Sudden cooling is detrimental to the good health of the piston aircraft engine. Lycoming Service Instruction 1094D recommends a maximum temperature change of 50˚ F per minute to avoid shock cooling of the cylinders.
Operations that tend to induce rapid engine cooldown are often associated with a fast letdown and return to the field after dropping parachutists or a glider tow. There are occasions when Air Traffic Control also calls for fast descents that may lead to sudden cooling.
The engine problems that may be expected when pilots consistently make fast letdowns with little or no power include:
- Excessively worn ring grooves accompanied by broken rings
- Cracked cylinder heads
- Warped exhaust valves
- Bent pushrods
- Spark plug fouling
Generally speaking, pilots hold the key to dodging these problems. They must avoid fast letdowns with very low power (high-cruise RPM and low manifold pressure), along with rich mixtures that contribute to sudden cooling. It is recommended that pilots maintain at least 15” MP or higher, and set the RPM at the lowest cruise position. This should prevent ring flutter and the problems associated with it.
Letdown speed should not exceed high cruise speed or approximately 1,000 feet per minute of descent. Keeping descent and airspeed within these limits will help to prevent the sudden cooling that may result in cracked cylinder heads, warped exhaust valves, and bent pushrods.
The mixture setting also has an effect on engine cooling. To reduce spark plug fouling and keep the cylinder cooling within the recommended 50˚ per-minute limit, the mixture should be left at the lean setting used for cruise and then richened gradually during the descent from altitude. The lean mixture, maintaining some power and using a sensible airspeed should achieve the most efficient engine temperatures possible.
The operating techniques recommended in this article are worth consideration as they will be a positive step toward saving dollars that might be spent on maintenance. Whatever the circumstances, pilots must plan their flight operations so that the potential damage caused by sudden engine cooling can be avoided.
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