Russian investigators have concluded that a KD-Avia Boeing 737-300 crew accidentally landed with the gear retracted after being distracted by an apparent asymmetric flap problem which, in reality, did not exist.

The aircraft had been preparing to make a night approach to Kaliningrad in poor weather last October, when an attempt to extend the flaps from 2° to 5° resulted in a mismatch indication, suggesting the left-hand flaps were stuck in the 2° position.

Russia's Interstate Aviation Committee (MAK) says its analysis of the aircraft's motion points to there being "no real disagreement", and that examination by Boeing pointed to a probable flap sensor failure from moisture, producing an incorrect indication to the crew.

Crucially, during the attempt to resolve the apparent flap mismatch, the co-pilot activated the 'flap inhibit' and 'gear inhibit' switches in the cockpit. This "incorrect" action, says MAK, effectively disconnected the ground-proximity warning system.

With the flap situation unresolved the pilots opted to leave them at the 2° setting and land at a higher speed. Combined with the darkness and heavy rain, says MAK, this approach required increased attention and created a "supplementary psychological burden".

KD-Avia 737 prang
 © Interstate Aviation Committee

The final critical error came about two minutes before touchdown when a landing-gear warning alarm was quickly cancelled by the crew. MAK believes this was done "automatically" and "without analysing the situation", the result of a "negative stereotype" formed by this alarm routinely being silenced during landing.

Neither pilot realised that the landing-gear had not been lowered, and the earlier flicking of the 'gear inhibit' switch meant the ground-proximity warning alarm did not sound.

The 737 touched down at 158kt and travelled 1,440m (4,720ft) along the runway before coming to rest. It remained on the runway and there was no fire. None of the 138 passengers and six crew was injured.

Included in MAK's final report into the 1 October 2008 accident is the observation that KD-Avia's fleet of 17 Boeing 737s featured three different cockpit layouts, containing different numbers of switches, for the 'gear inhibit' and 'flap inhibit' functions, but all carried identical quick-reference handbooks.

Source: Air Transport Intelligence news