Differential Compression Test (AC 43.13-1B)

The differential pressure tester is designed to check the compression of aircraft engines by measuring the leakage through the cylinders caused by worn or damaged components.

The operation of the compression tester is based on the principle that, for any given airflow through a fixed orifice, a constant pressure drop across that orifice will result.  The restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:

(Although AC 43.13-1B was revised in 1998, the following information was revised in Chg. 1 in September 2001. The identification criteria was revised from using “engines with a certain cubic inch displacement” to cylinder bore)

(1) For an engine cylinder having less than a 5.00-inch bore; 0.040-inch orifice diameter;
.250 inch long; and a 60-degree approach angle.
(2) For an engine cylinder with 5.00 inch bore and over: 0.060 inch orifice diameter, .250 inch long, and a 60 degree approach angle.

Remember, some of the smaller production engines have 5 inch or larger cylinder bores (e.g. Lycoming O-320-A1A has a cylinder bore of 5.125 inches).  Therefore, an orifice of 0.060 diameter should be used to perform the compression test.

See AC 43.13-1B, Chapter 8, paragraph 8-14 for more information.