Propellers are unique in the world of aircraft maintenance because they tend to defy the symptom- or condition-based maintenance philosophy that works so well with engines and airframes. With engines, we can use borescope inspections, compression testing, oil filter inspections, and oil analysis to evaluate the health of the engine as the years and hours build. Using these indicators, the risk of catastrophic failure is low and we can monitor most forms of wear and corrosion to protect our investment. The same can be said of airframes; field inspection and wear monitoring is the standard to reduce risk while also making sound economic decisions. This approach works well with fixed-pitch propellers because all inspections are external, but constant-speed propellers are a different animal entirely.

The challenge with symptom- or condition-based maintenance of constant-speed propellers is that there are a fairly small number of external indicators that a mechanic can use to evaluate the health of the propeller. We can certainly inspect the blades externally, check for play, look for leaks, and evaluate if the propeller is operating properly. However, none of this tells us much about what is happening inside the hub and how close we are to failure. The good news is that catastrophic propeller failures from internal wear or corrosion are extremely rare. In practical terms, you are generally not putting your life at risk by exceeding time between overhauls (TBO) on your propeller. You may, however, be putting your wallet at risk.

There are several factors that work against us when delaying an inspection or overhaul of a propeller.

Cost

Constant-speed propellers and the parts that make them up are very expensive. Anything that we can do to preserve their service life is paramount because the cost of replacement adds up quickly. A standard overhaul parts kit for a McCauley three-blade propeller, for example, costs about $1,500. Adding a piston or other internal wear component could easily double that in parts cost alone.

Modularity

While engines and airframes have many components, propellers have relatively few. Individual blades can be replaced (at $2,500 or more each), but the hub is the core of the propeller and carries the serial number of the propeller as a unit. If the hub cannot be repaired, a replacement will be even more expensive and may not be financially practical. This makes it critically important that the major propeller components be carefully maintained and inspected to remain serviceable.

Tolerances

The difference between a repairable propeller and a scrapped one can be a thousandth of an inch. I discovered that years ago when overhauling my last propeller. Upon inspection, corrosion was found in the snap ring grooves that retain the blades. There is a process that allows for the corrosion to be removed and oversized snap ring retainers to be installed at fairly low cost. However, this is only possible if the hub remains within specific tolerances. My prop was caught just in time to make the repair possible. Had I waited any longer, a $3,000 overhaul would have turned into a $15,000 prop replacement.

Mark Hahn is general manager of Sensenich Propeller Service Inc. in Pennsylvania. With locations in Pennsylvania and Georgia, Sensenich has been in the propeller maintenance business for over 50 years (and has seen just about everything). According to Hahn, catching wear, corrosion, or other propeller issues in time can make all the difference in the service life of the propeller, and the resulting hit to your wallet. An added benefit is that a propeller repair station can dress out nicks, corrosion, and other minor blade damage in a way that simply cannot be done in the field. I recently sent a propeller to Sensenich that had a fairly noticeable "dip" where a nick was dressed out in the field. After overhaul, the blades looked like new and it was impossible to identify where the field repair had been done. For owners of fixed-pitch props, Sensenich has a specialized, hydraulic tool to set the blade angles for pitch adjustments. It's the same machine used by the propeller manufacturers—a vastly different process than manually twisting the blades into submission using six foot-long hand tools.

The overhaul process

The overhaul process is complex, but it can be broken down into four major parts: cleaning, inspection, repair, and reassembly. Upon receipt, the propeller is disassembled and all parts catalogued, stripped, and cleaned as necessary. Then the parts are dimensionally checked to tolerances and carefully inspected using dye penetrant and other methods to look for cracking and damage. The blades are profiled to remove surface damage and reestablish the proper shape and airfoil, and other parts are replaced or repaired as necessary. According to Hahn, most propeller blades have enough material on them to make it through the re-profiling process of at least three overhauls, barring any serious damage. Finally, everything is painted, reassembled, tested, and statically balanced. The product you get back looks as good as new with a logbook entry, restarting the TBO clock.

Overhaul vs. IRAN vs. reseal

If you want to protect your propeller investment while minimizing the cost of maintenance, the "Inspect and Repair As Necessary" or IRAN process may be your best bet. While most IRAN processes do not include re-profiling of the blades, they do include replacement of wear components such as O-rings and seals; a thorough visual inspection ensures that you catch any issues in a timely manner. In addition to cost, an upside is that you won't be removing significant material from your blades. However, this is only possible if you don't have significant erosion, corrosion, and wear that really should be removed. Also, it won't add the same "logbook value" to the aircraft if you are considering selling soon. That said, performing an IRAN is excellent insurance against small problems escalating into a trip to the junkyard for your prop. A reseal, on the other hand, is more of a quick fix. In a reseal, the O-rings and seals are replaced in an effort to simply fix a prop that is leaking oil. Unless there is an obvious problem spotted during the process, you don't have the benefit of a thorough inspection to protect your investment.

One final thought: If you've ever removed a constant-speed prop that's been on for some time, you've seen how much oil and lead sludge can build up inside the hollow crank. Since the prop generally stops in the same position every time, this can build up asymmetrically and affect propeller balance. Inside the prop, the effects of excessive sludge buildup can cause problems for the seals, oil passages, and prop mechanism.  It's just another reason to consider an overhaul or IRAN on your propeller.

The decision to overhaul, IRAN, or reseal a propeller depends on a number of factors including its exposure to the elements, erosion, damage, hours, and time in service. A Florida airplane stored outside for five years may need its prop inspected more urgently than an Arizona airplane stored inside for 15 years. The key is to protect your investment and make a plan that's sensible for you. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.