A330 crash probe struggles to explain premature descent

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Investigators have traced the loss of an Afriqiyah Airways Airbus A330-200 at Tripoli to the aircraft's premature descent while preparing to approach runway 09.

But the inquiry struggled to determine the precise reason why the pilots failed to follow the correct descent profile for the non-precision approach.

Libya's Civil Aviation Authority, in its inquiry into the 12 May 2010 crash, notes that the same crew made a virtually identical error two weeks earlier.

The approach required the aircraft to descend to 1,350ft until reaching a locator, designated TW, situated 3.9nm before the runway threshold. This would then have allowed the jet to follow a normal 3° glideslope to touchdown.

But the inquiry found that the first officer, having levelled the aircraft at 1,400ft, selected flightpath-angle mode on the instrument panel while still 1.3nm from the TW locator.

Investigators concede that "it is not possible to determine with certainty" the trigger for this decision. But the inquiry suggests he might have misinterpreted a preceding remark from the captain as a change in strategy regarding the selected guidance mode.

The first officer dialled in a 3° glideslope and the aircraft commenced its descent, passing over the TW marker at 1,020ft - some 330ft below the normal profile.

Libya's inquiry notes that the top of the final descent started 5.2nm from the runway threshold, a distance which coincides with the distance of 5.2nm between the TW locator and the Tripoli VOR/DME beacon.

It theorises that the first officer could have erroneously interpreted the DME distance as an indication that the aircraft was passing over the TW marker - particularly if this 5.2nm figure was memorised by Tripoli-based crews.

But the inquiry also considers whether the first officer might have deliberately set the 3° glideslope ahead of the marker but "unintentionally" engaged the flightpath-angle mode, leading the aircraft to descend early.

The premature descent spurred a poorly-executed attempt to execute a go-around, and the aircraft crashed short of the runway threshold.

Fourteen days before the crash the same crew flew an identical approach, in the same aircraft, and also started the final descent from 1,400ft before reaching the TW marker.

During this incident the descent started 2nm ahead of the marker and it was overflown at 1,050ft.

"Overall the approach was never stabilised," says the inquiry. The crew executed a go-around some 70ft above the minimum descent altitude, but it was also poorly conducted, with the A330 initially ascending, then descending, and dual inputs being recorded from the captain's and first officer's sidestick controls.