Clear prop

As pilots we have a tendency to treat an aircraft propeller casually.

We tend to take more interest in the engine and its operation than the prop. Sure it is the prop that turns around and bites into the air to push us forward, but there is more to it than that.

A propeller or prop is typically made of an aluminium alloy but these days there are more composite blades appearing. Often these are of wooden construction with an outer covering of fibreglass. Sometimes the entire blade is of carbon fibre construction.

The material that has been around for 100 years is wood, the stuff that the Wright Brothers used in their Flyer aircraft. There is a whole hidden art to propellers that we sometimes do not hear about.

Wooden props usually have a metal leading edge to cope with dirt, dust and small stones.

On the subject of stones a prop should be checked before each flight to ensure that it has not been damaged by a stone or foreign object and thus developing a nick.

If left undetected and untreated that nick could turn into a crack. Thatcould develop into a portion of the prop blade breaking away under high rotational forces. When this happens the resultant unbalanced forces can do major structural damage to the engine, engine mount and airframe.

When pilots find a severe nick in a prop it may be a case of seeking an opinion from an engineer. If severe enough the engineer can either use a file to blend out the stone nick or, in a worst case scenario, ground the aircraft and have the prop overhauled or replaced. Props cost big money so pilots should be aware of where and how they operate their aircraft.

Treat an aircraft propeller like the back end of a horse – with a lot of caution.

When doing a run-up try to position the aircraft over a clean patch of tarmac or concrete. If not available it may be that the mag check at high rpm is done on the actual takeoff roll as the aircraft moves forward and thus is not sucking stones back or up into the prop.

As pilots we need to exercise prop caution, not only in the inspection of the prop blades during the daily and pre-flight inspection but even in a post‑flight inspection. A metal alloy prop should best be left in a horizontal position to prevent rainwater running down into the hub, particularly a constant-speed unit.

A wooden prop should be checked

A pilot should always treat an engine as live at all times.

to ensure that it has not shrunk in hot weather and become loose on the hub. The pilot may pick this up when he smells wood smoke from the prop movement friction. Some composite props are prohibited from being flown in rain.

In operation it is not impossible to have a prop strike the ground. This could occur when the aircraft goes across a depression or a tail dragger that noses over – time to let an engineer have a look at the machine. Propeller strikes usually result in a strip of the engine and run-out check of the crankshaft.

There are still a number of aircraft, antiques and homebuilts, that are started by hand – that is the propeller is rotated by the pilot to get a start. This is sometimes called an Armstrong starter. It is unfortunate to record that hand starting an aircraft is a dying art. It is still the method that most Tiger Moth biplanes are startedwith but it should not be encouraged by pilots who have a flat battery in their Cessna and want to do a hand start. Yes, it is possible in most cases but it should never be done unless the pilot has had adequate instruction from an experienced hand.

A pilot should always treat an engine as live at all times and avoid turning the prop over, regardless.

With Piper Warrior aircraft the pilot should never encourage or set an example by hopping forward off the wing. This sets up a scene where a passenger can inadvertently get off the front of the wing and, unbalanced, stumble into the prop of a turning engine. Good reason to not let the passengers leave the aircraft until the engine has been shut down. Cessna singles are the same with more than one passenger exiting the cabin and walking forward into the still moving propeller. This instructor has heard some pilots reciting a practice forced landing (PFL) brief saying that they would meet the passengers ahead of the aircraft after it stops moving. Better to meet off the wing than to go forward and into that prop danger area.

Each year in Australia there will be several prop and helo tail rotor strikes involving pilots and passengers. They are usually fatal or at best can disfigure one for life.

Propellers are critical for piston and turboprop-powered aircraft. They do a great job but should be checked on a regular basis for possible damage and the pilot should always exercise caution around them to ensure that no-one gets involved with a moving propeller. The result is unthinkable.