Investigators believe the crew of a crashed Antonov An-148 did not carry out a crucial checklist which should have included a deferred confirmation that the pitot-static heating system was active.
The Saratov Airlines aircraft came down a few minutes after departure from Moscow Domodedovo on 11 February. None of the 71 occupants survived the accident.
Federal air transport regulator Rosaviatsia has disclosed that the crew contacted apron control at 14:00 for permission to start engines.
The crew performed the 'before engine start' checklist and – after starting the engines at 14:02 – carried out the 'before taxi' checklist and commenced taxiing at 14:07.
Domdedovo tower cleared the An-148 to continue taxiing to runway 14R, and the pilots turned their attention to two further checklists, including that for line-up.
The line-up checklist includes an item to check the activation of the pitot-static heating system.
But Rosaviatsia says that, according to the carrier's flight operations manual, this item is deferred to the 'before take-off' checks.
Flight-data recording analysis shows that, before take-off, the aircraft's integrated information system indicated "no heating" on any of the three pitot-static sensors.
There was no take-off prohibition warning on the system.
"Almost immediately after taxiing the aircraft to the runway, the crew received take-off clearance," says Rosaviatsia.
It says that the 'before take-off' checklist was "not performed", suggesting that the pilots did not confirm whether the pitot-heating was active. The crew instead acknowledged the clearance and commenced the take-off roll.
The aircraft lifted off at about 14:21 and the crew engaged the autopilot at 130-150m, and completed flap retraction at 550m.
Some 2min 30s into the flight the aircraft began to display unreliable airspeed indications, and the crew eventually lost control of the jet before it dived into a field about 6min after take-off.