Crashed An-148 pitot-static sensors left unheated
Preliminary analysis of flight-data recordings from the crashed Saratov Airlines Antonov An-148 indicate that the heating system for the jet’s pitot-static pressure sensors was not operating at any point.
As the aircraft climbed out of Moscow Domodedovo its crew, presented with unreliable airspeed information, turned off the autopilot and attempted to fly manually before the jet dived into a field near the village of Argunovo.
Data from 16 flights has been downloaded from the recorder, and the Interstate Aviation Committee says that the heating system had been switched on before take-off for all 15 flights prior to the ill-fated service to Orsk on 11 February.
Investigators state that the aircraft took off at 14:21 local time. At a height of 130-150m (430-490ft) the autopilot was engaged, and the flaps were retracted at 550m.
Some 2min 30s into the flight – as the aircraft reached 1,300m – differences in the airspeed readings, between the captain’s air data system and that of the back-up system, began to appear at about 250kt. The recording did not include information from the first officer’s system.
There were no significant differences in the displayed altitudes from the systems.
After about 25s the captain’s system was reading some 16kt higher than the back-up, and the crew received a comparison alert lasting 10s.
As the aircraft reached 2,000m there was another comparison alert – but in this instance the back-up system showed the higher speed, which was increasing, while the captain’s was declining.
In response, the crew disengaged the autopilot and flew the jet manually.
The inquiry says the captain’s speed reading continued to deteriorate until it reached zero, 34s after the autopilot was switched off, while the reading from the back-up showed 290-300kt.
For some 50s after the autopilot was disengaged the aircraft flew at heights varying from 1,700-1,900m and experienced changes in vertical loading.
The captain’s speed reading remained at zero and that of the back-up system started to fall sharply, and the aircraft began to pitch some 30-35° nose-down.
As the An-148 dived its back-up speed reading increased rapidly, reaching some 430kt, before the aircraft struck the ground just after 14:27, having been airborne for only around 6min. The aircraft was 30° nose-down at impact and, in the final seconds of flight, had entered a 25° right bank. The speed reading on the captain’s system had remained at zero.
The inquiry says that this preliminary recorded information – and analysis of previous similar events – points to incorrect data on the crew’s airspeed indicators, apparently the result of pitot-static sensor icing.
Investigators have yet to determine why the pitot-static heating system was not active. The Interstate Aviation Committee says the inquiry will look at the cockpit-voice recording to understand the actions of the pilots, and will study the heating system for possible failure.