Propeller governor failure and overspeed on approach
A 1,350 kg retractable single-engine piston aeroplane (IO-550, three-blade constant-speed propeller, maximum 2,700 RPM) is established on a 3° ILS at 90 KIAS, 15 in Hg and 2,300 RPM. At 800 ft AGL, 3 nm from touchdown, the pilot pushes the propeller control fully forward to prepare for a possible go-around. RPM rises to 2,850 and remains there even after the pilot retards the throttle to 12 in Hg. The propeller lever moves normally but there is no RPM change; oil pressure and oil temperature are normal. The pilot suspects a governor fault. The aircraft is 600 ft AGL, 2.5 nm from touchdown, and the pilot must decide whether to continue the approach, go around, or shut down the engine. There is a 2,000 m runway ahead and the weather is VMC; the engine is still producing power.
1. Explain how a constant-speed propeller governor normally maintains RPM and why a governor failure can allow the propeller to overspeed when the propeller control is pushed forward. What blade angle does the propeller likely go to?
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The governor senses engine RPM with flyweights and controls oil pressure to the propeller hub. The propeller control lever sets the governor spring tension, which determines the RPM the governor tries to hold. Moving the propeller control forward calls for a higher RPM, so the governor reduces blade angle toward fine pitch, reducing propeller torque and allowing RPM to rise.
If the governor fails or cannot increase blade angle, the propeller can go to a low blade angle, or fine pitch, and overspeed. In this case the RPM rises to 2,850 against a 2,700 limit and does not respond to throttle movement, which is consistent with the propeller being stuck at fine pitch rather than the governor controlling it. Fine pitch means a low blade angle and a high RPM potential.
2. Evaluate the aerodynamic and operational consequences of continuing the approach with the propeller at 2,850 RPM. Include the effect on drag, glidepath, engine stress and go-around performance.
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At fine pitch the propeller blades take a small bite of air. At low manifold pressure the engine is not producing enough power to drive the propeller at that RPM, so the propeller tends to drive the engine, producing engine braking and high drag. The aircraft will need more power to hold the glidepath, and the drag rise will make the approach steeper or the speed less stable.
The overspeed also stresses the engine and propeller beyond the approved limit. A go-around is compromised: the RPM is already above redline, the propeller is at fine pitch, and adding power may produce a further overspeed or high engine stress. Climb performance and control of RPM are uncertain. Continuing to a landing with the engine running may be preferable to a go-around, but the approach must be flown with the overspeed and drag in mind.
3. State a safe course of action and the reasoning, including how to manage the throttle and airspeed if a go-around becomes necessary.
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The safest course is to discontinue any plan for a normal go-around and continue to a landing on the available runway, keeping the throttle low enough to limit RPM and using airspeed and glidepath control to manage the high drag. Do not shut down the engine unless there is a fire or another compelling reason, because a windmilling propeller and loss of power would make the landing less controllable.
If a go-around becomes unavoidable, lower the nose to maintain safe flying speed, apply power smoothly and only as required, monitor RPM continuously, and accept that the aircraft may be at or above the RPM limit. Fly a shallow climb and land as soon as possible. After landing, investigate the governor and propeller control system before further flight.