Magneto ignition & the mag check
Layout (TP 1102):
- Two magnetos.
- Two spark plugs per cylinder, firing simultaneously.
- Magneto makes its own spark current (TP 1102 l.1294).
- Running engine needs only the magnetos: independent of electrical system, fires with master OFF (TP 1102 l.1326).
Why two:
| Feature | Benefit | Cite |
|---|---|---|
| Two magnetos | Redundancy (safety): if one fails in flight, engine runs on other with only a slight loss of power | TP 1102 |
| Two plugs per cylinder | Improve combustion; slightly higher power output | PHAK ch.7 |
Mag check:
- First check at low RPM (idle or slightly above) before leaving flight line.
- Then full run-up mag check per POH/AFM (TP 1102 852).
- Select each single magneto from BOTH and back to BOTH before the other. PHAK order: right, then left ("from BOTH to RIGHT and then from BOTH to LEFT", PHAK ch.7).
- Run at the RPM POH/AFM specifies: it reaches maximum compression-stroke pressure; other RPM may not prove the check (TP 1102 834).
- Constant-speed propeller: pitch lever in full high RPM. At an intermediate setting the governor compensates for the power loss: drop still happens, tachometer will not show it (TP 1102 6818).
Mag check results:
| Result | Meaning | Cite |
|---|---|---|
| Small RPM drop on each single magneto | Normal (one plug per cylinder stops firing) | - |
| No RPM drop at all | Abnormal: do not fly | PHAK ch.7 |
| Too big a drop or differential | Fouled spark plug or faulty magneto on that side | PHAK ch.7 |
| RPM differential between the two magnetos | Must not exceed POH/AFM limit (number is aircraft-specific) | TP 1102 sets the rule |
Hot-magneto check (at idle, switch magnetos OFF momentarily):
- Engine should stop.
- Keeps running = live ("hot") magneto: engine can fire if propeller is turned by hand. Report at once (TP 1102).
- Usual cause: broken magneto ground wire (P-lead), so OFF switch cannot ground the magneto (engine manufacturer's manual, not in TP 1102; PHAK ch.7 describes the broken ground wire).
Separate hazard (not a broken P-lead): with battery and master OFF, engine can still fire and turn over if the ignition switch is left ON and propeller is moved. Magneto needs no outside electrical power (PHAK ch.7).
Trap: engine that keeps running with switch OFF is not "strong", it is a live magneto and a serious ground hazard.
References
- TP 1102
- POH/AFM
- FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25)