Light aircraft, which are for the most part rugged and reliable, rely more upon the quality and durability of components than redundancy of those parts. After all, singles have but one propeller, one crankshaft. Without the redundancy of multiple propellers and crankshafts, we must rely on the strength of those individual parts.
At some point the aircraft owner will need to replace these parts – an endeavor that is made difficult more often by availability issues than price. Replacement parts, by and large, come from the airplane manufacturers and their availability often is directly tied to the company's health. In the middleman role are parts distributors, who are the mechanic's link to the factory parts supply and who maintain a stock of common components. But first, the good news about new parts. According to a brief survey of parts houses and maintenance facilities, the majority of components needed to keep the light airplane fleet flying is readily available.
Before we get into the nuts and bolts, so to speak, understand that while model-specific parts (control surfaces and cowlings, for example) must come from the manufacturer or a supplier that has received Federal Aviation Administration approval in the form of a Parts Manufacturer Approval (PMA. The key here is that the part must conform to some sort of standard, usually a military specification, or mil-spec.
Where the parts picture turns from sunny-day bright to overcast gray is in airframe-specific parts. Owners of Cessnas, Beeches, Mooneys, and Pipers are in better shape than most because the parent companies are around to produce replacements. Cessna, Mooney, and Beech reportedly have a surprisingly good stock of parts on hand.
If the part is not in stock, one alternative is to have the factory make you one. According to one large distributor of Cessna parts, that company will make them to order, without a minimum purchase; for example, you don't have to buy two elevator skins if you only need one. But the wait could range from "next week" to "we'll call you when we find the tooling."
The spares outlook for more unusual and/or older airplanes often depends upon the degree of support provided by an owners' group. The Navion Society, to use one example, has in the past pressed vendors to make spares long out of production. The same can be said for the Cessna Pilots Association, which is now working on finding replacements for engine instruments originally produced by Stewart -Warner and found in a great many Cessnas. Stewart-Warner pulled out of the aircraft market, making replacement instruments and overhaul kits scarce. Also, the Mooney Aircraft Pilots Association helped convince a supplier to provide overhaul kits for the hydraulic flaps on early M20s; for a time, the only option was to convert the system to electric actuation at considerable cost.
Another source of new parts comes from companies that have obtained a PMA. A supplier of parts for the original airframe builder produces its wares using the manufacturer's production certificate, which makes the airframe builder responsible for a measure of quality assurance. It also provides a channel of product liability through the airframe manufacturer even though someone else built the part. Not so for a company building parts with a PMA.
An outside supplier can build replacement parts independent of the airplane's original manufacturer provided it proves to the FAA that the parts match exactly the specifications of the factory and these specifications will be maintained through an FAA approved quality control program. The cost of providing this proof can drive start-up costs sky high — and often deter companies looking at low-volume items from entering the market altogether.
Of course, if you can't find the part new, you could buy used. For some owners on a limited budget, this often is the only recourse to high-priced new-manufacture spares. This road, though often economical, is rockier than a Beirut expressway, and if ever the advice caveat emptor applies, it is here.
Used major components are supposed to carry a certificate of serviceability, otherwise known as the yellow tag. In fact, there are many types of yellow tags, but the most common are inspected, repaired, and overhauled. All the inspected tag guarantees (and some mechanics don't believe the tags guarantee anything, that the reputation of the shop is far more important than a piece of paper) is that the part was working, or serviceable, when it was removed from its previous installation. There is no indication of how much longer the part will work or even if it was originally the right part for the job. Inspection can entail nondestructive testing to determine its condition.
Should some aspect of the part be unserviceable, it can be fixed and would carry the repaired title. Along similar lines, a part might be overhauled, with any worn parts replaced or brought to new limits, and the yellow tag would indicate this. Both repair and overhaul of used parts must be performed by a repair station or signed off by an A&P with inspection authorization.
The origin of the overhauled or repaired part is of greater significance than the type and quality of the overhaul. If the part had simply worn and been replaced with a new one, it could, after an overhaul, live a long and happy life in your airplane. But if the part had been in a wrecked airplane, guarantees go out the window. Heat, as from a post-crash fire, could weaken heat-treated parts, as an example.
What's more, although the tag might say the part was from a certain make/model/year airplane, there's no way to be certain it was the right part for that airplane in the first place. Most shops will not accept parts from salvage unless they know the history of both the component and the company providing it. Naturally, the owner/operator is the one responsible for having the right parts on the airplane. The best advice we could gather was to find a reputable distributor or salvage company and believe, as Lou Reed says, none of what you hear and only half of what you see.
Fortunately, the situation has not degenerated to desperate "Mad Max" scrounging of parts, at least not for the more popular members of the general aviation fleet. And while there are arguably no unimportant parts in these airplanes, at least for now there are few unobtainable parts, either.