Saturday, November 21, 2009

High CHTs and cracked cylinder heads

A member of a German flying club wrote me about a Robin DR-300 that the club used in about a 50-50 mix of cross-country flying and towplane operations for the club's sailplanes.

When towing gliders, the airplane climbs at full power and very slow airspeeds, and CHTs regularly register in the high 400s Fahrenheit, close to Lycoming's 500F CHT red line. Once the glider releases, the engine is throttled back to idle and descends rapidly to land and hook up for the next tow.

The Robin's Lycoming O-360-A3A engine is now at 1,200 hours, just 60% of TBO. One cylinder started to show declining compression readins, so the mechanics reduced the compression-test interval from 100 hours to 50 hours. At the next check, the cylinder compression had fallen into the 40s, so the cylinder was removed and found to have a cracked cylinder head, with cracks running between the top spark plug hole and the exhaust valve seat.

    The club member's questions for me:

    1. Why did the cylinder head crack?

    2. Should the other three cylinders be removed?

    3. Should the mechanics have caught this earlier?

    I explained that any non-ferrous metal component (like an aluminum alloy cylinder head) will eventually crack if subject to repetitive stress for a sufficient period of time. The component will crack sooner if the repetitive stress is more intense and/or if the operating temperature is greater. Extended full-power operation during glider tows obviously increases the intensity of the stress, and the very high CHTs reduce the tensile strength of the cylinder head. (The head's tensile strength is reduced to 50% of its room-temperature value at a CHT of 400F, and to about 33% at 500F.)

    Although towplane operations are inherently very stressful on the engine, I suggested that cylinder longevity could probably be increased by making flatter, higher-speed climbs and less rapid descents. However, as a glider pilot myself, I understand that gliders generally have a fairly low Vne, and that glider pilots usually want to spend minimum time on tow, so engine longevity considerations may have to be subordinated to operational demands.

    I recommended to the German club member that the other three cylinders NOT be removed. Instead, they should be inspected for cracks internally using a good borescope and externally by direct visual inspection. Just because one cylinder cracked does not mean that the others will crack any time soon. In the 22 years I've owned and flown my 1979 Cessna T310R, I have had to retire 2 of the 12 cylinders because of head cracks. One crack occurred at about 2,000 hours time-in-service, and the second did not occur until about 4,500 hours. The remaining 10 cylinder heads have not cracked (yet), although they all will do so if kept in service long enough. It certainly would not have made any sense for me to pull all 12 cylinders when the first cylinder head crack occurred about 18 years ago!

    Finally, I suggested that it sounded like the club's mechanics had done a good job, but that had they performed regular borescope inspections of the cylinders they probably would have detected the cracks earlier. TCM requires at least annual borescope inspections (TCM SB03-3), but Lycoming does not. Nevertheless, I believe frequent and regular borescope inspections are an essential part of engine condition monitoring.


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