Sunday, March 7, 2010

Watch Your Language!

When an aircraft component becomes inoperative or unairworthy, we usually have a number of options. We can replace the bad component with a new one, a rebuilt one, an overhauled one, or perhaps send out our defective component to have it repaired. These four words -- new, rebuilt, overhauled and repaired -- have distinct, specific meanings in the context of aircraft maintenance, and it's important to understand precisely what they mean and how they differ:
  • New means never used. Dimensionally, a new component meets new fits and limits (obviously).
  • Rebuilt means a previously used component that has been overhauled to new fits and limits (possibly using approved oversize or undersize parts) by the original manufacturer. (For example, only the TCM factory can "rebuild" a TCM engine, although any A&P mechanic can "overhaul" one.)
  • Overhauled means disassembled, cleaned, inspected, repaired as necessary, reassembled and tested in accordance with the manufacturer's approved technical data (normally the overhaul manual). The word "overhaul" implies conformance to service limits, not necessarily new limits, so if you want new limits you have to specify "new-limits overhaul." (A new-limits overhaul is essentially the same as "rebuilt" except that it doesn't have to be performed by the original manufacturer.)
  • Repaired means inspected and repaired as necessary ("IRAN") to restore the inoperative compoment to proper working condition. This term implies nothing about fits and limits, because there is no requirement to measure anything when performing a repair. One could, for example, remove a cylinder, replace the exhaust valve and guide, and then put the cylinder back on the engine without measuring anything, and call it a "repair." A repair differs from an overhaul primarily in that there's no obligation to follow the fits, limits, mandatory replacements, and other procedures in the manufacturer's overhaul manual.
The words "overhauled" and "rebuilt" are defined in FAR 43.2, and have very specific regulatory meanings as described above. If a mechanic documents that something is "overhauled" and hasn't complied with every jot and tittle of the overhaul manual, he can lose his A&P certificate. However, if he documents it as being "repaired," he can do as much or little as he sees fit to do, so long as he is satisfied that his repair work is airworthy.

In short, if you ask for a "repair" you give the mechanic or technician considerable discretion to do only as much work as he believes needs to be done. If you ask for an "overhaul" you eliminate his discretion and require him to do everything precisely "by the book."

Repair is almost always the lowest-cost method to get a problem resolved. Often, the cost of having something overhauled will be much higher than the cost of having it repaired. For example, a propeller "overhaul" is typically twice as expensive as a "reseal repair." In some cases, having a malfunctioning gyro flight instrument "overhauled" can cost ten times as much as having it "repaired." The "O-word" is one of the most expensive and overused words in aviation maintenance. It is invariably a waste of money to have something overhauled if a simple repair will suffice -- often a lot of money!

Of course, the "R-word" is even more expensive than the "O-word" and the "N-word" is the most expensive of all. The object of the game is to use the least expensive word that will get the job done. So be careful to use these words carefully when approving maintenance work on your aircraft.

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