Magneto ignition & the mag check
A piston aero engine has two magnetos, and each cylinder has two spark plugs firing simultaneously (TP 1102). A magneto makes its own spark current, so once the engine is running it needs only the magnetos and runs independent of the electrical system: it keeps firing with the master OFF. Two of them are there for redundancy (safety), if one magneto fails in flight the engine runs on the other with only a slight loss of power (TP 1102).
Run-up mag check (BOTH, then L, then R):
- A small RPM drop on each single magneto is normal (one plug per cylinder stops firing).
- The RPM differential between the two magnetos must not exceed the limit specified in the POH/AFM (TP 1102 sets the rule; the number is aircraft-specific, so read your POH/AFM).
- Too big a drop or differential = a fouled spark plug or a faulty magneto on that side.
Hot-magneto check (at idle, switch the magnetos OFF momentarily): the engine should stop. If it keeps running, a magneto is live ("hot"): the engine can fire if the propeller is turned by hand, so treat the propeller as live and report it at once (TP 1102). The usual cause is a broken magneto ground wire (P-lead), which leaves the OFF switch unable to ground the magneto (engine manufacturer's manual, not in TP 1102).
Trap: an engine that keeps running with the switch OFF is not a "strong" engine, it is a live magneto and a serious ground hazard.
References
- TP 1102
- POH/AFM