Detonation & pre-ignition
Two abnormal-combustion faults; do not swap them (definitions per POH/AFM and engine manufacturer's manual):
| Detonation | Pre-ignition | |
|---|---|---|
| Timing | After normal ignition | Before spark plug fires |
| What happens | Charge explodes violently instead of burning smoothly | Charge lit too early by a hot spot (glowing carbon deposit or overheated spark plug) |
| Effects | Overheats; if uncorrected can fail piston, cylinder, or valves | Power loss, high operating temperature; can severely damage engine: expanding gases push on piston still on compression stroke |
(PHAK Ch. 7)
Causes of detonation:
| Cause | Detail | Ref |
|---|---|---|
| Mixture too lean | "The engine may be damaged due to detonation"; most likely at high power | TP 1102 (too-lean ratio); PHAK Ch. 7 (high power) |
| Fuel grade lower than specified | Especially in a high-compression engine | TP 1102 (never use a lower grade); PHAK Ch. 7 (higher-compression engines need higher grades) |
| Carburettor heat during take-off | - | TP 1102 |
| High cylinder-head temperature | - | PHAK Ch. 7 |
| High manifold pressure with low RPM | - | PHAK Ch. 7 |
| Extended ground operations or steep climbs | Cut cylinder cooling | PHAK Ch. 7 |
Avoiding detonation (PHAK Ch. 7):
- Correct fuel grade.
- Cowl flaps full open on the ground.
- Enriched mixture with shallow climb angle during take-off and initial climb: more cooling.
- Avoid extended high-power steep climbs.
Trap: pre-ignition fires before the plug, detonation burns explosively after it. High-power lean operation is a pre-ignition cause in PHAK Ch. 7 but a detonation cause in TP 1102.
References
- TP 1102
- POH/AFM
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25)