Then rotate the prop until the number one cylinder is at TDC (not the 20- to 25-degree timing mark). A spark plug dropped on concrete should be considered unusable due to possible internal damage. Immediately after shutdown, remove a spark plug from each cylinder, using care to not burn yourself or drop a spark plug. They are not approved for use on Lycoming or Rotax engines. Some testers designed for larger engines have an orifice size of. 040-inch orifice for all Lycoming and Rotax engines. Cold test results may be erroneous and without merit. A proper test should be conducted on a warm engine. Lycoming Service Instruction 1191A describes the test for their engines and how to interpret the results. The results of this test for each cylinder should be recorded in the engine logbook at least yearly. The results of the test are shown by placing the pressure the cylinder will hold on the top of the fraction (numerator) and the reference pressure on the bottom (denominator).Ī typical result would be shown as “#1-76/80” for an engine where the number one cylinder will hold 76 psi against a reference pressure of 80 psi. ![]() Ideally the differential is small-10 psi or less. The name differential compression test comes from the act of comparing the pressure a cylinder will hold to a reference pressure-typically 80 psi-thus allowing us to see the differential between the two pressures. The compression test measures the ability of an aircraft engine cylinder to hold air pressure at top dead center (TDC). Because of this, we sometimes do it by rote and forget why we perform this test, its benefits, and shortcomings. ![]() The differential compression test is a timer-honored tradition in aviation. The reference pressure will be on the left gauge, and the pressure in the cylinder will be on the right gauge. ![]() Longtime building buddy Ed Zaleski (left) performs a differential compression test on cylinder #1 while a friend hangs onto the prop to keep it from turning and injuring someone.
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