Newly-released cockpit voice-recorder transcripts from the crashed Polish presidential Tupolev Tu-154M show that the terrain-awareness system sounded a 'pull up' warning eight times before the jet collided with trees and disintegrated.
The first warning at 10:40:42, about 20s before the accident, came almost immediately after the crew had called the aircraft's height as 100m. This also happened to be the decision height for the approach, which was being attempted in dense fog.
Transcripts from the accident also show that, although the aircraft was descending, the height was still being called as 100m seven seconds later at 10:40:49. This could indicate that the reading was not the aircraft's height above the runway, but rather the immediate height above terrain as given by the radio altimeter.
Radio altimeter readings provide a 'snapshot' height above ground and can give a false impression of clearance margins in regions where terrain rises sharply ahead of the aircraft. The approach to Smolensk taken by the Tu-154 features a rising valley wall about 1km from the runway threshold.
The transcript reveals that the Tu-154 continued to descend through the 100m decision height, the navigator calling 10m intervals, with no apparent comment from the crew until at 80m the co-pilot said, "Odchodzimy" - or "We're leaving" - which may have been a reference to a missed approach. There is no subsequent evidence, however, that the crew opted to abort the landing.
At about 60m an 'unsafe height' warning sounded. The aircraft continued to descend, despite the continuing 'pull up' instruction from the terrain-awareness system, and a caution from the air traffic controller.
The transcript indicates the reduction in height from 100m to 20m took just 6.5s, equating to a descent rate of some 740m/min (2,400ft/min). This relatively high closure rate suggests that the clearance margin was rapidly being eroded by rising ground.
Just after the 20m height call from the navigator, the cockpit recorder picked up sounds from the aircraft's automatic on-board control computer as well as a middle-marker beacon signal. The middle-marker's location typically coincides with the point at which an approaching aircraft would normally reach decision height.
At 10:40:59 came the sound of a collision with trees, followed by the eighth and final 'pull up' instruction, and an order from air traffic control to execute a go-around.
The aircraft lost its outer left wing in the collision, rolled inverted to the left, and was completely destroyed.
None of the occupants, including Polish president Lech Kaczynski, survived the 10 April accident.
Source: Air Transport Intelligence news
