Pilots of the Aeroflot Sukhoi Superjet 100 involved in the fatal landing accident at Moscow Sheremetyevo did not request active measures to avoid a thunderstorm zone before the aircraft was struck by lightning.

Bound for Murmansk as SU1492 on 5 May, the twinjet had taken off to the south-west from Sheremetyevo's runway 24C and was tracking the KN 24E standard departure. This departure involves initially following a heading of 268° before a long right turn to head north-east.

Cockpit-voice recorder information shows the pilots had discussed "flashes" around the airport during pre-flight checks, and again – enough to provoke an expletive from the crew – as the aircraft lined up for take-off.

Moscow Vnukovo weather radar indicates that, as the aircraft lifted off, it was heading towards a thunderstorm region which was moving from the south-west to the north-east at around 40-45km/h.

Aeroflot's operations manual states that, in the event of towering cumulonimbus clouds around the departure airport, crews must use the weather radar to determine ways to avoid them, and co-ordinate avoidance with air traffic control if approaching a storm.

The Superjet was fitted with a Honeywell RDR-4000 weather radar. Its manufacturer recommends planning thunderstorm avoidance at least 40nm away from the storm cell, preferably by evasion on the upwind side, and to fly around 'red' and 'amber' zones at a distance of 20nm minimum.

At least three aircraft preceding SU1492, as well as 10 behind it, had requested active storm avoidance clearance. Most of these services were operated by Aeroflot.

While Russia's Interstate Aviation Committee says the Superjet crew "did not" request such measures, it points out that, shortly after the aircraft was cleared to 7,000ft, its selected heading was set to 327°.

This selection caused the aircraft to initiate the right turn – taking it away from the storm – earlier than prescribed by the KN 24E departure pattern.

The crew contacted approach control and was instructed to climb to 9,000ft, at which point they appeared to refer to the proximity of the storm.

"We're going to get shaken," the captain stated, but then reassured the first officer by adding, "Nothing to worry about."

But some 30s later, just after the approach controller cleared the flight to 11,000ft, the cockpit-voice recorder captured a sound effect lasting 1.5s, before the autopilot disengaged and an audio alert warned that the aircraft had dropped into direct law. There was also a temporary loss of radio communication.

"Wow!" exclaimed one of the pilots.

Investigators state that "most probably" an "atmospheric electricity impact" affected the aircraft. The Superjet had been airborne for about 5min at the time, and was banking 20° to the right and climbing through 8,900ft.

Subsequent examination of the airframe revealed evidence of lightning strike damage to various parts including the right-hand angle-of-attack sensor, right-hand ice detector, temperature probe, and upper sections of the fuselage.

Investigators list 16 prior instances of lightning strikes to Superjet 100s, not all of them to different aircraft. Most did not result in significant damage, although there were two instances requiring replacement of VHF antennas and one which led to a nose-cone change.

As result of the strike to flight SU1492, and its effect on the aircraft's systems, the crew opted to turn back to Sheremetyevo. The captain had stressed to the purser that there was "no emergency – we are simply going back". The aircraft, however, touched down heavily upon landing and caught fire, resulting in 41 fatalities.

Source: Cirium Dashboard