Use Case
To identify and eliminate the underlying cause(s) of improper cylinder compression in an internal combustion engine, ensuring restored performance, safety, and compliance with operational standards.
*** This is not meant to be authoritative, but a potential way of doing a RCA for loss of compression in a small aircraft engine. Please always consult your manufacturer's documentation and maintenance manuals.
Define the Problem
- Impact - Reduced engine efficiency, potential safety risks, and increased maintenance costs
- Symptom - Engine exhibits low or uneven compression across one or more cylinders.
- Indicators - Misfires, rough idling, power loss, increased fuel consumption, or failed compression test.
Data Collection
- Compression Test Result - Record PSI readings for each cylinder (dry and wet tests).
- Leak-Down Test - Identify where compression is escaping (intake, exhaust, crankcase, or coolant system).
- Engine History - Review maintenance logs, recent repairs, and flight hours.
- Oil Analysis - Check for metal particles or fuel dilution.
- Bore scope Inspection - Visually inspect cylinder walls, valves, and piston crowns.
Identify Possible Root Cause
| Category | Potential Cause | Diagnostic Indicator |
| Valve Train | Burnt or leaking intake/exhaust valves | Air escaping through intake/exhaust during leak-down |
| Pistons/Rings | Worn or broken piston rings | Compression improves with wet test |
| Cylinder Walls | Scoring or wear | Visible damage via bore scope |
| Head Gasket | Blown gasket between cylinders | Coolant in oil, bubbles in radiator |
| Timing | Incorrect camshaft timing | Valve overlap or late closure |
| Cracks | Cracked head or block | Persistent low compression, coolant loss |
Root Cause Verification
- Confirm with Secondary Tests
- Cylinder leak-down test
- Vacuum gauge analysis
- Dye test for coolant leaks
- Cross-reference with OEM tolerances and service bulletins
- Document findings with photos and test results
Corrective Actions
- Valve Issues - Regrind or replace valves and seats
- Piston/Ring Wear - Replace rings or pistons; hone or re-bore cylinders
- Head Gasket - Replace gasket; check for warping
- Timing - Re-time camshaft and verify timing marks
- Cracks - Repair or replace affected components
Preventive Measures
- Implement scheduled compression testing intervals
- Use oil analysis as an early warning system
- Ensure proper warm-up and cool-down procedures
- Train technicians on interpreting compression and leak-down data
Documentation & Reporting
- Maintain a detailed RCA report with:
- Test results
- Photos
- Corrective actions taken
- Recommendations for future prevention