What It Looks Like

You're conducting a preflight inspection and open the cowling to scrutinize the engine when you notice numbers scrawled in crayon on each of the metal valve covers - 75 on one, 68 on another. A closer inspection reveals that these numbers appear on top of other, similar numbers. What are these all about?

The numbers represent the compression in each cylinder. Each time an airplane undergoes a scheduled inspection - annually, or every 100 operating hours if the airplane is rented or used for commercial operations - a mechanic checks the compression in each cylinder. The object is to determine the extent of wear in the cylinder's piston rings and valves.

One of the cylinder's spark plugs is removed and, using a special tool, 80 pounds of air is pumped into the cylinder. The tool seals the plug opening so the compressed air cannot escape back through it. A gauge displays the pressure in the combustion chamber that remains after leakage across the rings and valves. That figure is what is written on the valve cover. A 75, for example, means the compression gauge showed that 75 pounds of air remained in the combustion chamber after 80 pounds were pumped in. Writing the compression number on the valve cover is a quick and easy way for a mechanic to see if the compression in the cylinder has changed since the last check.

Higher compression means more power. In general, any number in the high sixties on up is considered good. The minimum compression specified by engine manufacturers is 60 pounds per square inch (a 25 percent loss of compression), so a reading of 60 or below can indicate significant piston ring and/or valve wear. In that case, the mechanic likely will conduct additional tests to get a better idea of the condition of the cylinder walls, piston crown, piston rings, and valves.

Compression checks are not conclusive. For example, excessive oil in a cylinder can help seal rings and valves, which can result in a good compression check despite wear. That's why if there is any other symptom of a problem, the mechanic should do more extensive tests.