EAA Sport Aviation

MIKE BUSCH

COMMENTARY / SAVVY AVIATOR

Trust, but Verify
Before you approve any costly or invasive repair to your aircraft, make sure the discrepancy is real

ARGUABLY THE WORST PART of being an aircraft owner is the ordeal of putting your plane in the shop every 12 calendar months and then bracing yourself for bad news. Over the past four years, my company has managed more than 700 annual inspections and more than 6,000 lesser maintenance events, so it’s a safe guess we’ve dealt with tens of thousands of mechanical discrepancies. Sometimes the bad news is painful and costly to the aircraft owner. But surprisingly often it turns out to be nothing more than a false alarm.

A couple of months ago, for example, a client’s single-engine airplane went into a big, well-known Florida service center for its annual ordeal. Within hours, the shop reported that cylinder No. 3 measured 38/80 on the compression test, with air audible at the exhaust tailpipe. (This was a Continental engine, and the master orifice no-go limit was 46/80.) The mechanic attempted to “stake” the No. 3 exhaust valve but was unable to improve the reading. The shop said the No. 3 cylinder needed to be removed and sent to a cylinder shop to be re-valved and honed, and then reinstalled with a new set of piston rings. The estimated cost of this work was quoted at $2,200.

We checked the airplane’s downloaded digital engine mon-
itor data and found that the No. 3 EGT appeared normal, with
no hint of the slow, rhythmic oscillations characteristic of a
burned exhaust valve. We asked the shop to inspect the No. 3
exhaust valve with a borescope, and it reported back that the
inspection results were “inconclusive.” Further questioning
revealed that the exhaust valve had a symmetrical appearance
under the borescope (see Figure 1), with none of the lopsided,
green-tinged appearance characteris-
tic of a leaking exhaust valve.

The shop said the No. 3 cylinder needed to be removed and sent to a cylinder shop to be re-valved and honed, and then reinstalled with a new set of piston rings.