Brazilian investigators have been unable to determine why the crew of the TAM Airbus A320 which fatally overran at Sao Paulo Congonhas, two years ago, landed with only one of its throttles retarded.

The aircraft touched down at 140kt on the short runway 35L, which was wet at the time, with its left-hand throttle at 'idle' but the right-hand lever still in the 'climb' position.

With the A320 in this configuration, the spoilers and autobrake did not activate, and the resulting combination of forward thrust and diminished braking power meant the aircraft failed to stop before the runway end. It crossed a main road and hit a building, killing all 187 on board as well as 12 on the ground.

Brazilian investigation authority CENIPA has put forward two hypotheses for the sequence of events in its final report into the 17 July 2007 crash.

Since investigators could not verify the actual position of the right-hand throttle from examination of the wreckage, CENIPA considered whether the engine had received incorrect signals from a correctly-positioned throttle. But it concludes this was highly unlikely, with odds of a relevant failure during landing put at one in 25 billion flight-hours.

Only the left-hand thrust-reverser was operational and CENIPA's second hypothesis centres on whether the pilot - who had complained of a headache, and was under the pressure of a night landing on a short, wet runway - might have deliberately reverted to an outdated procedure in a bid to reduce slightly the landing distance.

This procedure, says CENIPA, involves reversing one, rather than both, engines and could have focused the pilot's attention on retarding only the left-hand throttle to 'reverse' - causing him to overlook the need for the right-hand lever to be at 'idle'.

CENIPA considers this possibility "undetermined" because of the "impossibility" of proving that it contributed to the TAM accident. But it says the scenario is consistent with the aircraft's behaviour, as registered by the flight-data recorder.

As the jet's nose-gear touched down, 2.5s after the main gear, the left-hand throttle was put into 'reverse', disconnecting the autothrust but locking the right-hand engine pressure ratio at 1.18 - where it stayed because the pilots did not touch the throttles again.

Six seconds after main-gear touchdown the crew started pedal-braking but the aircraft, drifting to the left, did not stop and was destroyed in the subsequent overrun and building collision.

While calculations indicate that the aircraft had sufficient stopping distance available, even without the use of reverse thrust, CENIPA says the crew demonstrated anxiety over the conditions at Congonhas. This concern might have led them to reach the wrong conclusion as the aircraft ran into trouble on the roll-out.

"Without the correct understanding of the behaviour of the aircraft, they may have been led to believe that the aircraft was hydroplaning," says the report, adding that the delay in pedal-braking could have been due to a fear of losing control.

Both pilots were captains, but the co-pilot had relatively little experience on the A320. CENIPA says the loss of situational awareness, combined with a lack of warning over the throttle position during the roll-out, "would have prevented adoption of any corrective action" to prevent the crash.

Source: Air Transport Intelligence news