August 2012
The pre-buy inspection is incredibly important, yet one expensive component that is largely overlooked is right out front: the propeller. What should a prospective buyer look for, and what options does he or she have?
Most buyers do give the prop a good visual inspection, at least from the spinner outward. That’s important, and it can reveal nicks and perhaps some cracks or a bad paint job.
What else is there to consider? The answer depends on the prop’s construction. Is it wood, metal, or composite? Fixed, variable-pitch, or ground adjustable?
Generally speaking, fixed pitch is the easiest propeller to inspect. It is one piece; there are no moving parts. Variable pitch propellers are more complex. With so many moving parts, there are many more things that can present themselves as problems.
In short terms, the simpler and newer the prop, the less we should expect to be troublesome. The converse is true, too. The older or more complex the prop, the closer the look we should give it.
DO YOUR HOMEWORK
Start with the obvious: logbook entries and compliance with any applicable ADs. Props have long histories—and some get substituted without proper notes—so be sure that the prop that’s on your airplane is the one that’s actually documented in the logbook, and that it’s approved for your airplane.
Since some aircraft have several approved props, be sure also that the prop on your new baby will do the job you need. If the seller did a lot of banner towing or recently sold his floats, you
may be getting a prop that, while approved for your model, may differ greatly from the prop in the POH, and the performance you expect may not match what the plane will deliver.
BE THOROUGH
Commit a thorough visual inspection of the propeller and spinner. Remove the spinner and have a good look at the spinner itself, as well as any plate(s). Cracks here can lead to unpredictable and dangerous failures, and cracks can happen any time since the last annual.
While you have the spinner off, pay careful attention to the hub area; this is the starting point for many problems. Remember, while that propeller is spinning and moving the aircraft through the air, that hub area is taking the thrust, gyroscopic effects and the centrifugal load of the blades.
Look for other obvious signs of trouble: fretting, oil streaks, metal dust, stripped screws or nut plates. Is there any sign that the backing plate has suffered damage, perhaps from a loose or over-tightened nut? Are all the bolts of uniform length, properly safetied?
If the propeller is wood, inspect the prop plate that the bolts go through. Look for signs of cracking or if the prop plate is warped, as this may be a sign that the prop plate is not of sufficient thickness—an unauthorized and probably undocumented modification or repair.
While it may not be a flight hazard (or may be!), an undocumented propeller modification should prompt you to look at other areas of the airplane, and at the very least, you’ll get some money off the price. Elongated holes or holes that are no longer perpendicular to the face are sure signs of previous trouble, and should
be checked before being accepted.
TEAM UP WITH THE MANUFACTURER
If the prop is metal, and you have any doubts about its provenance, you may be able to get more information from the manufacturer. A metal prop that has been returned to the factory for rework may be a few thousandths of an inch thinner than the original. That’s not a problem, but you will want to be sure that any reworked or rebuilt prop is consistent with its documentation. (Manufacturers actually shave a bit of metal during a typical “rebuild,” and they will give you a thickness and chord measurement, at a particular station, so that you can accurately measure the propeller there. A prop will never be larger than spec, but it may be a few thousandths smaller.)
Some props find their way onto experimental aircraft, and then back onto certified planes. That’s not a problem, either, if the documentation and inspections are proper—but it’s not uncommon to find a “76-inch” propeller perhaps suffered tip damage and was sold into an experimental application, where the new owner had a shop shorten it and reshape the tips. Even though the paint, stickers, model, and serial numbers match the correct and specified 76-inch propeller, a tape measure may show it to be 73 inches long!
While you’re corresponding with the manufacturer, inquire about what color the prop would have been when it left the factory. Many owners and mechanics routinely paint the back (pilot’s side) of the blades with flat black paint. Some factories never do. Some props get fancy tip paint, or a whole new color. The possible problem amateur paint can create has to do a little with balance, a little to do with protecting the prop, and a little more to do with documentation and “unauthorized repair” work.
REPAIR ISSUES
Manufacturers and authorized repair stations will never add material to a metal prop to compensate for a nick or chip. Though balance, aerodynamics, and cosmetics may be undiminished by skilled welding and machine work, the integrity of the prop definitely will be. If you detect any weld, you need to reject the prop.
Wood and composite props can have damage repaired by adding material. This is generally done by what is known in the industry as a scarf repair. There are limits as to the size and location of these repairs, so if you are unsure, check with the manufacturer.
Remove the wood prop in any pre-buy inspection. Look for signs of over-torquing (crushing and cracks) or looseness, which often results in burn marks at the prop hub/propeller interface. If you find these, you don’t want to fly with that prop. Look for any damage to leading edges, or evidence of finish cracking at the metal inserts or rivets. If you find any, get an expert opinion.
Composite blades are generally treated the same as wood blades when it comes to repair limits. The repair specs tend to be similar, but the technical know-how to accomplish such repairs is much more involved. If a composite propeller has been repaired, it is best to have an expert look at it.
IF IT SEEMS TOO GOOD TO BE TRUE…
As with most other aviation components, there are cheap knockoffs.
Case in point: Sensenich Propeller recently received a wood prop for inspection. The owner was proud of its new look and fine finish, but was a bit apprehensive, as the prop had never been flown and the seller, a reasonable and decent fellow, had come into the prop as part of a “hangar clearance sale,” and didn’t have any reasonable documentation.
The factory immediately recognized the fraud. The prop was laminated with green-colored glue. (Sensenich has never used a glue that wasn’t brown.) Stickers, a good paint job, even serial and model numbers… sometimes these are not sufficient for identification. If you have any doubt, have the factory check that prop!
And if you are buying a variable pitch prop, rely on an expert’s opinion. Beyond documentation and obvious faults, you probably won’t have enough to go on—and hidden trouble is expensive.
Tim Kern, CAM, MBA, has authored features in over 40 aviation publications. He writes technical, publicity and expository pieces for several companies in the aviation industry. He would like to thank Sensenich Propeller for many of the tips and much of the information in this article. Kern is a private pilot with a seaplane rating, and is listed as the manufacturer (“with a lot of help!”) of an experimental aircraft. Send questions or comments to editor@www.piperflyer.com.
