Propeller Overspeed in the Climb
Climbing through 6,000 ft in a twin with constant-speed, full-feathering propellers, the right engine RPM rises to 2,950 against a 2,700 redline, although the propeller lever is at maximum RPM and manifold pressure is 24 in Hg. Oil pressure is 65 psi and oil temperature is rising slowly. Reducing the right throttle to 15 in Hg brings RPM back to about 2,750; advancing it again reproduces the overspeed. The propeller lever moves through its full range with no change in the behaviour, and the left engine is normal. The aircraft is 20 NM from a suitable airport. Single-engine performance at this altitude and weight gives about 180 ft/min at 95 KIAS; terrain rises to 2,000 ft along the route and the aircraft is currently at 6,000 ft. The pilot must decide how to handle the engine and where to go.
1. What is the most likely cause of the overspeed and what risks does it create?
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RPM that responds to throttle but not to the propeller lever indicates the governor is not controlling blade angle - the propeller is running at a finer pitch than commanded, so the engine can overpeed at normal manifold pressure. Normal oil pressure and a lever with full travel make a simple linkage or low-pressure explanation less likely; the fault is in the governor or the propeller head. Advancing the throttle increases power and therefore RPM, which is exactly what is seen.
The risks are mechanical: sustained overspeed overstresses the blades and hub, and a propeller that is not governing can shed a blade or separate. That produces severe vibration, possible loss of the engine, and damage to the wing and fuselage. Oil temperature rising suggests the engine is working harder than the indications imply. Vibration, unusual noise or a change in oil pressure would push the decision toward an immediate shutdown rather than continuing.
2. Outline your immediate actions and the decision on whether to shut the engine down.
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First, reduce throttle to keep RPM below the redline - this is the only effective control available. Reduce airspeed by lowering the nose or reducing climb rate as required; higher airspeed drives the propeller faster. Do not chase the problem with the propeller lever; it is not responding and moving it repeatedly adds nothing.
The engine cannot be left in an overspeeding condition for long. With the aircraft 20 NM from a suitable airport and adequate single-engine performance, the sensible course is a precautionary shutdown and feather: reduce power progressively, then mixture to idle cut-off, fuel off, ignition off, and feather using the accumulator, since the engine-driven pump will stop. Feathering removes the windmilling overspeed and dramatically reduces drag. If the overspeed is uncontrollable even at low power, or vibration appears, shut down immediately rather than delaying. Then secure the engine, confirm the feather, and manage the asymmetric approach.
3. How do the performance numbers affect where you land?
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At 6,000 ft with about 180 ft/min available at 95 KIAS, the aircraft has very little excess performance. Terrain at 2,000 ft gives roughly 4,000 ft of margin, but a single-engine climb rate that small leaves no room for a downdraught, an icing encounter or a mistrimmed aircraft. The practical planning assumption is that the aircraft can maintain altitude only near its single-engine best rate speed, and any manoeuvring, gear or flap extension will erode or reverse the climb.
Therefore choose the nearest suitable airport rather than the original destination, and plan the approach so that the aircraft is not committed to a go-around on one engine. Fly a straight-in or a long final, extend gear and flap late, and accept a higher approach speed if needed. Brief the passenger, advise ATC of the shutdown and the intended diversion, and request priority. Landing overweight or with a tailwind is preferable to stretching the flight on marginal single-engine performance.