June 1, 1996
Marc E. Cook
Life is no day at the beach for a propeller. Consider this: In a maximum-effort climb, the propeller endures enough centrifugal and twisting force to fully un-shellac David Hasselhoff's hair. The leading edge of the typical aluminum prop blade suffers more abrasive action than a handful of sand in a contact lens. Far from coming through a tour of, say, Baja's beach strips tanned and rested, the propeller may look as haggard as a lifeguard at the end of a Memorial Day shift.
Despite its crucial role on the airplane, the propeller seldom gets more attention than a minor sunburn. Watch some pilots preflight and you'd think that the task requires little more than counting blades and looking for beachball-sized nicks. Think about it: Which would you rather lose, engine power or the prop itself?
Talk to prop shops and you get the idea that many owners feel that the propeller should be a lifetime device, and that the recommended time between overhauls has far less significance than the engine's TBO. In fact, the prop makers specify both a time-in- service TBO — usually 1,500 to 2,000 hours — and a calendar limit of five years. It's not too unusual for owners to acquiesce to a prop overhaul while the engine's undergoing a major, but how many times have you heard of the prop coming off a Part 91-operated airplane at the five-year mark? Not terribly often.
There are compelling reasons for looking into a constant-speed prop within that calendar-time limit. Corrosion is the prop's biggest and potentially ugliest bully on the beach. Because most designs of constant-speed props liberally mix and match materials inside the hub — aluminum blades mated to steel carriers riding in alloy hubs, for instance — the potential for dissimilar-metals corrosion is high.
Pete Paredes, service manager of Santa Monica Propeller, a repair facility 43 years in the business, confirms that corrosion is the most important reason to have a look-see into the constant- speed prop more than once a decade. "We often see 10-year-old props that have low time in service that are a mess inside. Owners don't seem to understand that just because it works and doesn't leak oil, you can't just leave the prop alone," says Paredes. "Also, we see a lot of props that haven't ever had service bulletin work performed. Many of the recommendations in the bulletins are more than make-work. They are real improvements."
So the first thing an owner should do to keep the prop shops happy is to overhaul every five years, regardless of time in service. Is this just an unnecessary expense? Not really. Especially if you consider that a good overhaul — which should run about $1,300 for a three-blade constant-speed prop — will usually implement all the evolutionary upgrades available. Moreover, money spent on proactive maintenance might be cheaper in the long run. (Aggressive marketing on the part of Hartzell and McCauley has resulted in many owners' upgrading to entirely different props at overhaul time, a good thing in Paredes' eyes. "The newer props are much better inside; they're much simpler. Any time you can simplify a mechanism, you're better off," he says.)
In the meantime, there are some steps you can take to preserve the life of your prop. Blades suffer great stresses, and it's critical to ensure that they are free from nicks and chips that could precipitate greater cracking and, in the worst case, departure of some part of the blade. We've long been taught to run our hands or fingernails along the leading edge to look for nicks, but you can take that a step further. At your periodic inspections — like the oil change or 100-hour — make sure that any pitting of the blades is lightly filed out. Corrosion at the bottom of the pits will weaken the metal and cause cracking. Letting the edges go until they look like the surface of a rippled potato chip is no way to improve blade life.
Likewise, keep your eyes peeled for signs of oil or grease leakage. A red-dyed oil showing up on the blades or cowling indicates an oil-filled hub leaking, perhaps around the blades or at the piston dome, and is reason for immediate attention. Grease staining — which is normal within the first 20 hours or so of a prop overhaul — should be viewed with concern, especially if it stops suddenly. It's more likely that the bearings have run out of grease than that a miracle cure has taken place. Engine oil making a grand appearance obviously indicates a leak at the crank flange or prop dome; either way, the source should be investigated without delay. It could be a minor leak at the flange or something as potentially troublesome as a crack in the hub. (A quick way of finding the source of leaks is to cleanse the area thoroughly, fly the airplane briefly, and then use the powder developer from a crack-checking system to locate the point of discharge.)
Inspect the prop spinner and backing plate carefully, too, because cracks here will quickly propagate into missing chunks and an attention-getting imbalance. Obviously all the hardware should be tight, with locking nuts or cotter pins securing everything. Look for fretting of the prop bolts and ensure that the right bolts are in place; there have been cases where bolt-bin stragglers have found new homes where they don't belong.
As much as you can coddle your prop, it's almost inevitable that you'll pick up a rock now and again — so be looking for serious nicks and chips. Advisory circular AC 43.13-1A outlines the procedures for dressing nicks in aluminum props and spells out general guidelines for rejection of a prop blade. (Of course, dressing the prop is the province of the A&P mechanic, although you can do it with your local wrench's supervision.) The basic idea is to dress the nick out to the bottom of the injury, which will keep stress risers from forming. Then the divot must be dressed smoothly into the material around it. While you're filing merrily away on the blades, keep in mind that the propeller has a minimum specified chord and thickness, so it behooves you to trim just as much as necessary to get to the bottom of the blemish. Many blades are rejected at overhaul time because they've been filed undersize. Also be sure that if you have to shorten one blade due to damage, you trim the opposing blade or blades accordingly.
You must keep the blades the same length for balance, but it's also a good idea to have the prop dynamically (on the airplane) balanced at every annual. You might also want to have the prop rebalanced if the engine's vibration signature changes suddenly, but make certain that all other causes have been investigated — like a change in ignition timing, sagging engine mounts, or interference between the engine and airframe. Prop balancing is a fine-tuning measure, not a cure-all. Also, be sure to have the accuracy of your tachometer checked to ensure that you are not operating the engine continuously in a yellow or red arc.
Finally your prop has lived to its fifth birthday or the engine major. A reputable shop will disassemble the prop and carefully examine all components for corrosion and cracking. The blades will be checked against the manufacturer's recommendations for size. The blades will be dye-penetrant inspected and all steel parts will be magnafluxed. Assuming your blades pass muster, they will be surfaced, reprofiled, and shaped to match each other. The hub will be similarly inspected and either alodined or painted. Many parts inside the hub will be new, including all gaskets and high-wear parts. Finally, the blades will be individually rebalanced — cavities in each blade accommodate lead wool for the purpose — and installed in the hub.
Then the prop will be reinstalled — followed by dynamic balancing — for its many more days in the sun.
FAA Information and Services,
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