Low Compression, Magneto Split and an Engine Past TBO
A flight school brings you a Cessna 172RG with a Lycoming IO-360 that has run 2,150 hours since major overhaul. The engine manual specifies a 2,000-hour TBO and states that continued operation beyond TBO requires the operator's approved programme to permit it. The operator is a commercial flying training organisation with an approved progressive maintenance programme that allows on-condition operation, but only with documented trend monitoring. Run-up results are: differential compression 74/80, 76/80, 52/80 and 78/80; oil consumption has risen from 1 qt per 8 h to 1 qt per 3 h over the last 100 hours; the magneto check shows a 60 rpm drop on the left and 150 rpm on the right, with the aircraft checklist limiting drop to 175 rpm and differential to 50 rpm. The oil filter was cut and shows a few fine specks, no flakes. The chief instructor wants the aircraft for a 0700 departure and says 'it has always burned oil'.
1. Interpret the 52/80 compression reading. What is the next diagnostic sequence before you condemn or release the cylinder?
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A single low reading is a data point, not a verdict - but 52/80 is below the manufacturer's typical 60/80 minimum, so it must be investigated before release.
Sequence:
- Confirm the tester itself is sound: calibrated gauge, correct orifice, master orifice check per the OEM procedure, engine at the specified temperature.
- Wet check - introduce a small quantity of oil into the cylinder and repeat. If the reading climbs substantially, the loss is past the rings; if it stays low, it is the valve, seat or head.
- Borescope the cylinder: look for a burned or tuliped exhaust valve, scoring, carbon bridging, or a cracked head.
- Check valve lift and wobble per the OEM service instruction; a sticking valve is common on engines run lean and hot.
- Cut the filter and check the suction screen.
Only then decide: rework the cylinder, or a documented 'serviceable within limits' if the OEM data allows it. The oil consumption trend, rising threefold, points the same way as the compression reading.
2. The magneto drops are 60 rpm left and 150 rpm right. Is the aircraft airworthy as it stands, and what is your troubleshooting order?
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Not airworthy as it stands. The 150 rpm right drop alone is inside the 175 rpm limit, but the 90 rpm differential exceeds the 50 rpm maximum, and that differential is the defect indicator.
Troubleshooting order, cheapest and most likely first:
- Spark plugs: fouling, wear, incorrect gap, or a plug swapped between positions. Clean, rotate and re-gap, then re-run.
- Ignition harness: chafing, resistance, moisture, or a loose lead at the distributor.
- Timing: check internal timing and the magneto-to-engine timing.
- If the drop persists after plugs and harness, remove the magneto for bench inspection - points, condenser, e-gap, coil. A failing condenser often shows as a drop that worsens as the engine heats.
Re-run the check after each step, record the figures, and only release when both the maximum drop and the differential are inside the checklist limits. A hot mag or a weak mag on a training aircraft will eventually bite at the worst moment.
3. The engine is 150 hours past TBO. What does the approved programme allow, and what must you document before signing the release?
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TBO itself is not a hard airworthiness limit in every operation - but here the permission is conditional, and the conditions are the question.
What the programme allows: on-condition operation past 2,000 hours only with documented trend monitoring. So you need oil analysis history, compression history, filter and screen findings, and consumption trend recorded and reviewed - not a verbal 'it has always burned oil'.
Before release, document:
- The trend data and your interpretation of it.
- The compression test, wet check, borescope and valve check results.
- The oil filter findings and any oil analysis result.
- A statement that the engine meets the on-condition criteria of the approved programme.
If the trend data does not exist, the on-condition permission does not apply and the engine is past a limit without authority - that is a grounding item, not a paperwork item. If this were commercial air transport under a programme without an extension provision, TBO would be a hard limit and the engine would come off. Escalate to the quality manager before the 0700 departure.