Detonation is Expensive

Prevention Requires Proper Fuel Flow During Takeoff

Detonation is the almost instantaneous release of heat energy from fuel in an aircraft engine. It is caused by the fuel and air mixture reaching its critical pressure and temperature. An explosion takes place rather than a smooth burning process. Detonation will cause premature cylinder damage, primarily in the valve train area. Severe cases of detonation will actually cause piston melt-down. Detonation can be avoided with proper attention to takeoff power fuel flow.

At RAM, we have noted that many Cessna T210 and T206 fuel flows have been adjusted downward to alleviate the possibility of an over-rich condition during the initial stages of the takeoff. An aircraft mechanic has been instructed to set the engine adjustments such that when the pilot places the panel mixture control full forward and applies takeoff power, the fuel flow goes up to red line only, rather than go over it slightly.

This just right situation is temporarily satisfactory, but shortly after rotation while climbing, the engine starts to reach in-flight operating temperatures. This is where detonation will occur. When the fuel flow (at full rich panel mixture control) is set to be just right during a ground run, the engine will be lean 10 lbs. to 20 lbs. once in-flight operating temperatures are reached. The elapsed time in attaining these temperatures will vary according to atmospheric conditions. Note: It is not possible to simulate in-flight temperature distribution on the ground.

To ensure proper fuel flow at all times, the pilot must be included in handling fuel management. The panel mixture control, at full rich, should permit fuel flow to exceed red line by several gallons. The pilot then has the latitude to add fuel or reduce fuel to ensure 186 lbs./hr. (32 gal/hr) is attained and maintained during takeoff power.

This pilot-in-control procedure is in conformity with the Cessna Owners Manual, which states “On any takeoff, the manifold pressure should be maintained and throttle set to provide 36.5 inches Hg. Then for maximum engine power, the mixture should be adjusted during the initial take-off roll to 186 lbs./hr.” RAM considers it essential that all pilots follow the recommendations of their Cessna Owners Manual.

A related subject is the propensity to over boost during the takeoff roll (exceed 36.5 inches of manifold pressure). This is caused by less than in-flight operating temperature of the oil in the wastegate actuator. It takes time for the engine systems to absorb and react to the heat that is being added so quickly. Pilot adjustments of the manifold pressure, during the early stages of takeoff, are considered normal and proper at RAM. The 310 hp Cessna Owners Manual for a Cessna T206/T210 can be valuable in helping you attain a good service life from your engine. Also, please see RAM Maintenance Tip “Shock Heating During Takeoff.”