Recip Technology: The Propeller, That Spinning Disc Attached to the Engine

Sept. 5, 2014
AMT spoke with propeller experts Michael Mayhill, manager of customer service and support, and Kevin Ryan, technical support representative, both with Hartzell Propeller Inc., and asked them what were the most common questions they received from general aviation (GA) maintenance technicians in the field.

The propeller: that long-thin spinning object mounted on the engine; generally on the front of the airplane, or on either side in a multi-engine aircraft. Except for a few examples like gliders and jets, many aircraft have one or more. There have been aircraft with propellers mounted behind, acting to push the vehicle through the air rather than pull it. Without a properly maintained and functioning propeller the aircraft isn’t going anywhere.   

AMT spoke with propeller experts Michael Mayhill, manager of customer service and support, and Kevin Ryan, technical support representative, both with Hartzell Propeller Inc., and asked them what were the most common questions they received from general aviation (GA) maintenance technicians in the field. Mayhill says, “You can place the most common questions into one of about 10 categories. Our technical support staff answers many of the same questions each and every day.” Ryan went on to say, “This is OK, that’s what we are here for; to support the owners and maintainers of our propellers.”

Q. What is the most common question asked by aircraft technicians in the field?

A. How big of a nick can I file out of the leading edge of an aluminum blade propeller? We are asked this almost every day, and it can be a difficult and challenging question to answer. The broad rule-of-thumb on our propellers is 1/16- to 1/8-inch deep nick or gouge can normally be repaired on the aircraft. However, if you have an old prop with blades already approaching the minimum width and thickness limits, you should consider sending it to a prop shop. By simply removing damage in this case, you could easily take the blade below minimum dimensional limits.

The minimum width and thickness criteria are part of the propeller overhaul manuals which are typically only available to an overhaul facility. Also, don’t ignore nicks and gouges on the face and camber sides of the prop blades. These sections of the blade frequently receive damage that is either ignored or simply sanded and repainted without a detailed inspection of the surface. Also, remember that removal of significant damage may result in an propeller imbalance situation requiring fine tuning on the aircraft with a propeller dynamic balance.

Q. What are the repair criteria for blade damage on the new Hartzell GA-class composite blades?

A. Most GA aircraft technicians have little experience dealing with composite blades, as composites have historically been used primarily on airline-class propellers. Hartzell published Service Letter HC-SL-61-294 to provide technicians with clear instructions on composite blade minor field repairs.

Q. What are the ground/object strike criteria? What has to be done and is my propeller repairable?

A. Depending on the nature and severity of the strike, a propeller’s repairability can range from very little needed to a complete restoration. Composite-bladed propellers are typically affected differently because by definition they do not bend like an aluminum blade. The three general criteria for determining whether a prop needs to go to a prop shop after a object strike event are: 1. Is the track or angle of the blade out of tolerance; 2. Has there been a diameter reduction; and 3. Has the pitch change mechanism been damaged, which can be first identified by movement of the blade inside the hub. Also, consult the propeller owner's manual.

Q. What are the ground run-up procedures? I’m having trouble with low pitch stop and governor adjustments.

A. Mechanics often contact Hartzell to discuss propeller rpm issues, particularly after a new propeller is installed. Static run-up is one area of particular concern and confusion. Many aircraft technicians do not appear to have a solid grasp of the propeller/governor relationship, and how adjusting one affects the other. Lack of basic understanding often results in unnecessary aircraft downtime, needless governor and/or propeller repairs, and excessive troubleshooting labor. This is true on both single and multiengine aircraft. The propeller owner's manual explains the proper procedures.

Q. How do I grease a propeller hub?

A. Regardless of what you may have been told, do not pump grease into an aluminum hub until it comes out the other side. On older Hartzell steel-clamped props there is a procedure to grease certain hubs by pumping grease in one grease fitting until it came out of the opposite removed fitting. This is not for aluminum hub propellers. The correct procedure is in the propeller owner's manual.

Q. Is an engine teardown inspection required following a propeller strike?

A. This isn't a question for the propeller manufacturer. Many technicians think that our repair or overhaul criteria "forces" the customer to tear down his/her engine. The answer to this question needs to come from the engine manufacturer.

Q. What are the proper procedures for propeller installation?

A. Propeller installation and removal questions are quite common such as correct torque and correct clocking angle. Generally speaking, the prop doesn't care about clocking, but the engine might. If replacing a two-blade with a three-blade, for example, if there are no directions in the STC installation instructions, ask the engine manufacturer about prop position on your particular engine model. Nut or bolt torque can be found in the propeller owner’s manual.

Q. Where do I find propeller maintenance and troubleshooting data?

A. In the propeller owner's manual. Such information includes: installation procedures, lubrication, troubleshooting, and much more. Technicians in the field need to be more aware of owner’s manual content, and manual availability. In our case all of the owner’s manuals are on the Hartzell web site. A lot of the technicians call requesting data they could easily find on the web site. Our web site contains all of the propeller owner's manuals including revisions. It also contains service bulletins, service letters, and airworthiness directives relating to our propellers. We provide almost all technical information needed by aircraft mechanics and end-users for care and feeding of our propellers. We do not provide overhaul manuals free on the web site, as they are specific to propeller overhaul and major repair, which may only be accomplished by a  certificated propeller repairman, usually at a propeller overhaul facility.

Q. What are the propeller overhaul requirements? Is my prop unairworthy if it hasn't been overhauled on time? What are the overhaul requirements for a Part 91 operator?

A. These are very common questions also, particularly from Part 91 operators. As far as Hartzell is concerned, propeller overhaul per Hartzell Service Letter HC-61-61Y is required, even for Part 91 operators. However, FAA regulations can be interpreted to indicate propeller overhaul is optional for Part 91 operators, so it is frequently done well past Hartzell recommended limits.  Our propeller overhaul recommendation on most modern GA propellers is six years or 2,400 hours of operations, whichever occurs first. Unfortunately, many people recognize the hourly requirement, but ignore the calendar limit. What you really need to consider is how many hours a year the aircraft is flown annually. If your customer is a typical GA pilot who flies 100 hours per year, are you and he/she both comfortable with not looking at the propeller for 24 years? 

Q. Should I overhaul the prop governor at the same time as the propeller?

A. This is a great question because prop governors often get neglected as they are out-of-sight and out-of-mind. Here’s the situation that all too often exists: An engine comes time for overhaul and it is removed and exchanged. Many times the accessories are removed from the old engine and held in the shop or hangar and later reinstalled on the exchanged engine when it is installed on the aircraft. Our advice is to not forget about the prop governor. Hartzell recommends governor overhauls be performed in conjunction with the engine overhaul. Remember that a neglected governor can fail, and when governors fail the result is metal makes its way into the entire oil system fast. Such contamination can lead to sudden and severe engine damage, as well as a complete loss of propeller rpm control.

Q. What can you tell me about dynamic balancing of a propeller?

A. A vibration issue really is a customer or pilot comfort issue. We are in favor of dynamic balancing, as really we see no negatives to it. The smoother the ride, the less equipment damage from vibration, and the happier the customer.

Q. Which propeller should I install and why?

A. These sorts of questions are often asked by technicians on behalf of aircraft owners. Aircraft technicians are a valuable resource for the owners and operators, particularly on the topic of propeller STCs. Usually you need to ask the customer some additional questions, like what are you looking for - less noise, faster cruise, shorter takeoff, operation on floats, etc. The actual STC holder can usually provide valuable technical information. Note that customers frequently want side-by-side propeller performance comparisons, which are only sometimes available. Regardless, as a primary source of advice on propeller options, it’s a good idea for technicians in the field to stay abreast of what propeller choices are out there, and their individual benefits.

Michael Mayhill graduated from Purdue University College of Technology with an AS in aviation management and a BS in aviation technology. He is an A&P and has been in propeller technical support for 23 years, six at Hartzell. Kevin Ryan received a BA from the University of Akron, Akron, OH. He is a licensed FAA propeller repairman and has been a Hartzell product support representative for 14 years. Hartzell Propeller is located in Piqua, OH. For more information visit