Russian investigators have disclosed that seven instances of unreliable airspeed indications involving Sukhoi Superjet 100s were recorded over a two-day period of heavy snowfall at Moscow Sheremetyevo.

Federal air transport regulator Rosaviatsia says the incidents were each characterised by "different speed readings" between the captain's and the first officer's side, either during the take-off run or the initial climb.

In all the cases – which occurred over 4-5 February – the crews opted to abort the take-off or return to the airport. The incidents occurred over 4-5 February, and are among several safety events detailed by the regulator over the period.

Preliminary information, says Rosaviatsia, indicates that inspection of the affected Superjets detected ice deposits on the fuselage "in front of" the air-pressure sensors.

Investigators suspect sensor icing, as a result of inactive heaters, contributed to unreliable speed readings on the Antonov An-148 which crashed after departing Moscow Domodedovo on 11 February.

Rosaviatsia says the icing on the Superjets could have been caused by the freezing, in heavy snow, of water trickling down from the crown of the fuselage and the cockpit windows.

It points out that the type has been involved in 14 incidents arising from speed-reading discrepancies since 2011 – three of which were related to water entering the pitot-static system supplying air-pressure data.

But Sukhoi has defended the Superjet's performance, pointing out that the incidents outlined by Rosaviatsia occurred during the "severest weather conditions" and stresses that the meteorological situation affected other aircraft types.

Weather data for Sheremetyevo over 4-5 February indicates constant snowfall, and temperatures falling to around minus 12C.

Sukhoi says the aircraft's airspeed warning systems in the Sheremetyevo incidents functioned as designed, activating "at the early stage" of taxiing or take-off, enabling the flights to be aborted.

While Rosaviatsia attributes one speed-reading incident to the crew's not using heating equipment, Sukhoi insists that heating of the pitot-static system activates automatically upon engine start.

"Owing to the aircraft's construction, full loss of speed indication for a long period on the [Superjet 100] is impossible," it claims.

"Certification tests carried out proved that the [Superjet's] systems successfully cope with icing."

Source: Cirium Dashboard