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.
