Compression testing is a way to determine engine health. We use differential compression testing in aviation because it is a more reliable method of testing engine compression than direct compression testing used in automotive applications. We will take a look at the basics of differential compression testing and share some tips on getting the most out of your tests.
Equipment
Differential testing uses two gauges to measure pressure. Compressed air is applied to the tester through the regulator gauge. This gauge indicates the air pressure being applied to the tester. The air then goes through a calibrated restrictor to the cylinder pressure gauge. This gauge indicates the actual pressure in the cylinder. Any compression loss in the cylinder would be indicated by a pressure indication in the cylinder pressure gauge lower than that of the regulator gauge.
Testing the Tester
In differential compression testing, the tester is a critical element to ensure accurate test results are obtained. There are a few things we should keep in mind in regards to the tester to ensure it will give us the precise measurements we need.
Dead-end check. A good way to test your differential tester for proper operation is to “dead-end” it. You basically cap off the end of the tester that would normally go into the cylinder and apply regulated air pressure to the tester. You want to ensure that both the pressure regulator gauge and the cylinder pressure gauge stabilize out at the same pressure reading. Any difference in pressure indication could mean leaks in the tester or defective gauges. The tester can then be repaired as necessary.
Your tester could have a valve installed between the cylinder pressure gauge and the cylinder. You should close this valve in order to perform the check. Keep in mind, this is another area that can leak causing a difference in gauge readings during the unit test. The valve should be eliminated as a leak source before looking at the gauges for leaks or inaccuracy.
Storage
Another factor that can affect the accuracy of your tester is debris. Any contaminants such as dirt and oil can adversely affect the readings. The tester should be treated as a precision measuring device, not as just a hand tool. It should be kept clean at all times to help ensure accuracy of tests.
Proper Orifice
An important thing to keep in mind is to ensure you are using the proper orifice in your tester for the engine being inspected. As a general guideline, AC 41.13-1B states that the restrictor orifice dimensions in the differential pressure tester should be sized for the particular engine as follows:
Engines up to 1,000-cubic-inch displacement: 0.040-inch orifice diameter, 0.250 inch long, 60-degree approach angle.
Engines in excess of 1,000-cubic-inch displacement: 0.060-inch orifice diameter, 0.250 inch long, 60-degree approach angle.
Engine manufacturers may have other requirements for their engines. Be sure that you consult the manufacturer’s maintenance manuals and service bulletins for specific testing requirements.
Special Needs
You want to make sure you have a source of dry compressed air capable of providing a minimum line pressure of 125 psi with a minimum flow capability of 15 cubic feet per minute. In addition, you need to have a differential tester. Refer to the engine manufacturer for approved testers. In addition, for Teledyne Continental engines, depending on the tester you are using, you may also need a master orifice tool P/N 646953A to determine the Minimum Acceptable Leak Limit for your engine.
Warm Up That Engine
It is recommended that the compression test be performed on a hot engine. This ensures that the piston rings, cylinder walls, and other engine parts are well-lubricated and at operating clearance. “Before doing a compression check, make sure to run up the engine so that it gets up to normal operating temperatures,” says David Noe, airframe shop manager for Poplar Grove, IL-based Poplar Grove Airmotive.
Some Safety Notes
Magnetos should be grounded and fuel should be shut off prior to performing a test in order to make certain that the engine will not accidentally start.
“Ensure the mags are not hot,” shares Noe. “Also, it’s good practice to disconnect all spark plug leads prior to doing a compression test.”
Another important safety note is to use two people whenever performing a compression test. “Two people are better than one when doing a compression test,” says Noe. “You always want to be on the ball.” Before pressurizing a cylinder, make sure that your co-worker has a good grip on the propeller.
Another couple of safety items are important to keep in mind. Since two of you will be working on the test together, one to operate the tester and the other to hold the propeller securely, clear communication is essential. Also, make sure that neither worker is standing in the path of the propeller just in case it slips away during the test. In addition, ensure the area around the propeller is free from any obstruction that could interfere with the test or cause damage should the propeller accidentally get away from the person holding it.
Noe shares another safety tip. “We installed unique couplings on our compression test adapter. They are different from the couplings on all our air hoses. That way, it is virtually impossible to connect it to un-regulated shop air.”
Interpreting Readings
Now you will take a reading on the tester. This is annotated as the actual amount of pressure in the cylinder over the regulated pressure. For example, if the cylinder gauge indicates 60 psi and the regulated pressure indicates 80 psi, then your compression is 60/80. In general, 60/80 is the minimum acceptable leak limit. Keep in mind if you are working on a TCM engine, you will use the limit determined using the Master Orifice Tool.
What Next?
Low compression test results don’t warrant immediate removal of the cylinder. A little more investigation is in order.
With your assistant holding the propeller, you can walk around the engine to listen for air leaks. This is a good way to determine where the problem is coming from. Exhaust valve leaks will be evident by air leakage at the exhaust. Intake valve leaks will be evident by air leaking at the intake. Leakage past the piston rings is evident by air leaks at the breather.
If you suspect a leak at the valves, you can stake the valves by gently tapping on the valve stems. This should dislodge any foreign debris present that could be causing a leak. Be sure to rotate the prop through before checking the compression again. If you don’t do this, you could hide a problem of egged or eccentric valve seats.
You also want to do a borescope inspection on any suspect cylinders. You can see the condition of the valves. This also gives you the opportunity to see the condition of the cylinder walls. You can see if there is excessive oil, or if there is corrosion.
The engine manufacturers have guidelines on inspection procedures to coincide with compression tests to help determine the health of the cylinder and the disposition should a reading be below the minimum limit. Be sure to refer to the manufacturer’s instructions for proper procedures and troubleshooting techniques and communicate clearly with the owner to best determine what action is required.
These have been a few tips on differential compression testing. For detailed inspection procedures and maintenance criteria, be sure to refer to the manufacturer’s instructions.
