Propellers are normally taken for granted on a general aviation airplane. They are almost invisible when they are doing their job to move the airplane through the air. It has been said that a propeller keeps a pilot cool because when a propeller stops in the air, the pilot starts to sweat.

As general aviation maintenance technicians we see many propellers. Some props are made of one piece of machined aluminum, some are made of wood with metal leading edges, some blades are made of wood and composites, some blades are all composites, some are constant speed, some are reversable, some are full feathering, some are pushers, and they are made with 2,3,4,5 or 6 blades.

During A&P training, our school had a video of propellers being tested to destruction. The slow motion videos showed the metal blades bending and twisting severely as the prop was subjected to loads that could never be achieved by any aircraft flight conditions. Even then, the blades were so tough that they did not fail. Another video showed a propeller that was not a good match to the vibrations caused by the engine it was installed on and the blades were amazingly bent during the harmonic imbalance. This type of testing and subsequent changes in prop design has made the modern propeller extemely reliable.

Some twin engine airplanes have one propeller that turns clockwise and one propeller that turns counterclockwise to minimize the difficulty of controlling the airplane if one engine fails in flight. When installing a pair of these propellers on the aircraft, one should be aware of which side each one belongs on. If they are installed on the improper sides, the aircraft will be pushed backwards rather than being pulled forwards. Unfortunately I have seen this done. That young general aviation maintenance technician learned a good lesson that day. When the props were installed correctly they worked perfectly.

Two common threats to the propeller are debris on the ground and attempting to land the aircraft when the landing gear is in the retracted position. During the preflight check, the engine is "runup" to about 2000 rpm to check the engine systems. Any debris in the area of the propeller may be drawn to the spinning blades and cause damage by striking the blades. (during a rainy day, a water spout often forms beneath the propeller and is fascinating to watch) These "nicks" can normally be filed out in the field without causing the propeller to be out of balance. During the pre-landing checklist, one of the check items is--landing gear down and locked. The propeller is not designed to be a landing gear and will have to be replaced when the ends of the blades are curled back on themselves.

The back of the propeller blades (called the face) is painted black to reduce any reflections. The spinning propeller is essentially an invisibe disk when operating and the black paint works well to keep it invisible. The ends of each blade on the front are normally painted with white stripes to make them visible to anybody in the area. It is a very bad idea to walk into a spinning propeller turned by 200 horsepower. Occasionally the general aviation maintenance technicial must stand directly behind a spinning propeller to make engine adjustments. It is unnerving and very windy (in the winter it almost unbearably cold) This is an exercise of trust in the person who is operating the engine from the cockpit. The cockpit person is constantly vigilant of the outside persons movements and is ready to kill the engine ASAP.

The general aviation maintenance technician is not authorized to make many repairs on propellers. The repair work is very specialized and requires special training and tooling. FAA Propeller Repair Stations do a good job of keeping these generally neglected masterpieces of engineering operating properly.