Braking Old Habit Patterns and Pulling a Stop to Runway Overruns

Hardly a day goes by without an airliner or bizjet sliding off the end of a slippery runway on landing (and now and again, off the side - probably after blowing a tire in a crosswind). In most cases the pilot will have made a simple error of judgment by arriving in disarray at his challenging runway environment. Rarely will the cause be attributable to an actual airplane unserviceability. Sometimes death and destruction will be the outcome, normally as a result of a marginal runway length, a contaminated surface and an obstacle-strewn or geologically inhospitable overrun area. Recent high profile cases have included the A340 at Toronto and the 737 at Chicago (Midway). Whatever the cause, the blame will eventually attach to both the pilot and the airline - so reputations are inevitably sullied. But why does it happen so repetitively? Is it just the "death of a thousand cuts" type of accident? Or is there something significant that is being overlooked? In ASW (08 Aug 05) we looked at the EMAS overrun bedding and in the 15 Aug 05 ASW, a landing performance monitor (LPM). In this discussion we will look at a largely overlooked pilot stopping technique that may just be a potent life-saver.

The Setup

If a pilot goes to his model's operating handbook, he will see what is expected of him for landing techniques - and his performance charts will tell him the landing distance required for wet and dry runways. However most operations into "contaminated" runways are just "not recommended" and it is there that pilots can enter unwillingly into a game of dungeons and dragons. 

The "thousand cuts" aspect begins when an airfield's ATC decides that even though its duty runway now has a tailwind, it cannot be changed because that would affect instrument approaches and departures at a nearby major airfield. Perhaps it cannot change because into-wind approaches to the reciprocal runway would have a higher minima and wouldn't work in the prevailing ceiling and visibility conditions, leading to mass diversions. So a pilot gets thrown in at the deep end, placed in the invidious position of "having a go" and often ends up role-playing the villain. Unfortunately the data upon which he was working may have quietly become vaguely ill-defined. Runway friction measurements are notoriously variable, whether made by instrument or opinion. If the preceding pilot reports after landing that the braking was "fair", the next man down the slot has little to hang his hat upon were he to judiciously decide to divert anyway. Commitment and challenge is the name of this game.

Unfortunately, one man's subjective "fair" may be based upon a slightly different wind, a lighter airplane weight or another type of mount altogether. Perhaps he's just enthused and infused by his own successful arrival and subconsciously laying down the gauntlet for the next man. It's invariably very precarious information upon which to make a potentially life-altering decision. As things now stand, land a little hot and a little too far in and you've just applied to join the rough-rider's Honor Roll of Infamy.

Down and Out (of Options)

Once a pilot is down, he must make an instant decision as to whether he can stop or should "go". That was the premise for advocating an LPM aid. Frequently a pilot will be quite unaware of how far in he's touched down and, early on, just how much runway remains (unless he's got sixth sense plus eyes in the back of his head). Tailwind, threshold crossing height, target threshold speed, flare technique and runway downslope can make that "distance in" quite grossly variable. At night and in rain, pilots can also suffer from optical illusions and land long. But nevertheless, once he has moved the throttles into reverse, the pilot has made (and effectively announced) a conscious decision to "stay" and stop. It is this point, its aftermath and the ensuing degree of pilot control that we are interested in here. What are the factors in play here for a halting success?  In both the SWA 737 accident and the AF A340 at Toronto there was an abnormal hiatus in achieving reverse. That can sometimes be a result of indecision and cross-cockpit last minute mind-changing or assumption of control by the captain (cf Qantas 747 overrun in Bangkok); yet it can also be a glitch by-product of a rushed premature grapple with the throttle interlocks that are there to prevent inflight reverse. The SWA 737 didn't achieve reverse until some 18 seconds after touchdown.

Meanwhile that airplane's headlong rush ate up a lot of landing real-estate and over that period it was also too light upon its wheels for effective braking. An airplane's weight-on-wheels ground/air sensing circuitry must wait for oleos to depress, radar altimeters to authorize and multiple microswitches to be "made". Once these pre-conditions are satisfied, hopefully the spoiler panels will spring up from the wings, reverser cowls can open and, once the mainwheels have spun up, the autobrake will quickly start "applying the anchors". It is the stuff of milliseconds. At this juncture is it all out of the hands of the pilot? - or is there something else determinative that he can do? - perhaps with his hands?

Slip-sliding Away

Think in terms of braking effectiveness; ruminate upon the criticality of "weight-on-wheels". Spoilers are designed to lift-dump a section of the wing and help move the airplane's weight onto its wheels for better braking traction, whereas thrust reversers are intended to kill speed (and indirectly, lift). Until the airplane's weight is mostly upon its mainwheels, the auto-braking is going to be initially restricted (for its effectiveness) by the anti-skid system. Anti-skid logic knows that if it allows a wheel-bogey's wheel-spin to even almost stop, the tires will be blown. Like an auto's ABS, its job is to detect any wheel's incipient wheel-skid and release the autobrake pressures sufficient to allow the wheels to maintain a threshold rotation rate. Thus the end result of a hyper-active anti-skid, because of a greasy surface, is much less braking effectiveness overall. So we need to get MORE of the aircraft's weight off the wings and onto its mainwheels - and to do that as early as possible. It will suppress the anti-skid's non-retardant interventionism. Is there a way to do this - i.e. get earlier weight-on-wheels?

Do (or Don't?) Hold Back?

Max effective braking requires ASAP max weight upon wheels

 There is a way - and it's one that has been largely overlooked by the civilian airline industry (although it is taught by the military). It is based upon the fact that reverse, braking and spoilers all serve to promote an effective weight-shift towards the nose. More precisely, all those stopping measures cumulatively cause a strong nose-down pitching moment. They collectively add greatly to the nose-oleo depressing moment that would be there, courtesy of the airplane's C of G, even if spoilers stayed down and reverse and braking wasn't being used.

But how can this cumulative weight-shift towards the nose help us stop? Quite simply, a pilot can confidently counter this nose-down pitch by introduction of progressive backstick once under reverse and braking. The pilot's up-elevator will oppose that nose-heavy pitch-down and push the main-wheels into the ground.... with no possibility of rotating the nose airborne again (a popular misconception and unfounded dread of some pilots). Now read through that again, just to make sure you understand.

So Non-Fatal a Traction

That up-elevator reaction will give greater rotational traction to the mainwheels and stop the anti-skid from interfering with the efficiency of the programmed auto-braking. It will significantly shorten the ground-roll. It will also assist greatly in avoiding aquaplaning by increasing each tire's foot-print.

Aquaplaning is an oft-encountered wet runway condition where the lightly laden tire is just sliding along (with little frictional incentive to rotate) upon a thin bow-wave of water, snow, ice or ultimately the thick wet rubber deposits at the runway departure end. The latter slide is called a reverted rubber skid (check Google). Effective braking just isn't happening under aquaplaning - yet blow-outs can.

Are there any other bonuses besides that all-important reduction in landing roll-out? Well yes. Greater weight upon wheels will tend to cancel the "getting blown sideways" effect of a stonking crosswind component (a big factor in AA1420's accident at Little Rock). Is there a downside? Well according to all those who spent a whole career unaware of this technique, there just MUST be. Otherwise the God-like manufacturer's God-like test-pilots would've recommended the technique... well wouldn't they? Unfortunately these skeptical individuals cannot offer a precise aerodynamic reason as to why the technique wouldn't work or (more ominously, rolling their eyes heavenwards) might indeed be dangerous ("you must only use the manufacturer's recommended techniques" they parrot). Some have ventured that the nose would rise once back-stick was introduced. Once they are challenged to delve into that mystic unaerodynamic development more deeply however, they tend to fall silent.

The advocated technique is NOT aerodynamic braking but this is (often used in conjunction with a drogue-chute)

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. Others have ventured vaguely that directional control would suffer. But actually the up-elevator re-distributes the aircraft weight back towards the mainwheels, restoring a directionally stable tricycle geometry. Conversely the forward stick (advocated by some) creates a nose-heavy (and directionally unstable) wheel-barrowing effect. Others have waxed on (confusingly) about the inefficiencies of aerodynamic braking, where the nose is held off (via earlier backstick) and aerodynamic drag is allowed to slow the airplane. By contrast, the technique of progressive backstick is initiated after the nosewheel is ON and other retardation systems have kicked in. It is thus nought to do with aerodynamic braking.... although the confusion is understandable.

In MANUAL braking, if you don't stop the wheels, you won't blow the tires. Under AUTOBraking it's a matter of reducing the intervention of anti-skid – a device which stops you from blowing tires but won't assist more effective braking on a greasy runway. That can only be done by maximizing the rotational traction of the tires. Progressive backstick (i.e. UP elevator) works well against the distant nose-gear fulcrum to lever the main-gear INTO the deck, vitally increasing that weight on the main-gear wheels. Of course it's something that needs to be introduced soon after spoilers are up and nosegear is down, as obviously the elevator authority will diminish with speed loss. Look for an improvement of the order of 20% better than book figures on dry runways and similar on wet runways, with maybe slightly less on really nasty rubbery and slushy surfaces. But runway surfaces vary so much over their length, particularly because of the rubber buildups in that critical last 2000 feet. Braking achieved early on, in the "clean" second third of the runway, is what it's all about. Early on and sustained is good, whereas introduced very latterly as a panicky afterthought is pathetic. That's like deciding to save fuel after the "tanks low" lights illuminate. The process is dynamic and what is being advocated is an early accelerated rate of deceleration. That dynamic process compares with the Air France, QF1 and SWA pregnant pause hiatus where there were interlock problems entering reverse and indecision/mind-changing. The latter process eats up landing real-estate at a great rate of knots and kills your options - whereas the backstick braking uses those arrival knots to achieve very early efficient deceleration before the aircraft reaches that last 2000 odd feet of rubberized runway remaining.

Opponents to the theory claim 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. The pea gets squashed of course.

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. 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 not squashed. That simulates the lack of anything other than the ruler's weight acting lightly upon the "maingear" pea (as pivot). Others quite rightly point out that reverse on some rear-engined jets has an early pitch-up effect - as do some spoilers. That may be, but their reverse is constrained to about 80% by the rudder blanking limit. Overall the effect of spoilers, reverse, C of G and braking is still strongly nose-down.

Both Dextrous and Ambidextrous

So will Airbus now be displeased to know that there is still a role for pilot manual input, particularly when in extremis? In due course they may quietly automate this process. Perhaps they should. Perhaps it's something that Boeing could also do in the interests of Flight Safety. Until then, any pilot quickly running out of bitumen, committed and with no other options remaining, would do well to whip out a photo-copy of this article, have a quick refresh and then pull the stick back into his gut. Backstick is a great corporeal leverage point for meaningful ergonomic toe-braking anyway. The alternatives, by then, are indisputably the stuff what nightmares are made of ..... so in for a penny, in for a pound (of backpressure). I might add that it works for all sizes. It's never the size of the dog in the fight, it's the size of the fight in the dog. At the very least it gives you something meaningful to do whilst motoring off the end. In the end, and particularly at the end, anything that you've neglected to do earlier just becomes simple wishful thinking. The status quo at present seems to be for the elevators to be left to seek their own equilibrium and the pilot to pay scant attention to yoke or sidestick pitch authority once the nosewheel is on. 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".

Marginality
There are many marginal length runways around. Arriving on "perhaps contaminated" marginal runways is a totally inexact science. In fact it is a "no mans land" full of hidden mines. Prudence dictates that pilots should be apprised of,and experienced in, all valid stopping techniques - including progressive backstick braking.

Not to do so is to push the envelope of commercial pressures to the scene of the next accident...and then the next. Threat and error management dictates frequent reviews of operating procedures before your next accident. Qantas found that out the hard way and changed its stopping policies after Bangkok. So, if you're still a non-believer, please put the weight upon the wheels in your organization to explain to you aerodynamically, in the words of Professor Julius Sumner Miller, "Please sir, why is it not so?"

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 physically taught to "the old school" military. In the age of autobrake and anti-skid it has now however "come of age" again. It would be surprising if, in the interests of improving the runway overrun statistics, one (or both) of the two major manufacturers didn't someday automate it.
Pprune Pilots's Forum Discussion link (4 pages) = www.iasa.com.au/backstick.htm 

SWA Pilots' Assn Observations on the SWA737 Overrun at Chicago Midway:  link 

relevant reading:

a.  www.pprune.org/forums/showthread.php?t=204112   (contaminated runway report reliability)

b.  www.pprune.org/forums/showthread.php?t=201568&page=13  (and subsequent thread pages)

c.  www.flightsafety.org/ppt/managing_threat.ppt  Approach and Landing Threats

"“Attempts to land on contaminated runways involve considerable risk and should be avoided whenever possible.” (UKCAA AIC 61-99)
"The friction level of a Concrete runway is not as good as ‘black top’ Tarmac, which is not as good as ‘high friction course’ Tarmac. Beware of rubber deposits on all surfaces, particularly on wet runways."

d.  Pprune debate thread (multi-page)

     www.pprune.org/forums/showthread.php?t=204897

e.  http://pprune.org/forums/showthread.php?s=&threadid=201987 braking conditions (defined subjective opinions for Good - Fair - Poor - Nil )

“it is not practicable at the present time to determine aeroplane performance on the basis of an internationally accepted friction index measured by ground friction devices”. i.e. “There is no overall accepted certification / operational correlation between mu meters and airplanes”
 

f.  www.pilots.or.kr/upfile/aip/9AAAA_StoppingonSlipperyRwys2.ppt

Boeing's "Stopping on Slippery Runways. "“If runway is reported to have slush/standing water covering, the flight crew should be suspicious of braking action reports and measured friction.”
and “Ground friction measuring vehicles are unreliable when the runway is covered with a depth of contaminant that exceeds: Water - 1 mm. Slush/wet snow - 3 mm. Snow - 2.5 cm”
 

g.  www.wingfiles.com/files/safety/gettingtogripswithcoldweatheroperations.pdf

Airbus perspective on contaminated runways operations