EAA Sport Aviation - April 2010

MANUFACTURER’S GUIDANCE

What do engine manufacturers say about the importance of compression test results and oil consumption? If you fly behind an engine built by Teledyne Continental Motors (TCM), the manufacturer’s guidance is remarkably consistent with Bob Moseley’s.

COMPRESSION TEST RESULTS

The bible for compression testing of TCM cylinders is TCM Service Bulletin SB03-3. In it, TCM throws the old 60/80 standard right out the window, and establishes a variable go/ no-go compression limit based on what your mechanic’s compression test gauges read when hooked up to a calibrated standard known as a “master orifice tool.” For most compression testers, the no-go compression limit is somewhere in the low 40s. Thus, according to TCM, a cylinder that measures 50/80 is every bit as airworthy as one that measures 75/80.

SB03-3 goes even further, saying that even if a cylinder measures below the no-go limit (say 35/80), it should not be removed from the engine unless a borescope inspection of the cylinder reveals some obvious reason for the low compression (like a burned exhaust valve or excessive barrel wear). If the low-compression cylinder looks okay under the borescope, TCM says the engine should be returned to service, flown for a while, and then the compression retested. Only if the cylinder flunks the compression test a second time should it be pulled for repair.

OIL CONSUMPTION

How much oil consumption is too much? TCM has two things to say on this subject. In the type certificate data sheet (TCDS) for most of its engines, TCM establishes a maximum acceptable oil consumption rate by means of a formula based on engine horsepower. For a 300-hp engine, that number is about 1. 5 quarts per hour! (For lower-power engines, it is proportionally less.)

TCM also has a service bulletin that suggests that oil consumption greater than 1 quart per three hours is “cause for concern” and warrants further investigation to determine the cause.

This guidance from TCM is astonishingly liberal. It’s clear that TCM—like Moseley—does not consider low compression or elevated oil consumption to be major airworthiness issues. If your TCM engine has compressions in the low 50s and is burning a quart of oil every four hours, TCM says “nothing to worry about.” —Mike Busch

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