Low differential compression on one cylinder
A six-cylinder Continental IO-520 in a commercial touring aircraft has a differential compression check during a 100-hour inspection. Cylinder No. 4 reads 58/80 with air escaping at the exhaust; the other cylinders are 70-74/80. The engine has 1,150 hours since major overhaul, oil consumption is 1 US qt per 7 hours, and the oil filter was cut and found clean. A borescope shows a narrow, dark exhaust valve seat contact line on No. 4 with a small area of missing ash deposit but no visible crack; the cylinder walls show cross-hatch and no scoring. The aircraft is scheduled for a 300 NM night freight run in two days. The manufacturer's service instruction requires investigation below 60/80 and permits continued service if the leak is not at the exhaust valve and the cylinder passes a borescope. The operator wants to defer the cylinder to the next inspection in 50 hours.
1. Interpret the compression result and borescope findings. Is the leak likely at the rings or the exhaust valve?
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The leak is most likely at the exhaust valve. Air escaping at the exhaust pipe during a differential compression check indicates leakage past the exhaust valve and seat, not past the rings. Ring leakage normally appears at the oil filler, dipstick or crankcase breather. The normal oil consumption, clean oil filter and visible cross-hatch in the cylinder walls further reduce the likelihood that worn rings are the main problem.
The 58/80 result is below the manufacturer's 60/80 investigation threshold, and the borescope shows a suspicious exhaust valve seat contact line. Even without a visible crack, a leaking exhaust valve can progress to valve burning, loss of compression and a fragments-in-cylinder event. The fact that the other cylinders are 70-74/80 makes No. 4 the outlier rather than a general engine condition.
2. Evaluate the operator's request to defer the cylinder for 50 hours.
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I would not approve a 50-hour deferral on these findings. The service instruction allows continued service only if the leak is not at the exhaust valve and the borescope is satisfactory. Here the leak is at the exhaust valve, so the stated allowance is not met. A leaking exhaust valve can deteriorate quickly, especially in a high-power touring engine, and the consequence is not just low compression: it can lead to valve failure, cylinder damage, power loss and a forced landing at night.
The operator's schedule pressure is not an airworthiness factor. The correct decision is to remove the cylinder for valve and seat inspection and repair or replacement. If the operator wants to ferry the aircraft, that would require a separate ferry approval with the risk understood and the engine restricted; it would not be a normal release.
3. What maintenance action and return-to-service checks would you require?
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I would remove No. 4 cylinder and inspect the exhaust valve, seat, guide and spring. The valve and seat should be refaced or replaced as needed, guide clearance checked, and the cylinder inspected for cracks and wall condition. I would replace gaskets and seals, reassemble to the manual, and check valve clearance and pushrod condition. After reassembly, I would perform a differential compression check, borescope the repaired cylinder, run the engine and check for leaks, abnormal temperatures and oil consumption.
I would document the compression readings before and after, the borescope findings, parts used and the operational check. I would not release the aircraft until the repaired cylinder meets the manufacturer's compression and Leakage limits and the engine runs satisfactorily.