Norwegian investigators believe that a fatal stall involving a Fairchild SA-226 Merlin regional turboprop illustrates the need for changes in taught recovery techniques.

The aircraft - a coastguard airframe - was being used to conduct pilot flight-skills tests for the Trondheim-based operator Helitrans.

Two candidates were undergoing the tests. On the day before the accident, 19 June 2008, the first candidate was asked to demonstrate a stall. But the poor weather conditions - turbulence, rain and low cloud - tended to trigger the stick-pusher, and the commander pulled the stall-avoidance system circuit-breaker to prevent nuisance activation.

"The candidate found this exercise frightening as she experienced great difficulties, having to use all her available physical strength to regain normal flight with the engines on full power and in [instrument] conditions," said Norwegian investigation authority SHT.

For the second candidate's test the following day - also in instrument conditions, and with stronger winds - the examiner instead requested slow flight up to the first indication of stall, and a recovery with minimum loss of altitude. The stall-avoidance circuit-breaker had not been reset.

While the commander added power and retracted flaps at the candidate's request, the pilot "lost control of attitude and airspeed". Altitude increased by 200-400ft (61-122m) and airspeed dropped to just 30kt (56km/h), even though the stall warning activated and full power was applied.

Just 37s after the control loss, and with an eventual sink rate of 10,000ft/min, the turboprop hit the sea in a near-horizontal attitude, 18nm west of Bergen, killing all three on board.

SHT found that modification of the Merlin airframe for coastguard duty had not degraded its stability, but that the stall recovery training focused too much on maintaining altitude - an issue which has become significant in the wake of the Air France Airbus A330 stall in June 2009.

"This [Merlin] accident highlights the need for a change in the current training on initial stall-recovery techniques," said SHT, "especially the focus on minimum loss of altitude at the expense of breaking the stall by lowering the nose and thus reducing the angle of attack."

Source: Flight International