-gear, with the nose-gear acting as a pivot-point. John Farley has endorsed the ability of backstick to "load up" the main-gear for more effective braking on mush - so perhaps we can leave this nicety to be thrashed out by the aerodynamicists (who will see it as the resultant of couples (nosedown pitching moment and taildown pitching moment). I've personally never been able to raise the nose whilst under backstick braking. As a pilot it's obviously something that would be quite attention-getting. Because the ergonomics are optimal, you find that the more you haul back, the greater the braking that you can apply without wheel-skid. The dynamism of backstick braking is such that any minor degree of "nose-rise" induced mainplane AoA would be more than offset in the lift equation by the rapidly diminishing IAS.....However if anyone is into cheap thrills, try removing the backstick whilst under concerted backstick braking. I guarantee that you'll only try that once, if at speed on a wet or slushy runway.

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

The NASA study on a tyre tread's effect on hydroplaning and braking friction coefficients places great emphasis upon the grooved circumferential treads being in contact with the runway, and that being a function of weight-on-wheels. For more groovy contact I'd imagine that you'd need a larger "footprint". However we are all agreed (I think) that weight-on-braked-mainwheels is the name of this game.
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HYDROPLANING is a condition that can develop whenever a tire is moving on a wet surface. The tire squeezes water from under the tread generating water pressures which can lift portions of the tire off the runway and reduce the amount of friction the tire can develop. On a runway contaminated by rain or wet snow, it can be impossible for an airplane to accelerate to take-off speed and then to stop on the remaining runway in an aborted take-off. During landing, deceleration and stopping an airplane can be similarly compromised.
Just to be clear (due to some PM's Rx'd). There are three types of hydroplaning. We're interested in #2 below and R-R hydroplaning to a minor extent (because of heavy rubber deposits).
Viscous hydroplaning occurs when there is a thin film of water and relatively low tire speeds. The water lubricates the surface and decreases traction. A water film of only a tiny fraction of a centimeter will drastically reduce the friction between the tire arid the pavement and double the stopping distance.
Dynamic hydroplaning requires deeper water and results in complete loss of tire contact with the pavement. The tire lifts off the runway and rides on a wedge of water.
Reverted-rubber hydroplaning can occur when a locked tire skids on a wet or icy runway. Frictional heating raises the tire temperature causing rubber particles to shred off the tread. These particles accumulate behind the tire forming a dam that blocks the escape of water. The trapped water heats and turns to steam. The steam pressure lifts the tire from the surface.
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Your counter-analogy to the "ruler and pea" is totally and transparently specious. One nose-heavy moment versus a tail-heavy moment and the main-gear as pivot-point..... that is adequately illustrative of the forces involved.
worth reading
also worth a look

[Dagger Dirk]

Making Sense of it ALLF

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

[alf5071h]

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

[Dagger Dirk]

See You and Raise you a more "Apt" Quote

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

[Mad (Flt) Scientist]

Originally Posted by OVERTALK

But many newer design rear-engined smaller jets such as the CRJ seem to have similarly mounted engines....so perhaps someone can comment on whether the CRJ has a similar caution written into the Pilots' Notes.

Actually, I can, because your assumption that the manufacturer I work for is based south of the "49th Parallel" is misplaced. So while I don't know what McDD (why does it censor the 6 letter version?) may have recommended in the past for their product line, Bombardier (in the form of Canadair, specifically) does have such a recommendation.

At present the FAA is very very circumspect in defining techniques and configurations for establishing "book" distances - so I'd see that view as being unnecessarily alarmist.

If we were allowed to take credit for all available braking devices, rather than having to leave one 'in reserve' as it were, I can assure you that simple commercial pressure would REQUIRE us to do so, or lose performance and hence sales to our competitors. So if there were a reliable technique for increasing braking performance that we could take credit for, there would be very power motivation to use it.

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

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

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

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

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

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

Oh, and ...

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

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

[Sick Squid]

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

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

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

Squid

[LEM]

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

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

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

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

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

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

See what I mean?

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

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

Or have I missed something?

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

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

What's the difference in lift generated?

Here we need the math people!

But only after we have the right picture in mind!

LEM

[Centaurus]

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

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

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

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

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

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

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

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

[OVERTALK]

A Principle Still Misunderstood

I thought what was the major reason for pushing the stick while braking has been almost totally overlooked: to decrease the residual lift from the wings, thus getting MORE weight on the main gear.
In simple words: while it's true that UP elevator will tend to increase somewhat the apparent weight for reasons discussed at length here, it will also increase the incidence of the wing, while elevator full down will reduce the attitude by maybe a couple of degrees.

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Elevator full down (i.e. stick full forward) will only effectively weight transfer onto the nosewheel, leading to "wheel-barrowing" (a condition of directional instability). It won't, by any useful measure, decrease the wing's AoA. Why? Think of a depressed oleo as only being able to soak up shock i.e. weight-bearing oleos are effectively incompressible to flight-control inputs - therefore fully-down elevator will just load up the nosewheel. It won't depress the nose oleo appreciably more, nor take any angle-of-attack off the wings. By contrast backstick will load up the mainwheels, increasing traction, enhancing directional stability and you will achieve effective braking much earlier - particularly on a wet or contaminated runway. Why wouldn't backstick raise the nose? Don't forget the combined pitchdown effect on nose touchdown of engine reverse, braking and spoilers. During the important period for effective braking this pitchdown couple enables the backstick's effect of loading up the mainwheels - by stopping the progressive up elevator from raising the nose. Why progressive? Well obviously a pilot is not going to immediately put his yoke to the backstops after nosewheel on. But as speed decreases, inevitably, if going for the maximum backstick braking effect, the yoke will end up fully back. Obviously differently configured airplanes will have slightly different characteristics.
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...by pushing the stick (almost) fully forward, as I've done since now, what do we get out of the balance?
More lift from the tail, or more-reduced lift from the wings?

You will note that internationally well-known Experimental Test-pilot John Farley came out in support of the backstick braking technique but was rudely rebuffed, essentially in mid-post, by one of those who cannot accept the practical facts..... and who instead waxed on with ever-confusing hypotheticals. Backstick braking is a proven effective stopping technique. Unfortunately it's not yet been automated and, because it is a dynamic process, it is easily misunderstood. Those who normally oppose change have been well represented on this thread and have used quite illogical reasoning in an attempt to deny its effectiveness and conjure up fanciful possible dangers.
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This lift reduction on the wings by a pitch decrease is the key to this issue, and I'm really surprised nobody has given this the due attention.
Or have I missed something?

You most certainly have missed the big points essential for understanding how the technique works.

[LEM]

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

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

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

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

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

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


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

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

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

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

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

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

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

LEM

[L337]

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

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

L337

[Mad (Flt) Scientist]

Originally Posted by OVERTALK

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

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

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

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

You will note that internationally well-known Experimental Test-pilot John Farley came out in support of the backstick braking technique but was rudely rebuffed, essentially in mid-post, by one of those who cannot accept the practical facts..... and who instead waxed on with ever-confusing hypotheticals. Backstick braking is a proven effective stopping technique. Unfortunately it's not yet been automated and, because it is a dynamic process, it is easily misunderstood. Those who normally oppose change have been well represented on this thread and have used quite illogical reasoning in an attempt to deny its effectiveness and conjure up fanciful possible dangers.

I for one prefer a soundly based theory to hero-worship - with all due regard to JF, he cannot possibly know the characteristics of every type and I will repeat once again for those who missed it:
THE SPECIFIC ADVICE TO CREWS FOR MY COMPANY'S MAIN PASSENGER AIRCRAFT IS PROGRESSIVE FORWARD STICK DURING BRAKING. This is based upon flight test experience of the aircraft, not upon a theory. I would hate one of our crews to decide to go against the advice of Test Pilots who flew our certification testing and try out some technique of their own.

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

[UNCTUOUS]

Pitch UP effect of Runway Reverse???

LEM says:

And another point where we disagree is that reverse thrust does not produce a pitch down effect.
Actually, the pivot being the main wheels, reverse thrust produces a pitch up effect, even on low mounted engines like a B737, unlike in flight where it would produce a pitchdown effect.

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Beg to differ here (and I did notice that MFS decided to gloss over this basic point).
But there is so much fundamental confusion about cause and effect in this thread that it wouldn't be worthwhile embarking upon any indepth explanation.
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MFS said:

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

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There's a significant difference between a compression able to be induced by flight control loads and the additional shock absorption capabilities of an oleo.
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MFS says:

THE SPECIFIC ADVICE TO CREWS FOR MY COMPANY'S MAIN PASSENGER AIRCRAFT IS PROGRESSIVE FORWARD STICK DURING BRAKING. This is based upon flight test experience of the aircraft, not upon a theory. I would hate one of our crews to decide to go against the advice of Test Pilots who flew our certification testing and try out some technique of their own.

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It has been conceded in ths thread that there is a difference between your tail-mounted engines types (now a dying configurational breed) and others. BAe146/RJ85/RJ100 pilots have said that they use backstick braking because it works and therefore it's an endorsed handling technique. At some time in the future we will probably see the practise automated (in Airbus A340, A380, A330, and 737, 767, 787 etc). However by that time there will be but a few aft-mount beasties around (Embraers and Bombardiers). They will be trapped in the technology of their era.
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MFS said:

ANY transference of load from nose to main MUST raise the nose. The less load you transfer, the less the nose will raise and the less use also will be the backstick method....

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Another confusing injection. The up elevator's aerodynamics is exerting a download on the maingear, not off-loading the nose-gear by any "transference". It's a relative change (nose to mains loadings) only. As speed dissipates, that up elevator ability will be progressively diminished - however it is at the higher speed that we would like (and benefit from) maximized maingear wheel traction, so that's the beauty of "backstick braking".
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[Mad (Flt) Scientist]

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

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

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

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

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

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

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

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

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

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

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

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

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

[LEM]

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

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

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

[UNCTUOUS]

MFS - I'll Try and Simplify it for you

MFS said (unQUOTEd) <<Far too much in his confusing post above to bear repeating so...>>(/UNderQUOTEd)

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

[UNCTUOUS]

Spoiler Effectiveness and Yoke Authority in Pitch

LEM queried:

(1) is it true or not that spoilers kill only a fraction of the residual lift?
(2) Is it true or not that we can change our pitch by yoke input during rollout?

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