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9/27/01 AC 43.13-1B CHG 1 a. Differential Compression Test. The most common type of compression tester cur- rently in use is the differential pressure-type tester. It provides a cross-reference to validate the readings obtained and tends to assure that the cylinder is defective before it is removed. Before beginning a compression test, consider the following points: (1) When the spark plugs are removed from the engine, identify them to coincide with the cylinder and location from which they were removed. Close examination of the plugs will reveal the actual operating conditions and aid in diagnosing problems within each individual cylinder. (2) The operating and maintenance rec- ords of the engine should be reviewed. Rec- ords of previous compression tests are of as- sistance in determining progressive wear con- ditions and help to establish the necessary maintenance corrective actions. b. Differential Pressure Compression Test. The differential pressure tester is de- signed to check the compression of aircraft en- gines by measuring the leakage through the cylinders caused by worn or damaged compo- nents. The operation of the compression tester is based on the principle that, for any given air- flow through a fixed orifice, a constant pres- sure drop across that orifice will result. The restrictor orifice dimensions in the differential pressure tester should be sized for the particu- lar engine as follows: (1) For an engine cylinder having less than a 5.00-inch bore; 0.040-inch orifice di- ameter; .250 inch long; and a 60-degree ap- proach angle. (2) For an engine cylinder with 5.00 inch bore and over: 0.060 inch orifice diame- ter, .250 inch long, 60 degree approach angle. (3) A typical schematic diagram of the differential pressure tester is shown in figure 8-1. FIGURE 8-1. Schematic of differential pressure com- pression tester. (4) As the regulated air pressure is ap- plied to one side of the restrictor orifice with the air valve closed, there will be no leakage on the other side of the orifice and both pres- sure gauges will read the same. However, when the air valve is opened and leakage through the cylinder increases, the cylinder pressure gauge will record a proportionally lower reading. (5) While performing the check the following procedures are listed to outline the principles involved, and are intended to sup- plement the manufacturer’s instructions for the particular tester being used. Par 8-14 Page 8-7PDF Image | AFS-640
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