Propeller Overspeed on Takeoff
A turboprop aircraft is powered by a PT6A engine driving a four-blade constant-speed propeller. Maximum propeller speed is 1,900 rpm. During the takeoff roll, at 1,850 lb-in torque, propeller speed rises to 2,080 rpm and remains there for about 4 seconds before the pilot aborts the takeoff. There is no abnormal vibration or noise after the event. On inspection, the propeller governor control linkage rod-end bearing has 1.2 mm of radial play; the limit is 0.3 mm. The propeller blade-root seals show minor oil seepage. The aircraft is at an outstation with a 1 h 10 min revenue flight planned with 9 passengers. The propeller has 300 hours since new.
1. What is the most probable cause of the overspeed, and how does the linkage play produce it?
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The governor control linkage is the most probable cause. The rod-end bearing has 1.2 mm radial play against a 0.3 mm limit, so the linkage has lost rigging precision. During the takeoff roll the governor receives a control input that does not correspond to the intended on-speed setting. With the linkage play, the governor can command a finer blade angle than intended, or it cannot increase blade angle fast enough as RPM rises. The propeller then overspeeds until the pilot reduces power. The minor blade-root seal seepage is a secondary finding and does not explain a 180 rpm overshoot by itself, but it should be corrected.
I would also test the governor and the propeller pitch-change mechanism, because a governor internal fault could produce the same symptom. The linkage play is the primary finding because it is measurable and beyond limit.
2. What inspections does the overspeed require before further flight?
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An overspeed to 2,080 rpm for about 4 seconds is outside the 1,900 rpm limit and requires the manufacturer's overspeed inspection. That normally includes:
- Visual and dimensional inspection of all propeller blades, including leading edges, shanks and blade roots.
- Eddy-current or ultrasonic inspection of blades if required by the propeller OEM.
- Inspection of the hub, pitch-change mechanism, counterweights and spinner.
- Governor removal, bench test and linkage replacement/rigging.
- Inspection of the engine reduction gearbox and propeller shaft for distress.
- Oil system inspection for debris and a ground run with propeller balance check.
The absence of abnormal vibration does not waive the inspection. Overspeed can initiate fatigue cracks that are not detectable by feel or sound.
3. Can the aircraft operate the planned revenue flight? Justify your answer.
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No. Two independent reasons prevent the revenue flight:
- The governor linkage bearing is beyond its 0.3 mm limit, so the propeller control system is not airworthy and the overspeed could recur.
- The post-overspeed inspection has not been completed, so the propeller and reduction gearbox condition are unknown.
A revenue flight with 9 passengers is not permitted. The aircraft may be ferried only after the linkage is replaced and rigged, the governor is tested, and the OEM overspeed inspection is completed, or under a specific ferry approval with the propeller system made safe. Otherwise the aircraft stays at the outstation until repaired.