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.

Source: Air Transport Intelligence news