A constant-speed unit malfunction in flight may cause the prop blades to move into full fine/flat pitch and overspeed, or run-away. The engine RPM may rapidly increase and exceed the maximum limits. This maybe accompanied by a high-pitch whining sound caused by the very high prop tip-speed. The engine should be throttled back and shut down immediately. If this is not done, the engine may burn up due to failure of the lubricating system in an over-heated engine and followed by a possible engine fire. This can of course lead to the total loss of the aircraft. Alternately, the high centrifugal force on the prop blades caused by the high engine RPM may result in throwing a blade. With the prop now out of balance the engine could be torn from its mounts, with fatal results. However, it may be possible in some cases to throttle back the engine to a low power setting and air speed and land at the nearest suitable airfield.
The WW II, Boeing B17F Flying Fortress bombers were notorious for propeller run-away problems. These bombers were retrofitted with Hamilton Standard paddle-blade propellers and Hydromatic CSU’s. The low temperature at high altitude caused the oil to congeal preventing it from flowing freely to the CSU. This resulted in difficulty in feathering the prop, which induced prop run-away problems, causing some engine to be torn from the mounts with the loss of the whole aircraft in some situations. If the prop over speeds on a twin-engine aircraft, the increase of thrust produced by the extra high RPM may cause a yaw towards the good engine, the opposite way to an engine failure. On the other hand, the high RPM may cause a tremendous drop in prop efficiency and thrust. This could cause high drag resulting in a yaw in the usual direction for an engine failure. Therefore, check both engine tachometers and the vertical speed indicator. If an engine fails during the climb, the rate of climb will greatly reduce, whereas a prop overspeed will maintain, or nearly so, the rate of climb near its normal limit. The American FAA certification requirements state in the event of a governor failure the static RPM should not exceed 103% of the engine’s rated RPM. This requirement determines the position of the prop’s fine/flat pitch stop.