BA 747 Crew commended
 

BA 747 crew commended for escaping near-stall on take-off
By David Kaminski-Morrow


South Africa's Civil Aviation Authority has praised the airmanship of British Airways Boeing 747-400 pilots who battled to prevent a low-altitude stall after the leading-edge slats unexpectedly retracted during lift-off from Johannesburg.
At 167kt on the take-off roll, fractionally below rotation speed, all the leading-edge slats inboard of the engines on each side automatically retracted, after receiving a spurious indication of thrust-reverser activation.
As the aircraft tried to climb out from Tambo International Airport, known for its 'hot and high' environment, the jet lost a "significant amount of lift", says the CAA, and the stick-shaker immediately engaged, warning of an approaching stall.
Instead of following the typical climb profile, the first officer - whose aerobatic experience meant he was familiar with buffet - controlled the aircraft through the stall warning and buffeting by executing a shallower climb, while the commander supported the manoeuvre by calling out heights above ground.
The slats stayed retracted for a total of 23s. They started to redeploy 7s after the jet became airborne - as the undercarriage was retracting, at a height of 56ft - and were fully extended 9s later. The stick-shaker, which had activated intermittently over a 15s interval, stopped as the airspeed rose to 186kt.
In its inquiry report into the 11 May 2009 incident, the CAA says the crew had "no notion" that the slats had retracted before rotation. There is no separate indication in the cockpit for leading-edge slat position.
"The flying crew should be commended for the professional way that they controlled the aircraft during a critical stage during take-off," it adds. "During [the incident] the flight-deck crew had no indication or understanding of what had caused the lack in performance of the aircraft."
After stabilising the 747's climb, the crew declared to air traffic control that they were experiencing problems with two engines and would be returning to the airport. The aircraft, which had been bound for London Heathrow with 265 passengers and 18 crew members, landed safely.
Investigators have concluded that, during the take-off roll, the slats retracted - as designed - in response to signals indicating deployment of thrust reversers on the two inboard Rolls-Royce RB211 engines. The right-hand reverser signal was triggered at 125kt and the left-hand at 160kt.
But neither reverser had been activated, and British Airways engineers examined the aircraft (G-BYGA) to trace the source of the false signals. The inquiry concluded that, although the reversers were stowed, their translating cowls were nevertheless seated relatively far rearwards.
As the 747's engines wound up to high power, and the aircraft accelerated, sensors monitoring the cowl positions transmitted incorrect 'reverser' signals. The slats retracted because of a logic process designed to prevent them being struck by efflux air from activated reversers.
Boeing subsequently developed a safety bulletin for Rolls-Royce-powered 747-400s to disable this reverser-based automated stowing.


SF

Good job. :DThat's why we put "pilots" in the pointy end. thank god they weren't button pushers.

Yeah - Antonov, tell us again about pilotless aircraft....

I thought that the auto-restow thing on the thrust reversers was necessary to prevent in-flight deployment. So I wonder how they did the risk balance of robbing peter to pay paul:confused:

At risk of sounding vastly ignorant, might a design strategy for such a capability, thrust reversers, tie in to a weight-on-wheels switch?

I guess the risk there is the circuit goes bad, and you don't get them when you need them. One could design the circuit so that unless powered, it can't turn thrust reverser function off ... where are the holes in this idea?

For one, which wheel? :)

Am I right in guessing that an auto-thrust reverse function is very handy in some tight landing situations?

Wake up at the back! It is of the leading-edge flaps.

I know of one fatal accident where the SOP for crew was to pre-select reverse on finals - KNOWING, FOR SURE - that the weight switch would prevent reverse thrust until touchdown.

Guess what ?

But as just stated - we're not talking about use of reverse in this incident.

My understanding of the incident was that the sensor on the thrust reversers caused the slats to retract. This has nothing to do with an interlock preventing the thrust reversers from deploying in flight, but instead would be related to braking efficiency once on the ground.

The technical aspects were discussed on this thread;

http://www.pprune.org/rumours-news/3...-incident.html

Dave

Old Piedmont crews used this unauthorized 'technique' on the classic 737's, weight on wheels would pop the buckets at touchdown to give best short field perfomance at places like Roanoke, Virginia.

Along came the 737-300 and 10 feet radar altitude was used to arm the reversers, inevitably a few very hard landings occured as the reversers deployed prior to touchdown.

why?

why is the slat system hooked to the thrust reversers in any sense at all? I haven't flown the 747400.

The last jet I flew...if you wanted thrust reverse with slats deployed, you got it.

In the BA incident, the autostow feature of the slats was triggered by false signals that the reversers were deployed. It is a little wacky and you would think this automatic leading edge retraction would be inhibited in the air. I suppose it gives the 744 better landing distance numbers somehow.

The claim that aerobatic experience helped with recovery from the improper leading edge configuration is remniscent of Captain 'Hoot' Gibson's high altitude TWA 841 upset years ago:

http://en.wikipedia.org/wiki/TWA_Flight_841_(1979)

Hoot said at the time that his aerobatic training allowed him to regain control of the aircraft.

protectthehornet

The inboard and midspan leading edge flaps retract automatically when on the ground with reverse thrust deployed, so as to prevent the reverse thrust air damaging the flaps. It is inhibited in the air, which is why they redeployed when airborne.

http://www.caa.co.za/resource%20cent.../2009/0717.pdf

Dave

Why were the false signals generated on two engines? As this is a rare occurrence for one engine, to have two in fault seems very curious.

What, if anything, occurred between landing and takeoff that might have contributed?

Sir George Cayley

I know this will generate a lot of flack but these guys certainly the same type of medals and props as the BA038 chaps...

you mean they did not look at the QRH and actually thought for themselves and used excessive airmanship...bah sack'em;)
and a computer controlled [pilotless] plane would simply say "not in data bank does not compute...Crash"


:D

:hmm:
Had Capt. Hoot not been one to play around with trying to deploy trailing edge flaps (by pulling circuit breakers and repositioning the flap handle) to gain more altitude capability at the existing gross weight (which, by the way, was beyond the authorizations in the manual), he wouldn’t have had to rely on his aerobatic experience to save his backside and that of the rest of the airplane's occupants!
:*

Sir George

See 1.16.2 in the report.

Dave

Rigged by the same engineer, that's why.

My thoughts exactly.