May 10, 2019, ©. Leeham News: An Aeroflot Sukhoi Superjet 100 crash-landed Sunday at Moscow Sheremetyevo Airport and burst into flames.
We shall look closer at the likely cause of the accident, which involves the SSJ100 Fly-By-Wire (FBW) control system working in Direct law.
The flight, Aeroflot 1492, started from Sheremetyevo for a flight of two hours to Murmansk, 800nm away. During climb out the aircraft was hit by lightning at 6,900ft and leveled off at FL100.
Normally an airliner recovers after a short hiccup after a lightning strike, but this time the aircraft’s radios and autopilot would not function after the strike. The crew, therefore, turned back to land on the runway they departed, Runway 24 at Sheremetyevo airport.
As the crew had intermittent radio contact with ATC on the emergency frequency 121.5 MHz (probably over the standby radio) they decided against long time circling over Moscow to dump fuel due to the heavy traffic in the area.
Instead, they prepared to land with an aircraft which was over Max Landing Weight but not more than a couple of tonnes. In normal landings, this creates no problems on runways the length of Sheremetyevo’s runways (shortest runway of three is 10,500ft, longest 11,650ft).
The lightning strike had put the FBW in its backup mode, Direct law. This is an emergency mode which bypasses the aircraft’s flight law computers and controls the aircraft’s control surfaces directly. One can say the aircraft is controlled by electrical wire instead of steel wire.
A FBW aircraft like the SSJ100 has a sophisticated digital feedback based FBW. This means the Pilot commands the aircraft movements he wants by sidestick and pedals. The FBW computers then move the aircraft’s control surfaces to achieve the commanded movement.
If the Pilot commands a pitch up to flare during landing, the aircraft pitches up at an adapted rate for the flare situation which is the same regardless of center of gravity or flap setting. The aircraft is nice and predictable to fly.
When such a system is forced to backup mode, Direct Law, in this case by a lightning strike, the aircraft becomes more difficult to fly. The movements of the sidestick and pedals are now moving the control surfaces directly. There is no damping nor any adaptation of stick sensitivity to different flight phases.
Full deflection of the stick must command full deflection of the control surfaces in case it’s needed to control the aircraft. This makes for a sensitive stick when making the small corrections needed for normal flight. Large movements of the stick and the aircraft can run into a Pilot Induced Oscillation, PIO.
Having watched the video of the landing I guess this is what happened. The Crew conducts an ILS approach on RWY 24 (the Nav receivers worked apparently). As they approach the runway the Pilot prepares to flare the aircraft.
The aircraft is about 15 to 20 knots over normal landing speed according to FlightRadar24 traces, which is plausible. They are overweight, on backup systems and might even be on backup instruments. A padding of the approach speed when landing overweight on a long runway is then natural.
As we only have video when the aircraft has started the flare over the runway it’s difficult to say if the PIO had started on short final. I would not be surprised if it had. When one sees landings with a runway gallop like for the SSJ, the PIO normally start before the runway threshold, on short final when the Pilot starts the flare.
Either he flares too little or too much initially. In both cases, he must correct and now he is stressed. His feel is, the aircraft is not responding to his commands. This pumps adrenalin into the muscles which get stiff. Stiff muscles increase the PIO and the over corrections further.
The gallop gets worse and worse until at the third touchdown the main landing gears, attached to the rear spars, ruptures the wingbox and with it the wing’s fuel tanks. Now fuel sprays out into the engine exhaust and the rear of the wing and fuselage is quickly engulfed in flames. The catastrophe is a fact and 41 of 78 onboard the aircraft perish in the smoke and flames.
I refrain from commenting on the evacuation with all its facets, I want to focus on why the aircraft burst in flames. One can talk of Pilot error but there are reasons for what happened. Direct mode is a certified flight control mode but it’s not certified to the same standards as full FBW mode.