Learn how a Cessna ignition switch controls the magnetos and starter, what OFF, R, L, BOTH and START mean, and how to spot common faults.
In real-world aviation, a typical Cessna ignition switch controls the engine’s magnetos and starter. OFF grounds both magnetos; L or R leaves one operating; BOTH uses both; and START energises the starter before springing back to BOTH. The magnetos then keep the engine running independently of the aircraft battery.
What do OFF, R, L, BOTH and START mean?
The labels show which magnetos are allowed to produce ignition and when the electric starter should turn. Unlike a car ignition switch, the Cessna switch does not normally supply battery power to the spark plugs.
Each magneto generates its own ignition electricity. The switch controls low-voltage grounding wires called P-leads: grounding a magneto stops it producing sparks, while opening its P-lead leaves it live. The separate START contact energises the battery-powered starter contactor.
| Position | What the switch does | Result |
|---|---|---|
OFF | Grounds both magnetos | Neither magneto should produce a spark |
R | Grounds the left magneto | The engine runs on the right magneto only |
L | Grounds the right magneto | The engine runs on the left magneto only |
BOTH | Leaves both magnetos ungrounded | Both ignition systems operate |
START | Energises the starter and configures the starting ignition | The propeller turns until the key is released |
On many Cessna piston singles, START leaves the impulse-coupled left magneto active while grounding the right magneto during cranking. The exact arrangement depends on the engine and ignition installation, so the aircraft’s POH and cockpit placards take precedence.
The five-position rotary switch is common in Cessna 172s and other piston singles. Diesel, turbine, twin-engine and electronically modified Cessnas may use different controls. Our walk-through of the Cessna 172 cockpit controls explains how the key relates to the master switch and engine instruments.
How do you use a Cessna ignition switch to start?
The pilot turns the key to START, releases it as soon as the engine fires, and confirms that it springs back to BOTH.
- Complete the aircraft checklist. Set the fuel selector, mixture, throttle and primer as specified for that model, secure the brakes and make sure the propeller area is clear.
- Switch on the required electrical power. The magnetos do not need the battery, but the electric starter normally does.
- Turn the key to START. Hold it there only while cranking and within the starter limits stated in the POH.
- Release the key immediately after the engine fires. It should return automatically to BOTH. If it remains at START, shut down because the starter may stay engaged.
Do not keep cranking continuously after an unsuccessful start; starter duty cycles and hot- or flooded-start procedures vary by model. Simulator pilots can use our practical Cessna 172 cold-start sequence for MSFS 2024, while checking that a hardware key or magneto binding is not overriding the virtual switch.
Why does a Cessna keep running with the master switch off?
A running piston engine normally continues after the master switch is turned off because its magnetos generate their own electricity.
The master controls the battery, alternator and most aircraft electrical equipment, not the conventional magneto ignition. Turning it off may darken the instruments and disable the starter without stopping combustion. Our explanation of the Cessna battery, alternator and electrical buses covers that separation in detail.
Normal shutdown is usually performed by moving the mixture to idle cut-off and then selecting ignition OFF and master OFF in the order specified by the POH. The master switch alone is not an engine-stop control.
What should happen during a magneto check?
At the specified run-up power, selecting L or R should produce a small RPM drop because the engine is operating on one ignition system instead of two.
Check each position separately, returning to BOTH between selections. The maximum drop and permitted difference between magnetos are aircraft-specific; use the POH rather than a generic RPM figure. Excessive drop, rough running or a large difference can indicate fouled spark plugs, a faulty magneto, incorrect timing or damaged ignition leads.
No RPM drop is not automatically good. It can mean the selected-off magneto was already contributing nothing, or that the switch or P-lead failed to ground it. Either condition requires investigation before flight.
If the key is accidentally moved to OFF at run-up power, close the throttle before restoring BOTH. Switching the ignition back on while unburnt fuel is passing through the exhaust can cause a severe afterfire.
What if the engine will not start or stop?
The symptom usually separates a starter-power problem from an ignition, fuel or grounding fault.
- Nothing happens at START: check the master, battery condition, starter circuit and any simulator control conflicts. Live magnetos do not mean the battery-powered starter can turn.
- The engine cranks but does not fire: check fuel, mixture, priming and ignition selection. On installations that start from an impulse-coupled left magneto, leaving the key at R can prevent normal starting.
- The engine runs roughly on L or R: some RPM loss is expected, but severe roughness or an excessive drop points to an ignition fault or fouled plugs.
- The engine continues running at OFF: one or both P-leads may be open, or the switch may have failed. Stop the engine using the approved mixture procedure and have the ignition system inspected.
- The simulator switch moves by itself: duplicate keyboard, controller or hardware assignments may be repeatedly selecting OFF, R or L. Our guide to simulator engines that will not start or stay running covers these control conflicts.
A broken P-lead can leave a magneto live even with the key at OFF or removed. Moving the propeller may then produce a spark and start the engine, so a Cessna propeller must always be treated as live.