RRAAMJET

Just read those replies - thanks for that. Interesting....as someone said earlier, 18 seconds is an inordinate amount of time if rev. were required.

The Good Samaritan

Smooth landing is not fit on runway conditions such as prevailed, and probably it explains the 18 seconds delay to deploy thrust reversers.

the_hawk

While the pilots said

Now one could say they are not telling the truth, but one could also say the flight attendants can't judge a landing as good as the pilots. This leads us nowhere, FDR will tell...

Airbubba

>>The aircraft involved is a 737NG. The reversers on 737NG can be deployed if either radar altimeter senses less than 10 feet altitude.

Seems like there was a mishap years ago where a B-737-300 crew (Piedmont or USAir?) used an unauthorized short field technique from the -200. They closed the throttles and pulled up on the T/R levers in the flare. Unfortunately, as you mentioned the glass 73's use RA instead of weight on the wheels and a very hard landing ensued after the buckets popped while still airborne. ValuJet had similar incident where they pulled breakers on a DC-9 to try to fix a ground-air sensing problem with pressurization. Unfortunately autospoilers were fooled too, another hard landing. Ground-air sensing has subtle and significant effects to many systems on current airliners.

Here's the report on the ValuJet incident:

http://www.ntsb.gov/publictn/1996/AAR9607.pdf

With the NG's I think the reversers will still come out with weight on the wheels if the 10ft RA signal is not present due to a fault, does this sound right?

The Good Samaritan

Attn DEFPOTEC,

The thrust reverser can be deployed when either radio altimeter senses less than 10 feet, or when the air/ground safety sensor is in the ground mode.

This is the exact quote from Boeing technical manual.

411A

<<...autospoilers were fooled too, another hard landing. Ground-air sensing has subtle and significant effects to many systems on current airliners.>>

Older ones too.

As in... full ground spoilers on an L10 when landing flaps selected on final.
Just what you need after a long 10 hour flight, at 5AM in the morning...

Belgique

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.
_
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.
_
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:
_
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.
_
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.
_
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).
_
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 .
_
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.
_
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?
_
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).
_
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.
_
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.
_
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.
_
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.

boofhead

A lot of what you are saying has already been considered.
The auto brake does not bring the nose down with a thump; it is easily controllable. Having the brakes come on early, as soon as touchdown occurs, is a good thing under all conditions. Once the airplane is slowing the pilot can take over manually just by pressing the brake pedals, if that is what he wants to do. Point is it should always be armed for landing, wet or dry. Boeing philosophy. Idiot proof, it was thought, but apparently not completely so, especially when airlines think they know better.
Spoilers are essential, which is why they are automatic and if forgotten, will automatically deploy when the reverse is selected.
The problem with late reverse is due to improper operation, not design. We have all done it at some time, but usually learn how to do it right early on. Hopefully during the initial simulator periods, and not on the airplane. If the reverse is pulled with the throttle(s) not fully closed, the linkages lock up and it takes some force to push them back down, properly close the throttles, and try again. Takes time, during which the residual forward thrust, if above idle, will retard deceleration, or at best will not help to bring the airplane to a stop. Some airlines even mandate that the reverse should not be pulled until the nosewheel is on the runway, contrary to Boeing procedures, and another of the many examples of idiocy in airline management.
All accidents can be traced to management; poor policy, training, supervision, support or whatever. In this case, it is apparent that a lack of understanding of Boeing procedures, and adoption of their own variations thereof (how many other deviations from standard do they have, in addition to this one? Most (all?) airlines have this insane idea that they know better than Boeing how to operate the airplanes, never mind that Boeing is staffed by experienced people and have the advantage of knowing what works in ALL fleets, not just the tiny sample the particular airline has to work with) led directly to this accident.
Root cause. Fix it and prevent similar accidents forever.

dallas dude

Boofhead said..."Having the brakes come on early, as soon as touchdown occurs, is a good thing under all conditions..."

Boofhead,

I respectfully disagree with your comment. When runway length [and condition] is more than adequate, in my experience, passengers do not always appreciate such high energy retardation.

Also, as turn times become shorter (35mins) brake energy/cooling is also a [secondary] consideration.

dd

boofhead

Follow the manufacturer's guidance. Changing the procedures arbitrarily leads to errors, which lead to accidents.
Use an autobrake setting appropriate to the conditions. Less for dry runways and more for wet/contaminated runways.
It has been suggested that accidents or incidents occur when something happens to distract the crew after touchdown (reverse fault, speedbrake failure, crosswind, traffic etc) causing them to miss starting the braking process until later in the roll out. This might be the one thing that leads to an accident.
You can always cancel the autobrake once the landing is seen to be normal, by pressing the brake pedals or turning the auto brake selector back to a lower setting (or off).
Modern autobrakes are great, giving a steady deceleration which is much better from the passenger point of view than many pilots manage with manual braking, especially if the manual braking is left too late and a big bootfull is required at the last second to make the planned turnoff.
All very smooth, does not upset the passengers, who are more upset by crews that leave the seat belt sign on for hours in smooth conditions, or use full reverse thrust and hard braking to make the earlier turnoff solely because they are in a rush, or chatter unmercifully on the PA when they are trying to sleep, etc etc.
If you, the pilot, vary the procedures given to you by the company and something goes wrong, you carry the can. If the company varies the procedures given to them by the manufacturer and something goes wrong, they carry the can. Unfortunately, both of those cases happen all the time. If we all used standard procedures (ie manufacturer procedures as varied from time to time by input from operators) we would have a much better safety record.

visibility3miles

dallas dude

Is brake cooling as much of a factor when there is an extremely heavy snowfall?

Some passengers reported that they couldn't even see the runway for all the snow after evacuating the plane.

The turn around time may have been more than 35 minutes regardless based on the weather.

dallas dude

v3m,

I was responding to the blanket statement made by boofhead, not the conditions specific to this event.

I regularly fly into airports with 10,000' runways so ,again, I was referencing personal experience, not yours or boofhead's. I wouldn't dream of telling you how to retard your equipment. I don't know what environment you operate in. I do, however, understand mine and accept the valid reason why the mandatory use of autobrakes only applies in certain conditions at my carrier.

My carrier 'inherited' some airplanes without autobrakes installed. I could go into detail but the point is it's not always possible to teach reliance on a system if there are such fundamental differences in equipment within fleet types.

Brake cooling as I previously stated is of secondary importance (and likely not such a problem if it's below zero anyway!).

dd

Airbubba

>>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).<<

Seems like QF1 at BKK was using the wacky "no reverse" policy that was all the rage in Asia at the time. The idea was that brakes are cheaper than engines so some operators were not even cracking the nacelles on landing to save the engines from spooling up. I remember riding SQ into SIN and watching in amazement as we demoed this on a wet (but long and grooved) runway.

>>Having the brakes come on early, as soon as touchdown occurs, is a good thing under all conditions.<<

Boeing seems to think a few seconds of programmed delay is a good idea at some settings in their widebody twins to prevent pitch instability as the spoilers deploy. Of course, many folks become impatient and kick them off before the four seconds is over (it is the longest four seconds of your life on a short runway <g>).

TheShadow

From Belgique's post: The bits in blue (increasing effective weight on wheels via backstick under braking) is something I'd not thought about or been taught. T'would seem quite logical and valid as far as techniques go.

Thoughts?

Airbubba

>>Thoughts?

Kinda like aerodynamic braking, might work in lightplanes, don't believe I'd try it on a modern widebody where tailstrikes are soooo easy...

barit1

In a sense I suppose...

Except instead of going for mainplane lift (Cl + Cd), you are using tailplane lift (negative) to increase the MLG load. As long as the nosegear remains on the deck you don't put the wing to work, and you get whatever nosegear steering is available.

Airbubba

What I meant was it is a technique that may work on smaller aircraft but does not always scale up well to larger ones. The longer widebodies are notorious for wanting to do a tailstand on rollout if you're not careful, particularly with the fuel efficient aft cg that many operators strive to achieve in their loading.

At least that's how it works on my MS Flight Simulator <g>...

UNCTUOUS

Works for me as advertised.

There's no CofG effect as the nose-down pitching moment due to an effective weight-shift forward (due braking) keeps the nose-gear pinned. It's taught in the military - at least it was when I went through advanced flight school.

It effectively increases the MLG footprint and weight on wheels - and so stops the anti-skid from cycling excessively. The end result is much more effective braking - particularly on slippery runways in the wet.

I recall seeing a study on aquaplaning where it was a recommended braking technique.

Airbubba suggests that it's an effect that cannot be scaled up to heavy metal. Don't think so.........

And it's got nought to do with aerodynamic braking because that's all about holding the nose-gear off...... and generating some aerody drag.

Dream Land

Agree with boofhead ,

Where I'm at our company too has adopted a use auto brake for all landings policy, unfortunately I have yet to find a runway turnoff that corresponds with the braking that takes place, LOW is too low and MED is too much too soon for normal dry and wet runways. I am always forced to manually intervene.

D.L

amos2

barit1 & UNCTUOUS have a neat little discussion going on here about,err, I'm not sure what!

Strange don't you think that neither Boeing or Airbus say anything in their manuals about this "new revelation" of aerodynamic theory that they're going on about!

As a matter of fact, I can't remember Douglas, Convair, Fokker, or anyone else for that matter, being aware of this amazing radical landing technique for heavy jets!

Still, we're never too old to learn are we?