Investigators have criticised Scandinavian Airlines’ Bombardier Q400 training and monitoring procedures after inquiries into a serious unstable-approach incident discovered that Q400 crews repeatedly failed to follow emergency checklists when handling propeller overspeed.

But Swedish investigation agency SHK has also questioned the clarity of these checklists in the wake of the incident at Kalmar on 6 April last year.

SHK found SAS Q400 pilots had experienced six propeller overspeed incidents in the preceding two years – four of them, in the space of three weeks, involving the same aircraft. But it says the emergency checklist was not followed properly in any of these cases.

The last of these incidents drew SHK’s attention because the pilot’s attempt to deal with the overspeed resulted in the aircraft deviating widely from the centreline, and dipping far below the glideslope, at it attempted an ILS approach to Kalmar. Such was the instability of the approach that the air traffic controller at Kalmar, who was watching the aircraft, alerted the crash-rescue services.

Propeller overspeed requires pilots to execute a memory-based checklist procedure which involves retarding the throttle, feathering the propeller and shutting down the engine. Critically, while the procedure requires the throttles to be retarded towards flight-idle, the checklist at the time did not specifically warn pilots against actually selecting flight-idle.

During the Kalmar incident – unlike the previous instances – the aircraft was being operated with its throttles already in the flight-idle position. When the overspeed occurred the captain, instead of following the checklist procedure, increased the power of the left-hand engine but left the right-hand throttle at flight-idle.

This created a dangerous asymmetry in which the aircraft’s left-hand engine was generating thrust while the right engine’s propeller was left unfeathered, creating substantial drag, yawing and loss of lift. The autopilot disconnected and the aircraft sank far below its assigned height of 2,000ft, triggering a ground-proximity warning, and leaving the crew struggling to maintain control.

While the Q400 managed to land, with no injuries to its 73 occupants, SHK says “at no stage” did the aircraft meet the requirements for a stabilised approach.

“[We] cannot judge how close the aircraft was to a crash in respect to height, speed and controllability, but can conclude that both pilots on separate occasions during the approach were convinced that they would not reach the runway,” it says, highlighting the “balanced” crew resource management which ultimately kept the Q400 under control.

SHK says the overspeed checklist procedure was “not completely clear”, notably regarding the crucial issue of retarding the throttles.

“There is no information in the checklist to say that a power lever should not be placed at flight-idle,” says SHK. “Nor was the company informed of the potentially dangerous situation that can arise by having an engine power-lever in flight-idle if the propeller is not feathered.”

Scandinavian Airlines only trained its pilots to cope with overspeed during the critical take-off and climb phases of flight, and that emergency checklists were sometimes not followed even when instructors were on board aircraft performing line checks.

“Against this background it was natural that the behaviour of not following the emergency checklist was perceived as acceptable,” says SHK. Despite three similar incidents on the same aircraft in the preceding three weeks, it says, the carrier did not assess the risks of crews not following the checklist and there was no reaction from its flight operations department.

“With poorly written documentation, the lack of applicable training, deficient information from the company and signals that the problem could be solved without following the checklist, the conditions for [the Kalmar pilots’] making a correct decision were simply not there,” it adds.

Following the Kalmar event, SHK and SAS suggested to the aircraft’s manufacturer that the checklist could be misinterpreted. While the manufacturer largely disagreed, says SHK, it nevertheless provided a degree of subsequent clarification.

SAS also improved communications with its pilots regarding turboprop characteristics, although the carrier has since opted to withdraw the Q400 from its fleet. A spokesman for the carrier says the incident was serious, and that the airline has accepted the findings of the SHK report.

SHK is nevertheless recommending that the European Aviation Safety Agency set up a working group with carriers to explore possible improvements to the content of the checklist.