Propeller Overspeed at Cruise Climb
During climb at FL240, the left propeller RPM rises from 100% to 106% while torque falls from 90% to 68%. ITT and Ng remain normal, oil pressure is normal, and there is no vibration. The propeller governor is in normal mode, synchrophasing is off, and propeller heat is off. The right propeller remains at 100%. The left power lever is at the same position as the right. Fuel flow is normal. The aircraft is 25 minutes from destination and 10 minutes from an alternate. The AFM says maximum continuous propeller RPM is 100%; an overspeed above 105% requires a precautionary landing, and if RPM cannot be controlled the engine must be shut down and the propeller feathered. The weather at the alternate is suitable.
1. What is the most likely fault and why does torque fall as RPM rises?
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The most likely fault is a propeller governor or pitch-control failure allowing the blades to move to a finer pitch. At finer pitch the propeller absorbs less torque from the engine, so RPM rises and indicated torque falls at the same power-lever position. Normal ITT and Ng make a gross engine-power malfunction less likely, and the absence of vibration argues against a damaged blade or spinner. Check that the power lever is actually at the same rig position as the right engine, then treat the governor or pitch-control system as the primary suspect. The overspeed itself is the urgent problem because it can lead to blade stress and loss of thrust control.
2. State your immediate actions and decision.
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Reduce the left power lever until RPM returns within the continuous limit, and avoid large power changes. If RPM remains above 105% or cannot be controlled, shut down the left engine and feather the propeller according to the AFM. Do not continue climbing or press on to destination with an uncontrolled overspeed. Divert to the 10-minute alternate, where the weather is suitable, and plan a single-engine approach. Brief the cabin and ATC, and expect reduced performance and asymmetric handling. The precautionary landing is required by the AFM and is the correct judgement call even though the aircraft is still controllable.