Not an ordinary crash

Maybe there is no such thing as an ordinary aircraft crash, but the Tatarstan Air Boeing 737-500 accident at Kazan was certainly not ordinary.

The instrument landing system was available for the approach, and all present assumptions are that the crew was using it. But the crew decision to go around was made on the basis that the approach was not stabilised, according to ATC at Kazan. Whether the crew faced other problems is unknown.

But a short time after the go-around was initiated, with the aircraft still over the airfield, security video shows that the aircraft’s attitude had become about 80deg nose down with no height left for recovery.

That huge attitude transition is impossible to achieve in such a short time/distance within the aircraft’s normal flight envelope. So either the aircraft went rapidly outside its flight envelope – for example entry into a stalled condition with no stall recovery procedures being applied, or there was an unlikely crew command for positive or negative G forces well outside the structural limits. Beyond those, we are looking into realms of serious technical or structural failure.

The latter seems unlikely if only because that sort of malfunction just doesn’t happen in modern aviation. We are not used to it any longer. Industry people talking about this accident are asking whether this could be a return of the 737 “rudder hardover” phenomenon. The last high profile rudder hardover crash involved a USAir 737-300 in the early stages of its approach to Pittsburgh at night in September 1994.

I don’t think it was rudder hardover: the impact attitude, given the low altitude at which the aircraft was flying, was too extreme even for that, and besides which the hydraulic valves deemed to be responsible for the phenomenon have been redesigned and replaced since then.

The theory of an extreme pitch-up and speed loss during go-around is more plausible because there have been a number of such events, most of which the crews recovered from.  But it is still unlikely (although not impossible) that the almost vertical pitch-down impact attitude could have been achieved from that scenario.

That leaves us with catastrophic structural failure the symptoms of which the airline had failed to recognise. Sorry, I can’t believe that.

Or sabotage. That’s always the easy answer when you have no real ideas.  And I have no real ideas.

PS (information received about 3h after posting this blog entry): My colleague David Kaminski Morrow quotes the Russian investigators as saying that the time between the initiation of go-around and impact was 45sec. That’s quite a long time. For me that begins to add weight to the stalling theory but, like everyone else at this stage, I’m still guessing.

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