737 Question on emergency retraction of landing gear on ground

[A37575]

Hypothetical Scenario.

For whatever reason, assume a B737 has total brake failure on landing leaving only reverse thrust to slow the aircraft down. The decision is made to retract the landing gear to avoid a disastrous over-run.

Would the landing gear retract on the ground by using the landing gear lever over-ride trigger? Or would you need to pull the air/ground switch circuit breaker to permit the landing gear over-ride switch to do its job? I realise the landing gear over-ride trigger is to cater for a case where the gear does not retract after take off.

[STBYRUD]

The override trigger will be enough, but all that will happen is that the nose gear retracts and you get a nasty nose slide. The override trigger bypasses the lock that usually prevents the lever from being selected up by means of a solenoid as long as there is weight on wheels. The main gears won't budge most likely because of the excessive sideways friction, but the nose gear will probably move up. Just one question here from me: Why ? ;)

[Checkboard]

In this situation you would be better to simply steer onto the grass off the runway once the reverse has done most of the braking, then if necessary (if the ground is so hard that the wheels don't break through the surface, slowing the aircraft) ground-loop the plane. The energy absorbed by tearing off the gear increases the crash survivability, and keeping the aircraft on the airport maintains RFFS access.

[lomapaseo]

For whatever reason, assume a B737 has total brake failure on landing leaving only reverse thrust to slow the aircraft down. The decision is made to retract the landing gear to avoid a disastrous over-run.



I'm a little concerned about "out of the frying pan and into the fire" postulation here.

First there haven't been a whole lot experiences with large transport landings with landing gear up.

Second, whose to say that the braking differences between sliding on your belly across the grass is different than ploughing through the grass on your gear. Of course this presumes that there is a modicum of unpaved overrun area rather than a cliff.

Third you may be more likely to slide off the runway heading with a gear up (probably can't steer around an obstacle)

I suppose that there is an optimum speed vs remaining runway where a gear up might be postulated to be better, but who's to make that decision?

[Starbear]

The main reason the main gear won't retract in this scenario (in additon to side friction forces) is that the gear leg passes through the vertical to overcentre during extension. Therefore in reverse the gear and hydraulics would have to be able to "raise" the whole aeroplane a couple of inches as it scribes that arc in reverse.

As for the nose gear, same principle in terms of arc described but possibly because of the lever arm between it and the main gear it might just retract but I don't know.

The point of this design of course is to try to keep gear in correct postion if downlock should fail.

[FlightPathOBN]

first the gear, then the engines....lots of stuff in the way before you get to the belly....

[lomapaseo]

first the gear, then the engines....lots of stuff in the way before you get to the belly....


Yes, the engines or fixed landing gear. Once you yaw significantly you tend to shear a wing mounted engine off while sliding and then a spray of fuel on the trail of sparks. It's just a matter of the odds how controllable your slide out is.

[A37575]

but the nose gear will probably move up. Just one question here from me: Why ?

Why? Many years ago a 727 was on final to a Pacific atoll. Due to erroneous VASIS light signals (vandalism) it came in too high on a black hole approach and despite max braking on the 5600 ft length runway, the pilot feared the aircraft may not stop by the end of the runway. There was no suspicion that the VASIS was other than serviceable until too late. There was no prepared overrun and the hazard was a boulder strewn cliff 10 metres from the departure threshold. For a fleeting second the captain considered retracting the landing gear since he estimated max braking wasn't going to stop the aircraft. However the 727 was able to stop with the nosewheel right on the extreme end of the runway.

The incident brought up the technical issues involved with pulling the gear up using the over-ride trigger on the landing gear lever. Pprune is the best place to ask around as there are very experienced and knowledgeable people that read Pprune who do not necessarily contribute unless it is in their area of expertese. The various answers were most instructive especially the points about only the nose-wheel retracting by use of the landing gear over-ride switch. Thank you all, for your replies.

[Tee Emm]

For whatever reason, assume a B737 has total brake failure on landing leaving only reverse thrust to slow the aircraft down.

We practice total brake failure in the simulator. It is worthwhile keeping in mind if that you manage to stop right at the end of the runway by using full reverse, that cancelling reverse quickly will cause the engines to develop significant forward thrust and the aircraft could move forward again.

To prevent this happening, consideration should be given to cutting the start levers while still in in full reverse, the moment the aircraft stops under full reverse. That way any forward thrust vanishes and the aircraft hopefully will not move forward

[itsresidualmate]

Us engineers usually have call to power up the hydraulics and select gear up on the ramp after a brake change, many aircraft types make use of 'gear up' hyd pressure to apply wheel retraction brakes and we need to do a hydraulic leak check. The gear pins are fitted and these will prevent retraction but on a few occasions they've been left out; result is the nose gear collapses but the mains usually can't lift the aircraft. No matter how quick you are to put the lever 'down' again, the nose hyds can't lift the plane back up!
I believe BA at Cardiff had a partial retraction of the body mains on a 747 when leaving the hangar, but as far as I remember the wing mains didn't move.

[A37575]

Can 737 brakes fail with serviceable hydraulics.

If the simulator instructor panel has a selection for loss of all brakes left and right, then presumably the certification of the simulator required this function. It is doubtful if this function would have been part of the simulator package if the technical possibility had not been considered by the aircraft manufacturer?

[safetypee]

Whilst there could be some rare combination of failures which results in a total brake failure, it should not be assumed that this is necessarily part of the certificated simulation package (#12).
In the few simulator certifications in which I participated, the required list of aircraft / sim matches was surprisingly small and limited in scope, and often the accuracy of the match was poor.
It is more likely that human ingenuity has found a mechanism to simulate a failure, which perhaps is being used without sufficient thought to the realism or probability of the scenario.

Reviewing accident statistics it would be of greater benefit to consider an impending runway overrun, but there again the arguments predominantly against drastic action, as above, would apply. But as with many ‘accident’ scenarios the contributions to an overrun can and should be identified before the event – before landing; isn’t there something similar for brake (hydraulic) failure – a system warning.

An overrun situation might have grater benefit for safety training, but if the objective was to get the crew to think, then consider a pre landing decision where the brakes / hydraulics were suspect and a ‘total’ failure might occur (‘might’ as in probability, likelihood, and system knowledge), the form of discussion / consideration in this thread.

[davionics]

One example that comes to mind is the Gimli Glider, crash landing of a fuel starved 767; the collapsed nose wheel helped slow it down.

Gimli Glider - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Gimli_Glider#Landing_at_Gimli)

[sevenstrokeroll]

one thing to remember is that in this unlikely scenario, one might have sufficent OOMPH to get the plane an inch off the ground and then try to retract the gear.

but please...the way I was taught to fly was simply this...hit your spot, on target speed and try to be in an energy state to pretty much coast to a stop.
now I know jets have residual thrust at idle and if you were not able to deploy reversers, one plane I know states in the manual to shut down one engine on the runway to minimize residual idle thrust (nutty, I know)....while keeping one engine for hyd.

I have heard the scenario of retracting the gear as a desperate stop method for another type of plane...but please, just don't get yourself into a situation that you have to try it.