Propeller Overhauls

​Propellers have been used on aircraft for almost 115 years and have evolved to become more efficient and reliable, but do we really understand the tremendous forces and corrosion to which propellers are exposed? How frequently should an aircraft propeller be overhauled? Is the process absolutely necessary? What happens during an overhaul? To find answers to these questions, we paid a visit to Aero Propeller of Calgary.
 
Located near Calgary International Airport in Alberta, Aero Propeller of Calgary was founded in 1979. Two of its owners, Gord Thompson and Nash Javer, have been with the company since it first opened its doors nearly forty years ago. In 2008, they were joined by Kevin Samuel, an experienced aircraft maintenance engineer and structural technician. Combined, they have more than a century of experience in propeller maintenance. Most of the propellers that enter their workshop belong to general aviation and light commercial aircraft, but the team occasionally works on more interesting examples, such as that of a Hawker Hurricane and Douglas DC-6. When it comes to propeller maintenance, these men have seen it all and were happy to talk about overhauls in the context of aviation safety.

It is easy to underestimate the importance of overhauling a propeller, especially when a quick visual inspection makes it appear to be in excellent condition. But consider the bending, pulling, twisting and vibratory forces to which propellers and hubs are subjected during every flight. Propellers are among the highest stressed components of an entire aircraft. Centrifugal forces, which could be described as forces which try to pull propeller blades out of the hub, can be greater than 7 500 times the weight of the blades, amounting to thousands of tonnes per blade. To make matters worse, the extreme stresses imposed on the blades are concentrated on the weakest areas, which exist as a result of corrosion or seemingly insignificant cracks or damage. Gord Thompson mentioned that in recent years, he has seen considerably higher levels of corrosion in propeller hubs, as a result of deteriorating air quality. Therefore, more frequent inspections than those recommended by manufacturers and Transport Canada, may save aircraft owners money in the long run. As an example, Thompson pointed at a hub which had been rendered useless by corrosion. If the problem had been detected earlier, the component could have been saved. 

When determining the when a propeller, or related components, need to be overhauled, flight time or calendar dates are not the only factors one should consider. It is important to take operating conditions and the environment into consideration.

​That said, what does an overhaul entail? The first step is to mount the propeller and visually inspect it. The paint is removed and blades are examined to see if they had been damaged in any way, and to measure dimensions to determine whether they can be overhauled. Once the propeller has been taken apart, the basic components are cleaned and degreased. The next step is to repair the damage and ensure that the components are within the manufacturer’s dimensional limits. All major components are then sent to a certified workshop for non-destructive inspection. Next, components are polished and dipped in a solution for corrosion protection, before being painted and receiving a durable polyurethane coating. Finally, the propeller is reassembled and set according to the manufacturer’s overhaul manual.
 
Internal corrosion is extremely dangerous and can only be detected when all the components have been taken a part and cleaned in a workshop. The importance of propeller inspections and overhauls cannot be overstated. When experiencing an engine failure, for the most part, the aircraft can glide and complete a safe forced landing. If, on the other hand, a propeller blade separates, the remainder of the flight can be considerably more eventful, if not catastrophic.
 
For further information on propeller maintenance, or advice regarding purchasing or owning a propeller, please contact Aero Propeller of Calgary at 403-291-9400.
 
Information on how frequently propellers need to be overhauled can be found on Transport Canada’s website, www.tc.gc.ca, in the Canadian Aviation Regulations (CARs) section.