In the event of an engine failure, the prop will reduce speed to around 1200 RPM as it windmills. The power required to cause the prop to windmill is provided by the free air stream flowing through the prop disc. In an attempt to maintain RPM, the CSU will decrease the blade angle to the fine/flat pitch stop. However, a windmilling prop produces more drag in fine/flat pitch than it does in coarse pitch. Therefore, on a single-engine aircraft select full coarse pitch before the engine stops running. The required oil pressure to the CSU piston will be lost once the engine has stopped and then it is too late to select coarse pitch. Reduced prop drag improves the aircraft’s glide ratio enabling it to cover a greater distance in the ensuing forced landing. [See Windmilling Prop Forces].
The Feathering Prop
On multi-engine aircraft, the prop can be feathered in order to stop the engine to prevent windmilling drag and any further damage to the engine. Feathering must be achieved quickly before the engine stops, otherwise it maybe impossible to get the blades to feather. Opening the throttle a small amount maybe sufficient to increase the RPM above idle; alternatively, lowering the nose may help to keep prop windmilling long enough to feather it before reverting to safe single-engine speed (VMC). This can only be done if altitude and time permits. An engine failure, on or shortly after take-off requires immediate action to get the prop feathered.
The air/oil type CSU is more suitable for twin or multiengine aircraft; in the event of an engine failure, loss of oil pressure would allow the opposing air pressure acting on the CSU piston to turn the blades into the feathered position. The aerodynamic turning moment, which normally turns the blades towards coarse pitch, would no longer be of assistance.
In fact, it is more of a hindrance due to the force being reversed when the prop is windmilling and it attempts to turn the blades towards fine/flat pitch. [See Prop Stress].
A feathered prop has its blades turned to a pitch angle of approximately 90 degrees edge-on to the air flow to reduce aerodynamic drag and vibration caused by the disturbed slipstream flowing over the wing and tailplane. Due to the blade’s twist, only the middle portion of the blade is parallel to the airflow, while the blade’s inner and outer portions are presented to the airflow at a positive angle of attack in opposing directions; this will tend to rotate the prop in opposing directions with the net result, the prop remains stationary
Feathering the prop in flight can be achieved by various methods depending on the design installation. These methods are:
• Manually moving the prop pitch lever through a detent on the throttle quadrant
• The use of a feathering button to activate an electromechanical pump, or
• An auto-feathering system.
Some of the smaller and lightweight composite props employ a blade counterweight system to assist the pitch change mechanism. When an engine fails, the counterweights will automatically cause the blades to turn towards the feathered position instead of their natural tendency to turn towards fine/flat pitch.
The pilot should be familiar with the feathering and unfeathering procedure for his/her aircraft. If an engine is shut after a real failure, it is the usual practice to leave it so and carry out a single-engine landing as soon as possible. The decision to restart an engine after a failure should not be taken lightly, because of the risk of fire, further engine damage, or the inability to re-feather the prop again if a re-start is not possible.
It should be kept in mind when feathering a prop for training purposes, in cold weather the oil in the CSU may become congealed quickly. Congealed oil can impair the operation of the CSU and present difficulties in un-feathering the prop again at the end of the exercise.
Un-feathering the prop is a relatively simple procedure that can vary between different aircraft types. It also depends if an oil pressure accumulator is used for un-feathering the propeller. For a given type and model of aircraft, some have accumulators and others do not. It is imperative to know the correct procedure for the aircraft. The accumulator holds oil under pressure and when activated oil is directed to the CSU to un-feather the prop. This is a one-time method only, so if it does not work correctly the first time you could be stuck with a feathered prop until after landing.
After selecting the coarse pitch for less prop drag or fine/flat pitch if an oil pressure accumulator is used, the engine starter is engaged. Oil pressure returning to the CSU as the engine comes back to life, will move the blades out of the feathered position. With the slipstream acting on the prop blades, the engine will start easier than it does on the ground. However, expect a fair amount of vibration until the engine has returned to active duty at the normal engine RPM. Un-feathering can also be achieved on some aircraft by activating the feathering button to start the auxiliary pump, which will supply oil to the CSU. Un-feathering requires greater oil pressure (around 600 PSI) than that required for the initial feathering. As the prop blades move out of the feathered position, the air flow through the prop disc due to the plane’s air speed will start the prop windmilling. When the RPM passes through a pre-determined figure of around 800-1000 RPM, the feathering button is activated as the CSU returns to its automatic operation. Holding the feathering button in for too long will cause the blades to move through to the fine/flat pitch stop and cause damage. Hence, the need to deactivate the feathering button. Because there are different methods of feathering and un-feathering the prop, depending on the aircraft type, a full knowledge of the particular system and procedure is essential.
An engine shut down in flight will cool rapidly. It will need some time to warm again at a low power setting after re-start before opening up to cruise power; check oil temperatures and pressures and the cylinder head temperature gauge. The throttle should be set at the recommended manifold pressure and the prop control moved from the fine or coarse pitch setting, whichever was used for the un-feathering procedure.