When the Cessna Citation 500 crashed into houses near Biggin Hill aerodrome on 30 March 2008, it mystified all the aviators I spoke to who tried to understand what had happened. Now the Air Accident Investigation Branch has released its final report and we are not much the wiser despite their painstaking work. There were no recorders on board - there were not required to be.
The AAIB's verdict is that, two minutes after take-off while the aircraft was flying across the airfield on its cleared departure routeing, the crew misinterpreted vibration in the air conditioning unit as an engine fault, The Board says there was no evidence of a fault in either engine, ruled out fuel contamination and a host of other possibilities, but concluded that 70sec before impact neither engine was delivering any power. At impact both engines were powering up at different stages in their re-start procedure.
The Board believe that if the crew had restarted one engine only it would have supplied sufficient power in time for the crew to have recovered the aircraft, but that starting two together slowed the process.
What a tragedy. Imagine being in the left hand seat of this Citation on the kind of day when everything you do turns to dross. You misinterpret an "expensive" noise produced by the failing bearings of an ancillary component as impending or actual engine failure. What happens next is not clear and never will be, but maybe something tells you that your right engine is the problem and you shut it down. Then, with supreme irony (if the AAIB's theory is correct), reducing power on the left engine causes fuel cut-off because of a mechanical snag. You go for engine restart and it's too late - just. You tell Biggin tower "we're going in".
If there's a moral to this story for all pilots, it's don't shut down an engine that's producing power while you are still very low, and you're overhead an airfield you can land at quickly. Easy to say. But we never will know what the crew saw and heard that made them act as they did. We don't even know for sure exactly what they did.
It's time to demand cockpit voice and flight data recorders in turbine aircraft, however small, which is what the AAIB is calling for.