Lots of unconsolidated wisdom on this thread - about stopping. It may be incomplete however, there is some conflict - and it might bear summarising.
Could somebody clarify reverse thrust availability, spoiler auto-deployment and auto-braking on the 737-700?
Present beliefs (possibly incorrect):
1. Reverse is available below 10ft RA
and/or? when squat switches are made (but will become unavailable if actually selected
during an oleo rebound - due to one/both squat switches opening), requiring a reverse
re-selection (which is possibly why the F/O later succeeded.
2. Spoiler auto-deployment depends upon the same g/a sensing system - but spoilers will immediately auto-deploy
3. If auto-braking commenced immediately after G/A sensing was "made", then de-rotation would lead to a hard nosewheel oleo bash. I believe that wheel-spin-up is required for the autobrake/anti-skid to function and modulate the brake-valves (i.e. prevent MLG wheels stopping). But wheel spin-up shouldn't be the sole trigger for autobraking. I seem to recall that an early LH A320 ran fatally afoul of that design philosophy in Warsaw. Wheel spin-up didn't happen and so most retardation functions remained locked out.
An inability to enter reverse may have just been due to a sticky microswitch. It was reportedly a hard landing however, with nosegear on very promptly,_so there would have been an oleo rebound. If that upward leap coincided with the pilot's rushed attempt to enter reverse, that failure to get into reverse would be unsurprising. The fact that the delay concerned the F/O enough to reach across and try to assist may indicate (via the timing) that the pilot reverse selection was made during a momentary ground-air sensing relay cycle caused by the oleos' decompressive rebound.
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What a risk-ridden process stopping upon a contaminated short runway is - even without a braking policy that has nothing to do with stopping safely - but keyed to_turnaround times. Why is that? Well jet engine reverser effectiveness drops off steeply as speed reduces. Even short delays in getting into reverse can eat up a lot of runway at high speed (as the AF A340 proved in Toronto & the QANTAS 747 in BKK). Why wouldn't automatic spoiler extension have pinned any rebound by dumping wing lift? Well, firstly the rebound is due to the energy stored in the oleos (on top of any involuntary backstick).
AFAIK, the spoilers and auto-brakes would be energized through that
same g/a sensing circuit -so maybe the spoiler panels didn't pop up either, compounding the immediate after-effect of the heavy landing rebound. If they
hadn't been on the same G/A sensing circuit, then spoiler extension could part mitigate the bounce effect of a heavy landing and help pin_the airplane's MLG's to the ground, quickly allowing the wheel spinup that's required before auto-braking starts modulating the brake-valves.
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AA1420 revealed just how nasty the lack of spoilers could be. It causes the aircraft to remain light on the main-gear (because the lift stays on the wings)._In that scenario any attempts at hard braking just ends up with a directional instability as the airplane remains light upon its wheels and the weight transfers towards the nose. It's called "wheel-barrowing" and it can be a divergent and continuing process as the pilot attempts to use a combination of pos/neg thrust to offset crosswind effects (by steering with a cocked wrist on the throttles). In a rear-engined jet any attempts to achieve effective reverse results in rudder blanking - and that compounds directional control problems. No spoilers =>_Light on the main-gear, so with a wet runway and strong crosswind it's an ongoing_nightmare_due to_both directional control problems
and a lack of stopping power. It becomes a float onwards into oblivion (as it did for the captain of AA1420). What can be done to bullet-proof the stopping evolution -particularly on short contaminated runways? Well different airplanes have different G/A sensing mechanisms at present, but although
EMAS overrun provisioning is a great backstop (but greatly disruptive to ops) - an ideal system configuration standard for effective stopping would seem to be:
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a. To firstly have the spoilers deploy on main-gear touchdown via a L+R maingear G/A sensing microswitched relay and/or have an SOP that says the PNF can manually override (as a captain's prerogative/on command) to deploy them at maingear touch.
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b. Have the spoilers, even if manually deployed, auto-stowable by (say) any application of >60% throttle lever inc_movement (to cover the case of a decision to go -_and not stop -_due to having landed too far in). 757 had that retrofit after_AA's Flt 965_Cali crash - but for GPWS reasons.
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c. Have the reversers dependent upon MLG x 2 (both sides)_
first contact only (with any disabling_rebound effect factored out) - as few bounces will be so high that inflight reverse would be of significance. Also_provide a reverser entry_override switch_for RH seat actuation "not before" he sees the post-touchdown pitch attitude reducing towards nose-gear on. DC-4, DC-6, Constellation, SP2H Neptune etc all had this for prop reverse pitch selection. It would overcome those panic stricken moments that happened to all the above-mentioned accident crews. i.e._Just when you're really "into" stopping, the bastard designers have done it to you again (no wheel spin-up, oleo rebound, dropped throttle lever, no spoiler auto-arming, stuck microswitch or relay_etc). The pilot should always have the last word (in other words).
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d. Have the autobrake cut-in dependent upon G/A sensing circuit #1 plus G/A sensing circuit #2 (which is nosewheel oleo depression micro-switch actuation by NLG ON). That stops brakes cutting in while NLG is still airborne and causing a heavy nose hit de-rotation. And there is a difference between maximum
wheel-braking and maximum effective
aircraft braking. For the former, standing on the toe-pedals will just produce wheel-lock and tire-skid on a contaminated runway (i.e. ineffectual for stopping). In the latter case, minimizing the brakes release time of the anti-skid by maximizing the weight on the wheels will stop you in the shortest distance. Despite the NTSB finding on the 182kt Burbank touchdown, that max wt on wheels will be achieved by an autobrake married to some hefty back-stick introduced
after the nose-gear is on. MLG braking produces a strong nose-down pitching moment and the backstick counters that. The important and oft-overlooked nett effect is that the MLG gets pushed into the deck...... for the
very best auto-braking on slick runways .
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e._Increase spoiler
panel size and deployment rate => increased effectiveness for lift-dumping, => earlier weight-on-wheels for
avoiding aquaplaning and maximizing tire rotational torques (which minimizes auto-brake cycling). The less frequent the brake-valves have to modulate to permit continued wheel-spin, the more effective is the auto-braking. This would also make for safer take-off aborts. To blow the tire you must first stop the wheel (is the practical aspect). That's least likely to happen if you maximize weight-on-wheels courtesy of the spoilers. What else can you do to maximize weight-on-wheels? Per d. above, use back-stick once you are on
to stay and braking is heavy. Up elevator forces the main-gears down into the ground. Conversely, forward stick (down elevator) would induce wheel-barrowing. Stopping effectively is a real science.
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f. Add optional drag chute for when runway is both short and contaminated and there's absolutely no scope for getting things wrong (or things just going wrong -like the braking action not being what you were told by ATC). Last non-military airplane to have a drag-bag (
AFAIK) was the Dassault Mystere Bizjet (aka Fanjet Falcon). Drag chutes_are very effective but they_cannot be used in strong crosswinds due to induced directional control hassles. Pax might not like the strong decel, but who really cares about that when ****z is trumpz?
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g. Increase brake capacity and add brake cooling fans for regional operations like SWA's._Underpowered steel brakes plus short runways = very hot brakes at the gate -_and consequently give rise to ill-considered policies (like SWA's alleged nil autobraking one).
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h. Refuse to land on wet marginal runways with any tailwind component.... and always use full flap. Any counter-claim that switching to an into-wind runway wouldn't allow landing minimas (or affect O'Hare ops) is an unconscionable "fudge". If that's the case, you then go elsewhere. Any tailwind component on a marginal contaminated runway is unacceptable (and irresponsible) risk management.
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i. Don't add that 1000kgs fuel insurance for Mum and the kids when the weather's lousy and a diversion is possible. Heavier airplanes have inertia and a higher Vref - and are understandably_that much more difficult to stop.
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j. Don't get in the habit of adding airspeed increments (gust factors)_when they're non-essential. Some pilots are habitually incre"mental". Every extra 5 knots over the threshold will add 15% to the braked stopping distance on a contaminated runway (cumulative extended float + ground-speed) - and it could even be disproportionately more than that when landing with a tailwind. Excess speed also must be dissipated, delays the touchdown and reduces the runway available after touchdown for the "stopping" systems to act. I'm not aware of any STOL appch technique that arrives at the threshold anywhere above the airframe's 1.3Vs.
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k._Most factors influencing stopping power_will be additive (added weight, speed increment, excess height over threshold, induced delay, tailwind component, airfield elevation, malfunction, MEL'd reversers, company SOP's, misjudgment, poor tire tread, confusion_and surprise). It's also accepted that any cross-wind component on an icy runway will significantly detract from the efficiency of reverse (particularly for tail-mount engines) and manual braking, as well as decreasing traction for nosewheel steering.
l.
"when my company introduced the NG with winglets we saw more long landings. They reduced significantly when the transition had been completed and awareness of the fact was more common. Basically it's a question of how well you fly Vref when close to the RWY". A lurking factor for prolonged float? Larger spoiler panels won't fix that.
m.
"The first section of the runway had the best braking, and the last section had the worst."
To cover the possibility of snow-plowing still permitting a glaze-ice slippery slide at the runway departure end (via a quick temperature plunge), it should be mandatory for ATC to avoid this by compulsorily sand-salting the last 1000 feet to go. I'd personally prefer that measure to any reassuring RCR (or pilot) report on braking effectiveness.
_Well retarded pilots are always thinking impetus, momentum, inertia and options. They also evolve a personal STOL approach style because they know that everything's set in concrete once they've planted it on the bitumen. A safe landing starts with an appropriate approach.....for the conditions.
In true, tested, tried & trite fashion, a cavalcade of pprune experts will now point out any flaws in the above logic. But that's OK too. Overruns are commonplace & expensive. There seems to be very little fat built in to accommodate human errors (of judgment and visual illusion). The overall factors need to be re-examined and re-weighed. The review might also impact upon the confidence pilots should feel in their ability to reject take-off successfully (i.e. stop) in similar runway scenarios.
Despite: "Air traffic control reported runway braking to be fair on most of the runway and poor at the end. Touchdown was normal. There was a slight bounce. There was a 7- to 8-knot tailwind. There was 32 seconds from touchdown until the aircraft hit the fence.", it's still possible to run out of runway when things are marginal to start with. All factors must be favorably re-jigged. Max weight on wheels and early reverse is the answer.