Russian investigators have disclosed that the Sukhoi Superjet 100 involved in a fatal accident at Moscow Sheremetyevo was 1.6t over its maximum landing weight, and experienced two impacts in excess of 5g as it bounced on landing.
The Interstate Aviation Committee says it has completed an initial analysis of information from the flight-data recorder retrieved from the Aeroflot jet after the 5 May event.
Investigators have revealed that the crew received windshear warnings on approach and that the aircraft experienced pitch fluctuations just before the fatal touchdown.
Federal air transport regulator Rosaviatsia, in a detailed outline of the flight, states that the aircraft suffered an electrical failure at 8,900ft – about 5min after take-off from runway 24C – while following the KN 24E standard departure pattern for a service to Murmansk.
The aircraft’s autopilot disengaged and the aircraft’s flight control system dropped into direct law.
Rosaviatsia does not specifically state that the aircraft was struck by lightning, but it does point out that the aircraft was flying within a “zone of thunderstorm activity”.
The flight recorder registered disengagement of the autothrottle, and Rosaviatsia says the captain manually controlled the aircraft for the remainder of the flight.
Unable to communicate on the approach frequency, the crew restored VHF radio links using the emergency frequency 121.5MHz, and was vectored back to Sheremetyevo while transmitting the squawk code ’7600’ for loss of communication.
The aircraft conducted an ILS approach, in manual mode, to runway 24L.
Rosaviatsia says the aircraft had departed with a take-off weight of just over 43.5t and that its weight upon entry to the glideslope was 42.6t – which, it says, exceeded the maximum landing weight by 1.6t.
As required for the overweight landing, and the direct-law control, the flaps were set to 25°. The crew also upgraded the squawk code to the emergency setting ‘7700’.
The aircraft remained largely stable on the approach – performed in a crosswind from the left of up to 30kt – with an airspeed of 155-160kt.
As the Superjet descended through 1,100-900ft above ground, the crew received five predictive windshear “go around” warnings.
The aircraft began to dip below the glideslope at about 260ft and, at 180ft, a glideslope alert sounded.
Thrust was subsequently increased, with the throttle levers alternately advanced and retarded between 18° and 24° as the aircraft descended to 40ft. This resulted in the airspeed increasing to 164kt as it crossed the threshold and 170kt at 16ft from touchdown.
As the captain retarded the throttle to idle, says Rosaviatsia, he made several alternating inputs to the side-stick with “large amplitudes” – up to the maximum – which resulted in the pitch varying between 6° nose-up and 2° nose-down.
While the aircraft had appeared close to touchdown at about 700m from the threshold, Rosaviatsia says the first three-point contact with the runway occurred at 900m from the threshold at 158kt, when the aircraft experienced an impact of more than 2.5g, and bounced to about 6ft.
Rosaviatsia says the aircraft’s spoilers did not deploy automatically. Aeroflot stresses that its procedures do not require the manual deployment of spoilers until thrust-reverse is activated and the aircraft is settled and stable on the runway.
“In the absence of a stable course the release of the spoilers was impossible,” the carrier adds.
Having bounced, the aircraft touched down 2s later on its nose-gear at 155kt, with a heavy impact of 5.85g, causing the Superjet to bounce a second time, to a height of 18ft. The third, and final, impact occurred at 140kt – reaching at least 5g – and was immediately followed by damage to the aircraft’s structure, a fuel spill and fire.
As the aircraft decelerated through 100kt, sliding along the runway, a fire alarm was triggered in the aft baggage and cargo compartment, followed by a fire alarm in the auxiliary power unit 16s later. The aircraft’s PowerJet SaM146 engines continued operating until the end of the flight-data recorder trace just after 18:31.
Rosaviatsia says the captain had logged 1,570h on type out of a total of 6,844h while the first officer had 623h on type.
The aircraft (RA-89098) had accumulated 2,710h over the course of 1,658 cycles.
Rosaviatsia says the fatalities comprised 40 of the 73 passengers and one of the five crew members, while six passengers and three crew were injured.
Aeroflot stresses that the preliminary information disclosed by Rosaviatsia does not reference errors by the crew or any violation of procedures, and that final conclusions have yet to be released by the investigating authorities.