Plane & Pilot
Tuesday, January 26, 2010

Upgrade Your Plane! Part III

Firewall forward—life after TBO

The 1971 Piper Cherokee PA28 received a paint, panel and engine upgrade.
The field overhaul is unique among the aforementioned options because it doesn’t use 100% original equipment parts. Field overhauls are done by independent businesses, and they’re free to substitute parts that aren’t original equipment. While not OEM parts, aftermarket parts must be manufactured by a company authorized by the FAA with Parts Manufacturer Approval (PMA) to ensure each part is certified for installation on engines as a direct substitute for the OEM part. The use of PMA parts may help the field overhauler further reduce price, provided that the PMA parts are less expensive than the OEM parts. Generally, the consumer has no way of knowing which engine parts are OEM versus PMA unless the overhauler produces a complete parts list from the overhaul.

Consequently, there may be less than 100% OEM parts in a field-overhauled engine, which is okay, because the PMA parts are FAA-certified. Additionally, the field overhauler may use or reuse more cores than the factory overhaul if the subject parts are deemed to be “serviceable” or within the specified service limits. But remember, the engine manufacturer only publishes service-limit data/specs, not proprietary manufacturing specs, so the tolerances on a field overhaul may not exactly match the OEM spec on a manufactured or rebuilt engine.

Furthermore, just as with a factory overhaul, by definition, the field overhaul isn’t a zero-time engine. Thus, if the engine had 2,132 hours on it before the overhaul, it still has 2,132 hours on it after the overhaul. Although the engine may be rightfully positioned as a “zero-time-since-overhaul” engine, it’s still a 2,132 total time engine. This means that no matter how shiny and new the overhauled engine appears after the overhaul service is performed, the entire engine history follows the engine. Even if the engine was completely overhauled after a prop strike, the history on that engine’s data plate will reflect the prop strike.

• Do Nothing: Finally, one option is to simply turn a blind eye and do nothing. Since TBO isn’t a milestone for engine repair or replacement, the operator may opt to run the engine beyond TBO. While operating an engine beyond the manufacturer’s suggested limit is a personal decision, there are steps you can take to increase your odds of continued safe operation until such time when the engine itself decides that it’s time for an upgrade.

Since the FAA-mandated annual inspection includes a cursory engine inspection, early signs of engine wear may be detected through the basic compression test. In between the annual inspections, the prudent way to monitor engine health is through oil analysis. By continually monitoring the engine oil (with increased frequency as the engine clock ticks up), the amount and type of contaminants in the oil will indicate which engine parts are wearing and at what rate (normal wear versus trends that may indicate a component-specific failure is imminent). In fact, it may be beneficial to periodically submit an oil sample between oil changes if you push beyond TBO. In the grand scheme of things, oil analysis is inexpensive and can provide advanced warning about the health of your engine that you can’t possibly know until the fan stops turning.

Value Proposition
Clearly each aircraft owner must individually determine the most important elements of the upgrade—price, warranty term, warranty coverage, engine time, new part content, residual value, etc.—to determine which represents the best value. In my case, it became obvious after plotting the options in a matrix (page 57) that purchasing a factory-rebuilt engine was the clear winner. Purchasing a fully airworthiness directive– and service bulletin–compliant zero-time engine built on the same line as factory-new engines, under the scrutiny of an AS9100 Quality System and FAA production certificate, and using 100% OEM parts—all for less money than a brand-new engine—seemed like the greatest value. Plus, I was the beneficiary of some production improvements, like roller tappets that a field overhaul may not have included. Any way you spin it, upgrading an engine is a decision that you’ll be living with for a long time (almost as long as it will take for the bite in your wallet to heal). So the better you understand the differences between your options, the better informed your firewall-forward upgrade decision will be and the more satisfied you’ll be as an aircraft owner.

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