Turbocharged and Fuel Injected Aircraft Engines
Shock cooling is often thought of as a significant power reduction, aggravated by simultaneously dropping the nose, both causing the engine to cool rapidly and unevenly. The front of the engine is exposed to more cooling air than the rear of the engine. This is a frequent situation and certainly undesirable. However, there are two other forms of shock cooling that need to be understood. They are reduced power settings at full rich mixture, and excessive temperature variations during
1. Reduced Power Settings
At reduced power settings during descent, approach, and landing, the fuel flow per horsepower increases. A full rich mixture results in excess cold fuel entering the hot cylinders. The result is rapid and uneven cooling in the area of the fuel injection nozzle. This condition can cause a crack between the fuel injection nozzle and the spark plug hole.
Corrective action is to maintain an acceptable lean mixture during descent and landing, usually 1200° F to 1400° F. This can best be accomplished using a range marked and calibrated E.G.T. There should normally be no need to fully enrichen the mixture until on the runway with the throttles at idle.
2. Ground Maintenance Uneven Cooling
Although running the engines on the ground with the engine cowling off for maintenance is more likely associated with poor cooling air distribution and heating, unfortunately it is also an unfavorable conditions for shock cooling. Rapid and uneven power testing causes uneven cooling. During a ground run, the cooling air is not being evenly deflected and directed around all the cylinders.
If you must make a brief ground run, consider a limit of two minutes at 1200 rpm, 400° F CHT, and 200° F oil temperature to be the maximum allowable. Power test runs should not be considered.
For additional RAM information
RAM Maintenence Tip "Ground Runs At High Power - Caution."