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From Golden Standard to Second Opinion: Why the Differential Pressure Test Alone Isn’t Enough

/ General Aviation / By

Why the Differential Pressure Test Alone Isn’t the Best Indicator of Engine Health.

When evaluating the condition of your aircraft’s combustion chambers, the differential pressure test or cylinder compression test is often the go-to method. However, while this test has its place, it’s not without limitations. Misinterpretations of results or improper procedures can lead to costly and unnecessary maintenance. A more reliable approach complements the test with borescope inspections and considers real-world operational conditions.

The Limitations of the Differential Pressure Test

The differential pressure test is a non-destructive method of determining the internal condition of cylinders and cylinder components. As with any test or inspection, the differential pressure test has certain limitations that may necessitate its use in conjunction with other non-invasive inspections. The cylinder differential pressure test identifies leaks and the source of leaks, with the engine under static conditions (not running), using a regulated 80 psi pressure source.

While the differential pressure test measures the static leak rate in the cylinder using a regulated 80 psi air source, it doesn’t provide the complete picture of your engine’s health. Misleading results can stem from improper testing techniques, uncalibrated equipment, or reliance on outdated standards. For instance, earlier Continental guidelines recommended cylinder replacement for any valve leakage. This approach led to unnecessary cylinder removals and overhauls.

Continental's Guidelines for Differential Pressure Testing

Continental’s M-0 Standard Practice Maintenance Manual provides detailed instructions for conducting the test. Key steps include using calibrated equipment, such as the E2M Differential Pressure Tester with a master orifice, and supplementing results with a borescope inspection.

If the test indicates low cylinder pressure, Continental recommends:

  1. Performing a borescope inspection.
  2. If no issues are found during the inspection and oil consumption is stable, flying the aircraft at 65-75% cruise power for at least 45 minutes.
  3. Repeating the test immediately after landing while the engine is still hot.

This process can help eliminate false readings caused by carbon deposits or temporary seating issues, potentially saving you from unnecessary repairs.

Lycoming’s Approach and Recommendations

Lycoming’s Service Instruction SI-1191A sets fixed limits for compression test results but does not recommend follow-up flights to confirm low readings. Nevertheless, we suggest flying the aircraft at 65-75% cruise power for at least 45 minutes when safe and legal or performing an extended high-power ground run. Lean the mixture aggressively during these operations to help remove carbon deposits.

Why You Should Always Pair the Differential Pressure Test with a Borescope Inspection

Differential pressure tests can be unreliable if used alone, often leading to the premature removal of cylinders based on incorrect readings. Many perfectly functional cylinders have been replaced because of an over-reliance on this method.

A borescope inspection provides a detailed view of the combustion chamber, revealing issues such as burned valves, scoring, or piston crown damage that a pressure test might miss. By combining the two methods, you can achieve a far more accurate assessment of cylinder health and avoid costly mistakes.

Additionally, re-testing following a positive borescope inspection are equally important. Flying the aircraft at cruise power for at least 45 minutes and repeating the differential pressure test while the engine is still hot can confirm whether an issue truly exists. This practice allows potential carbon deposits or temporary sealing issues to resolve under normal operating conditions, preventing unnecessary repairs, overhauls, or cylinder replacements. By taking this step, you can save time, money, and extend the life of your engine.

How to Perform a Differential Pressure Test Correctly

Executing the test properly is crucial for obtaining accurate results. Follow these steps:

  1. Prepare the Engine: Perform the test as soon as possible after a flight or extended ground run while the engine is still hot. Ensure the cowling is installed during ground runs to simulate normal operating conditions.
  2. Inspect Spark Plugs: Remove the most accessible spark plug from each cylinder. Inspect the plugs for deposits or abnormalities as these can indicate underlying engine issues.
  3. Position the Cylinder: Rotate the crankshaft by hand until the piston in the test cylinder is just before the compression stroke.
  4. Connect the Tester: Attach the cylinder adapter to the spark plug hole and connect the E2M Differential Pressure Tester.
  5. Apply Pressure: Slowly turn the tester’s Slow Fill Valve to pressurize the cylinder to 20 psi. Continue rotating the propeller until the piston reaches Top Dead Centre (TDC). Adjust the pressure regulator to maintain 80 psi.
  6. Rock the Propeller: Gently rock the propeller back and forth to seat the piston rings and stabilize the reading. Record the highest pressure reading displayed on the gauge.
  7. Repeat for All Cylinders: Follow the same procedure for each cylinder. Record results along with the acceptable leakage limit determined by the master orifice.
  8. Document and Evaluate: Compare results against manufacturer’s limits and document them in the engine logbook. If discrepancies are noted, consult a maintenance professional.

The Importance of Using Calibrated Equipment

Using the correct, calibrated equipment is critical for accurate differential pressure test results. The E2M Differential Pressure Tester, for example, incorporates a master orifice to establish the acceptable cylinder pressure leakage limit. This tool must be calibrated annually to ensure accuracy. Without proper calibration, test readings can become misleading, leading to unnecessary maintenance actions.

When selecting test equipment, ensure it meets the following criteria:

  • Pressure Source: Dry, oil-free compressed air capable of maintaining 125 PSI with a minimum flow of 15 CFM.
  • Tester: A device with a master orifice, like the E2M Differential Pressure Tester, calibrated to account for atmospheric conditions.
  • Maintenance: Regular calibration and proper handling to prevent damage or wear.

Final Thoughts

The differential pressure test is a valuable diagnostic tool, but it should not be your sole method for assessing engine health. By combining it with a borescope inspection and following best practices, you can gain a complete understanding of your engine’s condition, avoid unnecessary expenses, and ensure continued airworthiness.

Stay tuned for our next post, where we’ll dive deeper into conducting borescope inspections and interpreting their results.

About Quest Aeronautics

Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.

About Author

Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.