Low Compression, an Oily Plug, and a Trip Tomorrow
A carbureted four-cylinder horizontally opposed engine, 1,900 hours since overhaul, comes in for an annual. Differential compression test: #1 76/80, #2 74/80, #3 58/80, #4 75/80. On #3, air is clearly audible at the exhaust pipe. Borescope of #3 shows a crescent-shaped eroded area on the exhaust valve face under heavy green-gray deposit; cylinder walls show normal crosshatch. The #3 bottom spark plug is oil-fouled; the others are normal. Oil consumption has risen from 1 quart per 10 hours to 1 quart per 6 hours. Magneto check: left drop 70 rpm, right drop 150 rpm; the manufacturer's limits are 175 rpm maximum drop and 50 rpm maximum difference between magnetos. The approved data gives 60/80 as the minimum compression, with lower readings rechecked using the master orifice tool. The owner has a three-hour cross-country tomorrow.
1. How do you interpret the #3 cylinder findings, and what is the disposition?
Read the worked answer
The number alone does not decide it: 58/80 is below the 60/80 minimum, so the first step is a recheck with the master orifice tool per the approved data. But the rest of the evidence points the same way regardless of that recheck:
- Air escaping at the exhaust locates the leak path at the exhaust valve.
- Borescope shows erosion of the valve face under deposit - that is valve distress, not a normal seating leak or carbon flaking.
- The oil-fouled bottom plug and the jump in oil consumption (10 to 6 hours per quart) are consistent with a valve/guide problem.
A master orifice recheck can clear a low reading caused by leakage past rings at test conditions; it does not clear a physically damaged exhaust valve. Disposition: the cylinder comes off, and the valve, seat, and guide are inspected and replaced per the manual. No approval for return to service.
2. Is the magneto check acceptable? How does the #3 cylinder work affect your ignition troubleshooting?
Read the worked answer
No. The right drop of 150 rpm is inside the 175 rpm maximum, but the difference between magnetos is 150 - 70 = 80 rpm, which exceeds the 50 rpm maximum differential. That is a failed check as written, even though each individual number looks legal.
However, do not chase the magneto first. Each cylinder fires on both magnetos through separate plugs, and #3's bottom plug is oil-fouled. That fouled plug can produce exactly this kind of uneven drop, so the ignition check must be repeated with clean, correctly gapped plugs after the cylinder work is done.
If the differential persists after that, troubleshoot the right magneto (timing, points, capacitor), the harness, and the ignition leads. Signing it off on the basis that '150 is under 175' would ignore the differential limit that the same data imposes.
3. The owner pushes to take the airplane and says he accepts the risk. What do you say, and what do you record?
Read the worked answer
He cannot accept this risk on your signature. The aircraft is not airworthy: a distressed exhaust valve can fail, warp, or drop, leading to pre-ignition, cylinder damage, and possible in-flight power loss. The magneto differential is an unresolved discrepancy as well.
What you do:
- Decline to approve for return to service and state the specific discrepancies.
- Record them per 14 CFR 43.11 in the inspection results, listing the low #3 compression with exhaust valve distress and the excessive magneto drop differential, with the work required.
- Record any work actually performed per 43.9 with the approved data referenced.
- If he elects to fly it anyway, that is his decision as operator, but it is outside your authority to approve and must not appear as an airworthy sign-off. Note the conversation in shop records if your shop's practice is to document refusals.