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Pulling a Stop to Runway Overruns

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Pulling a Stop to Runway Overruns

Old 11th Jan 2006, 03:04
  #41 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

Simple question.

Is the nose landing gear oleo FULLY COMPRESSED under braking?

If not then ANY reduction in nosegear download WILL raise the nose, by an amount equal to the reduction in download times the effective spring stiffness of the oleo. I would suggest that there are very few types which completely compress the nose oleos, if for no other reason than to completely compress on an oleo risks mechanical damage and significantly affects nose gear damping behaviour.

If that is a significant distance - and on some types it will be, you may rest assured of that, especially at more aft cgs and lighter weights - then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.

Just to be clear: I recognise that OVERTALK and others are suggesting a progressive increase in tail download. The reasons I will CONTINUE to harp on about "full back stick" are:

(1) a procedure which requires modulation of input will ALWAYS carry the risk of someone pulling full back on the "if a little is good, a lot must be better" reasoning

(2) in order to obtain a significant improvement in braking a LOT of tail download is required - to the extent that full back stick will be used at some point

There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load.
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Old 11th Jan 2006, 07:25
  #42 (permalink)  
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the writing's already on the wall......

MFS said:
Is the nose landing gear oleo FULLY COMPRESSED under braking?..............................................
....then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.
I think that we have lost the physics bubble here. The four retardation scenario factors discussed (a-b-c-d above) are producing a strong nose-down pitching moment. By opposing that moment with progressive backstick soon after spoilers are up and reverse kicks in, we must end up with an effective weight increase on the main-gear - which helps the "rubber more intimately meet the road" so to speak - instead of glissading over the wet and slippery surface. It makes early braking much more effective by minimizing the intervention of anti-skid; and it's even got the improved directional control bonus. If you multiply that tyre/bitumen area interface increase on one tyre by all the tyres on a plane's main-gear, you can appreciate that the overall effect will be worthwhile.
<<<"then there will be an increase in AoA and corresponding increase in AoA and REDUCTION in load on mains due to increased aerodynamic lift.>>>
This is a little confusing but I imagine that you meant to say that up elevator would cause the nose oleo to extend and consequently the wing's AoA to increase, thereby acting in an opposite sense to what is being claimed (i.e. reducing the weight upon the main-gear). That that is a fallacy can be exposed by a simple illustration. Put a soft pea (or jelly-bean say) under the midpoint of a 12" rigid ruler and press down on both ends of the ruler. Do it with roughly equal force, simulating a pilot tempering his backstick input to just below what "could" (not saying "would") possibly cause the nose to rise. In other words, braking "for effect" by flying the attitude out the front window. The pea gets squashed of course. That simple experiment replicates the 4 retardation factors producing a nose-down moment at one end of the ruler and, at the other end, the opposing nose-up (i.e. tail-down) moment induced by the backstick. It adequately demonstrates that the resultant will be a down-force upon the maingear (the pea). Now do it with a new "soft pea" yet without the simulated pilot's up-elevator input. The nose drops, tail rises and the pea is unsquashed. That simulates the lack of anything other than the ruler's weight acting upon the "maingear" pea. It emphasizes the need for early introduced backstick braking when the chips are down (i.e. you suspect that you may have landed too far in on a slippery runway and urgently need max effective braking ASAP).
If I was in the RHseat and the reverse had cut in and I knew that we were "down, come what may", yet running short of bitumen - well I'd be urgently calling for backstick and double-checking that the spoilers were up. I have a mind's eye image of both SWA 737 pilots urgently tromping their toe-brakes at Midway recently, the end looming large - yet salvation having been only a pole-grip away. That's the needless futility of not understanding the logic behind backstick braking.
Just to be clear: I recognise that OVERTALK and others are suggesting a progressive increase in tail download. The reasons I will CONTINUE to harp on about "full back stick" are:
(1) a procedure which requires modulation of input will ALWAYS carry the risk of someone pulling full back on the "if a little is good, a lot must be better" reasoning
(2) in order to obtain a significant improvement in braking a LOT of tail download is required - to the extent that full back stick will be used at some point
Disagree with your point (1) because I've been there and done that. Pilots that I have trained are told to introduce toe-braking first and then (but almost simultaneously) progressively feed in the backstick, ensuring that the nose stays down. Unless they soon ease up significantly on the brakes while maintaining "the pull", the nose won't rise - in part because, by that time, you will have slowed significantly. It is the hard braking that keeps the nose down (as well as the airspeed loss). The danger of teaching it in a jet without maxarets or anti-skid on a wet runway is perversely that BLOGGs will overcook on the toe-brakes without having adequate backstick in. It's the backstick that keeps the wheels loaded up and the tyres rotating. If you don't stop the wheel, you don't blow the tyre. I can recall writing that across the top of whiteboards in my briefing cubicle for many early type-conversion handling briefs. With autobrakes and anti-skid, the whole exercise is just too easy (unless you're ignorant of the technique and/or don't mind motoring mindlessly off the end). I reiterate that pilots are taught to use two controls at once (in fact directional rudder inputs plus toe-braking, aileron into wind and steering with cocked throttles to compensate for wind is a normal pilot activity that has four unique and discrete inputs). And if challenged, most could probably make an R/T call at the same time. You seem to have a picture of pilots as motor morons incapable of coping with a simple manipulative task. Landing a heavy-weight asymmetric airplane on a wet runway with 50 kts across used to make me salivate with anticipation..... not dread.
(2) <<<....to the extent that full back stick will be used at some point. Agreed (that full backstick may be used), but if that occurs more latterly in the piece, where's the harm? I reiterate that once heavy braking is underway, with or without reverse, that nose-rise just won't happen.
There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load.
Unfortunately that 67% is roll-out, if I'm not mistaken. Few pilots manage to get to grips with the first few feet of runway. Indeed many PAPI's and glidepaths would have you landing over 1500ft in. When pilots misjudge and make a name for themselves, it's usually because they've grossly misjudged. That can happen as a result of an optical illusion or down-sloping runway or a tail-wind. It's relatively easy to discard 2 or 3 thousand feet of bitumen, yet be unaware of it. Why? Because we haven't got eyes in the backs of our head and a touchdown too far in is not readily apparent. Distance-to-run marker boards are usually only found at military airfields. Landing 1/3rd into a runway at night off a low ceiling in poor vis? That's easily done. When the runway is additionally greasy and ATC has arranged a tailwind for you, don't start any wishful thinking after you've touched down and ripped it into reverse. At that point the writing's already on the wall at the far end of your marginal runway.

Last edited by OVERTALK; 11th Jan 2006 at 07:51.
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Old 11th Jan 2006, 11:42
  #43 (permalink)  
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Re: Pulling a Stop to Runway Overruns

(a) the landing distance factor is applied to the demonstrated flight test/aerodynamic model data as accepted by the Regulator and applies to the total demonstrated distance from 50ft, not rollout distance.

(b) may I suggest that, if the approach is flown accurately to a predetermined aiming point, few pilots with any experience to speak of will fail to detect that they have floated significantly beyond the planned touchdown zone ...
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Old 11th Jan 2006, 12:16
  #44 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

<<<total demonstrated distance from 50ft,>>> by test pilots, probably in good weather in a pristine jet.

<<<<if the approach is flown accurately to a predetermined aiming point, few pilots with any experience to speak of will fail to detect that they have floated significantly beyond the planned touchdown zone ...>>>>

Must be another explanation for all the overruns then, although most that I have read about or heard about seem to have landed well down the runway and frequently hot. Usually there's a weather component or tailwind. The Air France A340 in Toronto touched down around 2000m in.

Rejecting a proposed technique on the basis or with the assertion that pilots wouldn't have problems if they did everything correctly is to deny that fatigue, illusions, CRM breakdowns and mishandling regularly occurs.

That ruler and pea analogy above (in OVERTALK's post) works for me. What further proof does anybody need (beyond flight test)?
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Old 11th Jan 2006, 19:42
  #45 (permalink)  
 
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Re: A Momentun of Your Time

Originally Posted by UNCTUOUS
.
Perhaps you should elaborate on your point.
.
As I understand it, OVERTALK's suggestion (by contrast) is to propose a straight-forward means of stopping contaminated runway overruns (ie. accidents) by enhancing post-touchdown braking effectiveness. That might indeed be an example of something:
<<<which is not utilized in todays flight operations, to the detriment of flight safety.>>>
.
Would you not agree? Or are you disputing that backstick braking works? i.e. as a means of achieving effective braking on slick runways. That doesn't appear to be the case. But perhaps you didn't review the whole thread.
.
Wsherif1 I do agree.
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Old 11th Jan 2006, 21:03
  #46 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

There is general agreement about the theory of how progressive back stick during landing should increase main wheel load and assist braking, although there are some interesting and debatable explanations of the physics, actually mechanics, of how this is achieved.
There have been several parallel inputs about the overall effectiveness of the technique when applying it practically to a wide range of aircraft and operations, and many issues have been identified which could negate any improvement or even be detrimental to safety, especially in crosswinds.
Given these inputs, it is difficult to understand how individuals can persist with the view that their specific experiences on one or a few aircraft will apply universally to all aircraft and operations. There is an overwhelming need to remind operators to heed manufacture’s advice to ensure continued safe operations; the doubters should read the thread on the Emirates A340 incident.

I had hoped that from the posts of 10th Jan alternative aspects of the ‘interest in stopping pointless overruns’ would be explored.
There have been several threads on the hazards of contaminated runways (where 67% margin may not be available), and then there are the issues of landing long and / or fast, for which there are approved solutions with proven safety benefits.
In recent years, there has been more focus on human factors, particularly the frailties in awareness and decision making. Therefore, perhaps the safety focus that we require is ensuring pilots’ use of their ‘superior judgment’ before landing, rather than the superior skill on the runway, which some would claim.
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Old 12th Jan 2006, 12:00
  #47 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

Mad (Flt) Scientist said:

There is already a nominal 67% planning margin between your "landing field length" and "actual landing distance". Unless a technique is going to make a significant impact relative to that 67% margin, it's not going to do much. That means a lot of redistribution of reactions, and that means a lot of tail load.
My understanding of the margins built in to the Landing Distance Required (for the B737 at least) are:

DRY RUNWAY: Actual demonstrated landing distance from 50 ft to complete stop multiplied by 1.67.

WET RUNWAY: (no more than 3 mm of standing water): Actual demonstrated landing distance (on a dry runway) multiplied by 1.67, multiplied by 1.15.

CONTAMINATED RUNWAY: (more than 3 mm but no more than 13 mm of standing water, compacted snow, ice): "Calculated" landing distance for either Good/Fair, or Poor reported braking action multiplied by 1.15. So the margin for error and variables is only 15%, not 67%.

Interestingly, our manual states that contaminated runway performance data is only "Guidance Information".

So with the contaminated runway scenario in mind, any technique that improves the actual landing distance by just 1.5% increases the safety margin by a whopping 10% (1/10th of 15%)!!

It's been a very interesting thread so far. Thanks everyone.
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Old 12th Jan 2006, 14:01
  #48 (permalink)  
 
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Re: Pulling a Stop to Runway Overruns

Re BLIP's Post
Book figures for landing are good for planning and inflight review (except for contaminated runway performance data).
Unfortunately when flight crews get it wrong, they seem to do it big-time, with or without CRM. Or maybe it's just that many of the variables are actually indeterminants. As OVERTALK said in his first sentence at post #1 on this thread, landing overruns are happening all the time. It's only when they have a nasty outcome that goes beyond embarrassment, lost jobs and muddy, scrubbed or flattened tires, that we get to hear much about it. The fact that the industry is (or was) unaware of the value of progressive backstick braking on nasty surfaces is unsurprising. Offhand I can think of numerous things that the industry was apparently unaware of:
.
a. the dangers of certain types of wiring and neglecting wiring husbandry (EAPAS NPRM comments close 03 Feb 06)
b. the dangers of Concorde tire failures (court-case soon to begin)
c. the perils built into faux redundancy (two computers each empowered to take over from the other when it is assessed that the primary computer has failed) i.e. G-VATL's fuel transfer double-flameout.
d. the dangers inherent in forgetting to ARM spoilers (AA1420)
e. the lethal unpredictability of SLD icing (freezing rain)
f. the flammability of heated tank ullage
g. the dangers inherent in resetting CB's (because ensuing arc-tracking faults won't re-trip them)
h. the hazards of designing identical interchangeable fuel gauges for different models (ATR42/ATR72)
i. the hazards of sandpaper textured rime ice on supercritical wing sections. (CL-600)
j. the pitch-up illusion (GulfAir A320)
k. the flammability of metallized mylar thermal-acoustic blankets
l. the idiocy of having a take-off configuration warning horn identical to a pressn warning horn. One oft heard, the other rarely heard - and every chance that a hypoxic crew wouldn't make the right choice. Inaccessible hypoxic pilots behind an impenetrable reinforced door.
m. Software that can allow a fatigued crew to leave previous take-off calc figures in place (and operable) for their next departure (HFX 747)
n. A closed runway with lethal WIP that can be mistaken for the duty runway due to insufficient markings (no active alerting/just passive cues) SQ006
o. A bogus FMC fuel usage consumption that would suck in a crew transiting with the gear down (Hapag A310) or an FMC that allows a crew to enter nonsense take-off parameters (SIA 744 Auckland).
p. Fuel leakage scenarios and checklists that can sucker a crew into believing it's fuel imbalance
q. the ability to down a modern airliner by just bugging the pitot or taping over the static ports
r. A CRJ engine that requires a massive sustained speed increase to achieve sufficient fan rotation for an inflight relight
s. A rudder handling and RTL design flaw that allows a vertical fin to be torn off in the blink of two eyes. A rudder that can disintegrate in flight with little more than a shudder/shake (and no pilot input - Air Transat ex Cuba)
t. The design of a runway incursion system that is useless in rain and doesn't warn pilots directly (AMASS)
u. A Beech 1900 maint manual error that was ancient but would sucker engineers into fatally misrigging an elevator (Colgan and Air Midwest)
v. Standby horizons that are panel central nowhere near the scan of PF or PNF (aka "twinning, where any anomaly between roll or pitch-rate of an immediately adjacent STBY instrument and primary attitude reference is immediately apparent - KAL747F Stansted, Air India 747 Bombay and many others).
w. Using an a/c NAV configuration that is open to auto-resetting to a VOR operating on TEST (A320 North Africa)
x. Jeppesen databases that could dial up a distant NDB in error (AA 757 Cali) long enough to CFIT the crew
y. A speedbrake that wouldn't auto-retract in response to a full power GPWS response. (AA 757 Cali)
z. bits falling off shuttles and NASA conveniently assuming that high-speed light-weight foam was harmless (despite ample evidence over many flights that tile damage was occurring).
.
etc etc etc. Yes we're in great shape. Unawareness is blissful ignorance. But this thread has adequately demonstrated also how some people will argue on quite specious grounds for their right to not know or be told.
The only sure thing is that nobody will be surprised at the next jaw-dropping fatal revelation.
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Old 13th Jan 2006, 02:37
  #49 (permalink)  
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Re: Pulling a Stop to Runway Overruns

Dagger Dirk indirectly highlights a very important consideration ... SOPs seek to address the fact that the guy on the line doesn't have all the information or answers and, to a significant degree, can insulate himself from embarrassment by sticking with the published words of wisdom .. not a guarantee, just an example of sensible risk management/minimisation.

So far as landing is concerned, the principal hazards which the pilot can influence include

(a) approach profile control, minimising float .. ie land shortly after the aiming point ... ALL the time, even if the runway has 15000 ft to play with.

(b) getting the configuration/speed correct - autobrake, autospoiler, flap setting, speed additives, manual brake, reverse and spoiler use, and so on.

At the end of the day, the pilot is paid to think and be aware of what is going on around him .. the enquiry will never be complimentary if the aircraft was hot, incorrectly configured, floated way in (how about a miss ?), boards/autobrake/reverse failed to operate .. but the pilot just sat there with a dazed expression on his face.

Some of us wonder whether this will become an increasing philosophical problem as/if the depth of training and knowledge is watered down in the pursuit of profit ?
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Old 15th Jan 2006, 17:38
  #50 (permalink)  
 
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Re: A Momentun of Your Time

Originally Posted by UNCTUOUS
.
Perhaps you should elaborate on your point.
.
As I understand it, OVERTALK's suggestion (by contrast) is to propose a straight-forward means of stopping contaminated runway overruns (ie. accidents) by enhancing post-touchdown braking effectiveness. That might indeed be an example of something:
<<<which is not utilized in todays flight operations, to the detriment of flight safety.>>>
.
Would you not agree? Or are you disputing that backstick braking works? i.e. as a means of achieving effective braking on slick runways. That doesn't appear to be the case. But perhaps you didn't review the whole thread.
.
I do agree. wsherif1
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Old 15th Jan 2006, 18:47
  #51 (permalink)  
 
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Re: the writing's already on the wall......

Originally Posted by OVERTALK
I think that we have lost the physics bubble here. The four retardation scenario factors discussed (a-b-c-d above) are producing a strong nose-down pitching moment.
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.
b. Braking. As I noted above, the best you can hope for is 5% or so additional mass on the nose - under relatively good braking action. With poor braking action - say 0.10'g' or lower decel - you'll be lucky to see more than 1 or 2% mass 'transfer' occur.
c. c.g. - not sure exactly what you allude to here. Are you assuming that the landing cg is further forward than at takeoff? It hardly matters, for most practical aircraft configurations this is basically the static nosewheel load - usually, as discussed above, of the order of 5-10% of aircraft weight.
d. spoilers. Depends on their location on the wing, degree of sweep etc; entirely possible to have a nose UP moment due to spoilers, since they are AFT of the cg and above or aft of the main gear.

The best you could hope for is the 5-10% basic, static, nose loading, plus an additional 5% due to decel 'weight transfer' under good braking. There are other factors acting to raise the nose, on our types at least, so that represents an OPTIMISTIC assessment of the force on the nose gear.

If you multiply that tyre/bitumen area interface increase on one tyre by all the tyres on a plane's main-gear, you can appreciate that the overall effect will be worthwhile.
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.

This is a little confusing but I imagine that you meant to say that up elevator would cause the nose oleo to extend and consequently the wing's AoA to increase, thereby acting in an opposite sense to what is being claimed (i.e. reducing the weight upon the main-gear).
Absolutely; 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.

That that is a fallacy can be exposed by a simple illustration. Put a soft pea (or jelly-bean say) under the midpoint of a 12" rigid ruler and press down on both ends of the ruler. Do it with roughly equal force, simulating a pilot tempering his backstick input to just below what "could" (not saying "would") possibly cause the nose to rise. In other words, braking "for effect" by flying the attitude out the front window. The pea gets squashed of course. That simple experiment replicates the 4 retardation factors producing a nose-down moment at one end of the ruler and, at the other end, the opposing nose-up (i.e. tail-down) moment induced by the backstick. It adequately demonstrates that the resultant will be a down-force upon the maingear (the pea). Now do it with a new "soft pea" yet without the simulated pilot's up-elevator input. The nose drops, tail rises and the pea is unsquashed. That simulates the lack of anything other than the ruler's weight acting upon the "maingear" pea. It emphasizes the need for early introduced backstick braking when the chips are down (i.e. you suspect that you may have landed too far in on a slippery runway and urgently need max effective braking ASAP).
OK, let's stick with the 12" ruler, and make it more like the actual aircraft conditions.

Put the "mainwheel jelly baby" in the middle, and stick something fairly heavy on top of the ruler, pretty much right above it. Say a coffee mug or something. That squashed the baby a bit - representing the bulk of the aircraft weight on the mainwheels.

Now put a smaller jelly baby - just a head, perhaps? - near the "front" of the ruler, to represent the nose gear, and then slide the mug forward a little until there's a little bit of load on the "nosegear jelly baby" too. That represents the aircraft under normal static conditions.

Now take a small glass - say "shooter glass" sized and place it above the nose gear. That's our "weight transferece" due to various effects. Fill it with water (it might tip off, so don't risk alcohol, no sense in wastefulness) to simulate the effect of increased braking/increased nose weight transfer. What should happen is that the "nosegear jelly baby" gets more squished due to this increased load.

Now, apply back stick by pushing down at the back of the ruler with your finger. As you do so you will both squish the "mainwheel jelly baby" AND "unsquish" the "nosewheel jelly baby". The only circumstances under which you will NOT unsquish the "nosewheel jelly baby" is if the load was so high that it totally flattened it first. In which case you'd have some ability to apply tail download before affecting the angle of the ruler.

And THAT is why I mentioned complete nosegear oleo compression.

You seem to have a picture of pilots as motor morons incapable of coping with a simple manipulative task. Landing a heavy-weight asymmetric airplane on a wet runway with 50 kts across used to make me salivate with anticipation..... not dread.
That's all very well for a skilled pilot; procedures are written, have to be written, so that a pilot of minimal skill can apply them reliably. I'm afraid I've seen enough cases where everyone is sat round the table - our own training and test pilots included, I might add - looking at each other, wonder what the HELL was pilot X thinking when he did whatever the FDR is showing.

And, incidentally, let's assume that I'm wrong, and that a reliable and simple technique can be demonstrated to provide a, say, 10% reduction in landing distance. 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.
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Old 15th Jan 2006, 19:16
  #52 (permalink)  

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Re: Pulling a Stop to Runway Overruns

This thread has become rather personalised – but that so often happens here.

It started with an excellent proposal – pull the stick back to improve braking in the landing run.

This simple idea is in danger of being lost so (if I may be allowed) I will start the thread all over again.

A suggestion to reduce the chances of an overrun accident

On a tricycle gear aircraft pulling the stick back increases the effective ‘weight’ on the main wheels and in many circumstances this will increase the retardation available from the brakes. It will never make the brakes less effective. The military use this technique widely so does anybody know why the civil manufacturers apparently ignore it?

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?
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Old 15th Jan 2006, 20:41
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Re: Pulling a Stop to Runway Overruns

Civil practice is generally straightforward enough.

You define the practice used to achieve the scheduled field performance in the manual, and describe that in the POH.

It's actually common to have aircraft where it's known that the POH scheduled performance could be improved upon - but a conscious decision was made to use a more conservative (and thus predictable) set of handling actions to safeguard the company from liability in the event that the pilot failed to take the actions exactly as the company TP had.

Incidentally, the last light aircraft I test flew (a new French model) had all three wheels braked - it was like hitting a brick wall!

G
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Old 15th Jan 2006, 21:44
  #54 (permalink)  
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Re: Pulling a Stop to Runway Overruns

.. as with the nosewheel brake mod to the 727 ... gets the pilot's attention smartly if the pedals are pushed harder than intended in normal operation.
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Old 15th Jan 2006, 23:54
  #55 (permalink)  
 
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More Fun with Figures

Have finally been driven to pull out my '72 edition of Kermode's Mechanics of Flight. Appendix 2 presents acceleration formulae in various units.

Taking 4b. F = (W/g)a:
a 744 at 644,000 lbs with autobrakes producing a 10 ft/s/s deceleration and g of 32.2 ft/s/s produces a braking force of:

F = (644,000/32.2)10 = 200,000 pounds

Wildly assuming a CG 15 ft above the ground for this tall a/c (pending a better figure from the more knowledgeable) produces a nose-down moment of 3,000,000 ft-lb.

Assuming a mlg to nosewheel distance of 100 ft. produces a corresponding nosewheel loading of 30,000 (3,000,000/100) lbs from braking.
Assuming a static nosewheel loading of some 5% produces some 30,000 lbs of loading from the CG before the mains.

So braking near autobrake max potentially doubles nosewheel loading.

Poor braking action can drastically reduce the extra loading on the nosewheel, but there remains a potential that as deceleration increases as a result of some backstick, greater amounts can be progressively applied.

The maximum case is nosewheel loading reduced to zero by back stick with good braking action. The mlg loading will in this case be increased by:
  1. the static and dynamic loading previously on the nose wheel
  2. the tailplane downforce (approximately equal to the nosewheel loading)
which comes to some 120,000 pounds, i.e. some 20% increase in mlg loading.

I would welcome the application of precise numbers for any type.
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Old 16th Jan 2006, 16:03
  #56 (permalink)  
 
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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.
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Old 16th Jan 2006, 16:55
  #57 (permalink)  

Do a Hover - it avoids G
 
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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
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Old 16th Jan 2006, 18:20
  #58 (permalink)  
 
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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.
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Old 17th Jan 2006, 06:16
  #59 (permalink)  
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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.
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Old 17th Jan 2006, 14:09
  #60 (permalink)  
 
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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.
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<<<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.
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