THE pilots of a Russian airliner that crashed and killed 71 people failed to switch on a heating unit — leading speed sensors to ice up and provide wrong information, investigators said on Tuesday.

According to the New York Post, the Russian Interstate Aviation Committee said a factor in the crash “could be the wrong data about flight speed on pilots’ indicators”.

“[This] was likely due to iced Pitot tubes (speed probes) while their heating systems were shut off.”

“During all the other flights registered on the flight recorder [15 more flights], the heating of the total pressure probes was switched on before takeoff in its line-up position,” the IAC said, according to the Russian TASS news agency.

Iced-over Pitot tubes were cited as the likely reason behind the tragedy of an Air France flight that crashed into the Atlantic in 2009, killing 228 people on board.

The Saratov Airlines Antonov An-148 took off on Sunday from Moscow’s Domodedovo airport for a flight to the city of Orsk and went down in a field about 65 kilometres southeast of the capital.

All 65 passengers and six crew members aboard perished.

“A special situation began to develop about two minutes and 30 seconds after the takeoff at an altitude of around 1300 metres and the indicator speed of 465-470 km/h,” the committee said.

The flight recorders captured varying data from the plane’s two airspeed sensors — a result of the pilots’ failure to turn on the heating unit for the pressure measurement equipment prior to takeoff, the committee said.

The crew turned on the autopilot after taking off but took manual controls back when they saw the conflicting speed data, with one indicator showing the speed at 0 kmh and the other about 550 kmh.

The pilots performed a series of manoeuvres and eventually took the plane into a dive at 30 to 35 degrees before it plummeted into a snowy field six minutes after takeoff.

Before the plane slammed into the ground, one of the sensors continued to show a speed of 0 kmh while the other showed a speed of 800 kmh, according to TASS.

“Forty-five seconds before hitting the ground, the plane developed a right-side roll of 25 degrees,” according to the IAC, which said it has completed an analysis of the flight recorder and still needed to analyse the voice recorder that captured conversations in the cockpit.

It also planned to look at whether the Pitot tubes — vital pieces of equipment that measure airspeed — could have malfunctioned.

A criminal case on charges of violations of flight safety and aircraft operation rules entailing the death of two or more people through negligence has been opened.

The RBK newspaper reported on Tuesday that the captain refused the de-icing procedure before takeoff. The Kommersant newspaper previously reported that the de-icing was optional given the relatively mild temperatures.