At DFW Wednesday morning, inbound AA 777 rolled back 200 ft (really? isn't that a typo?) just before passengers started deplaning. You know where the door isn't now. Sky Talk: American 777 loses door at D/FW
TV news story says wheel chocks were not set, and plane rolled back 200 feet while passengers were disembarking, snapping off the door against the jet bridge. Amazing that no one was hurt. See: Door Flies Off American Airlines Plane
I have had one occasion when the chocks slipped on a greasy ramp after the brakes had been released! Fortunately we noticed as we were conducting post flight checks etc and were able to reapply the brakes before the a/c had rolled a short distance. Luckily there was no damage other than the workload of filing an occurrence report at the end of a very long day!
Even though a wheel or axle may seem to be completely rigid, there is an awful lot of torsional energy stored in one that is braked to a stop -- especially if the final braking was severe. My guess is that this energy was held in by the final braking, and was only released when pressure in the hydraulic system bled away after the engines were shut down. It probably took a couple of minutes before enough force to overcome the static friction of the tires was accumulated, but after it did, the roll began.
OK, I'll bite. I can visualize the torsional energy thing. But when the brakes are released the torsional energy would actually make the aircraft go forward until it is dissipated, not backwards. So either its the rocking out of snow effect (where going forward against an incline makes it easier to go backward etc.) or its more simple - its on a wicked slope and they forgot to chock the airplane. I go for simple.
DGS is used at DFW gates. No guideman required so chocks not installed until ground handlers show up. Parking brake valve malfunction and A/C rolled backwards. I imagine some procedures will change.
My good friend checking out in a DC10 at the same airline as a captain released the handle before the brake and it rolled into the jetbridge. He had been in the right seat so he forgot about how to properly set the brake. SHIT HAPPENS.
A couple of questions for the Euro crews that use the parking systems which I've seen in places like Madrid,
Is there a requirement for a member of the ramp crew to man some sort of a stop signal button as an aircraft approaches the gate? Is there also a requirement to chock an aircraft after parking with the auto system? Is an aircraft allowed to park using the auto system without any ramp personnel being present at all?
OK, I'll bite. I can visualize the torsional energy thing. But when the brakes are released the torsional energy would actually make the aircraft go forward until it is dissipated, not backwards
Yes it would. And the release would have to be sudden to generate an impulse strong enough to set the aircraft rolling. A slow bleed-off of brake pressure and the aircraft would simply ease forward a distance commensurate with the amount the axle was twisted. I would guess mere centimetres.
I thought that all you experts out there would realise that most parking bays are built on an upslope of about 2%.
Why?
It makes it so much easier for the tug to push the aircraft back and it also makes it easier for those aircraft that are capable of doing so to power back by using reverse thrust without using a tug.
This has been discussed many times in the past on PPRuNe (wake up those of you who are sat at the back of the class).
I thought that all you experts out there would realise that most parking bays are built on an upslope of about 2%.
Where? The UK?
Maybe at some brand new well designed terminals in the US, certainly none that I can think of. I think with lowest bidder contracts, we're lucky of the slope is +/-2% in any direction
The slope of the apron / parking bay is generally 1% or slightly less. The minimum we aim for is typically 0.4%. I think 2% is quite excessive; as a comparison, the runway transverse slope maximum is 1.5% for larger airplanes, and that is quite steep. To quote ICAO Annex 14:
Quote:
3.13.4 Recommendation.— Slopes on an apron, including those on an aircraft stand taxilane, should be sufficient to prevent accumulation of water on the surface of the apron but should be kept as level as drainage requirements permit. 3.13.5 Recommendation.— On an aircraft stand the maximum slope should not exceed 1 per cent.
I can understand how the aircraft rolled 200 feet then stopped. That is the depth/length of a typical large aircraft parking bay. The drainage is designed for water flow down to a defined low point or line (such as a drainage channel), and it is quite likely that the parking bay sloped down at 1% and outwards from the terminal to the low end at the end of the bay (which acts as a drainage gully) and then the pavement sloped up again to the next high point such as the centreline of the taxiway. The aircraft stopped at the bottom of the slope.
To get the water away from the low point, the end of the bay is either sloped transversely so the water turns 90 degrees and flows away, or there is a slotted drainage inlet running along the width of the bay. Cheers Overrun
I understood that the parking brake, when set, is backed up with a 3000 psi brake accumulator, and will, subsequent to the securing the relevant hydraulic system, maintain sufficient pressure to hold the brakes sufficiently applied to have these sorts of accidents not occur! I'm talking in the vicinity of 18 hours here!!
