REVISITING A PC-12 CRASH

Sometimes it's not the speed that breaks aeroplanes, it's the slowing down.

IN AN ARTICLE ON THE 2012 crash of a Pilatus PC-12, I faulted the National Transportation Safety Board for mixing up indicated and true airspeeds. Actually, it was I who misread the report. I am indebted to reader Timothy Burtch, an accident investigator with the NTSB, for pointing out that the maximum speed of 338 kt that the aeroplane reached in a spiral dive before it broke apart was, in fact, an indicated airspeed, not a true one, and that the aeroplane did, therefore, exceed its manoeuvring speed by 175 kt as the report stated.

The Pilatus, with a family of six aboard, was climbing through FL250 in IMC. It was at 109 KIAS, in a 25-degree right bank, deviating to avoid an area of rain, when the autopilot disengaged for unknown reasons. Presumably a chime sounded and a warning light illuminated, but the pilot seemingly did nothing to take over control of the aeroplane. A baffling aspect of the accident was the pilot's apparent failure to act even when the aeroplane was vertically banked and plunging downward at an horrific rate.

Within 10 seconds of the autopilot disconnect, the angle of bank had increased to 50 degrees and the plane had begun to descend. After 30 seconds, the bank angle was 100 degrees and the plane had lost 2,600 feet. In the next 13 seconds, it lost another 5,900 feet while the positive load factor increased to 4.6 G.

At 36 seconds the indicated airspeed, having peaked at 338 kt - 430 true -- dropped to zero, suggesting that the breakup - the aeroplane lost its horizontal stabiliser and portions of both wings had taken place ant somewhere around 15,000 feet.