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Summary

Summary

The SAAB SF 340's departure from Wagga airport had been delayed for 4.5 hours due to ground fog. As the aircraft taxied for departure, the crew completed the pre-flight checks. Incorporated within these checks was the requirement for a "first flight of day" propeller governor overspeed test. As it was the aircraft's first flight for the day, the check was carried out.

Following take-off, and shortly after landing gear retraction, with the Constant Torque On Take-off system engaged, the crew noted that the right engine propeller RPM was low; at approximately 1,100 RPM. The left propeller was within the normal operating range at an indicated 1,378 RPM.

As a return to Wagga was unavailable, due to ground fog, the crew contacted air traffic control indicating their intention to divert to Albury Airport. An Alert Phase was declared and Albury Emergency Services were on stand-by for the aircraft's arrival.

The crew then carried out the appropriate abnormal checklist actions for a propeller underspeed, shutting down the right engine just prior to the top of descent. During that time, the crew briefed the cabin attendant on the engine problem, before informing the passengers of the situation via the aircraft's public address system. Following an uneventful single engine approach and landing, the Alert Phase was cancelled.

An investigation by the aircraft's operator, included analysis of the aircraft's flight data recorder readout. The analysis indicated that during taxi the right propeller RPM had reduced from 1,040 to 990. That RPM drop was consistent with the crew carrying out the propeller governor overspeed test. However, unlike the left propeller, the right propeller RPM had not fully recovered at the completion of the check.

Both crew members reported that on completion of the propeller overspeed governor checks, once they had observed the propeller indications returning towards normal, their attention was diverted towards other checks. The crew also indicated that during the take-off they did not normally check the propeller RPM indications, instead monitoring the engine parameters of "torque and inter-turbine temperature". Consequently the low right-propeller RPM had not been initially detected.

During the take-off roll the crew noted that the right engine torque had lagged behind the left. That was considered to be due to not having the right power lever pushed far enough forward, as the Constant Torque On Take-off system only engages after advancing the power lever past 64 degrees. The crew had then pushed the right power lever further forward in order to equalise both engine torque indications.

After the flight, the operator completed a thorough maintenance check of the right engine and propeller systems. No unserviceabilities were found during that check. Following consultation with the aircraft's engine manufacturer the aircraft was returned to service. The problem did not re-occur.

The operator attempted to replicate the problem in their SAAB 340 flight simulator. That attempt was observed by a representative from the Civil Aviation Safety Authority. After thorough investigation, the operator was unable to repeat the occurrence.

 
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