What does the avionics master switch do and when is it used?
Learn what the avionics master switch powers, why it stays off during engine start, when to switch it on and how split avionics buses differ.
The avionics master switch energises the electrical bus or buses feeding most radios, navigation units and related avionics. In real-world aviation and flight simulators, leave it off for engine cranking, then turn it on after the engine is running and the alternator or generator is online—unless the aircraft checklist says otherwise.
What does the avionics master switch control?
The switch connects one or more avionics buses to the aircraft’s electrical supply, usually by operating a relay rather than carrying the equipment’s full electrical load itself. It does not generate power: the battery, alternator or generator must supply that. Our explanation of how a typical Cessna distributes battery and alternator power shows where the avionics buses fit into the system.
Depending on the aircraft, an avionics bus may feed:
- COM and NAV radios
- GPS and multifunction displays
- The transponder and audio panel
- The autopilot, DME or ADF
- Other installed navigation and communication equipment
The exact equipment is aircraft-specific. Essential instruments, engine displays, standby equipment or one glass-cockpit display may use a separate bus and remain powered with the avionics master off. Lights, ignition, the starter and other general electrical loads are normally controlled elsewhere.
When should I turn on the avionics master switch?
The normal light-aircraft sequence is to switch it on after engine start, once oil pressure is satisfactory and the alternator or generator is supplying stable power. The aircraft’s POH, AFM or approved checklist always overrides this general sequence.
| Stage | Typical position | Reason |
|---|---|---|
| Initial cockpit preparation | Off | Prevents unnecessary battery drain |
| Temporary preflight avionics check | On as required | Allows equipment testing or setup |
| Immediately before and during cranking | Off | Reduces starter load and avoids voltage sag or reboots |
| Engine running and generation confirmed | On | Supplies normal avionics power |
| Before engine shutdown | Off | Isolates avionics and leaves them off for the next start |
In a simulator, the same logic belongs in the wider cold-and-dark aircraft start sequence: establish electrical power, start the engine or APU as applicable, confirm generation, then energise the avionics buses.
Why should it stay off during engine start?
Leaving the avionics master off reduces the electrical load while the starter is drawing heavy current and isolates equipment from the resulting voltage drop and possible electrical transients. Otherwise, displays may dim or restart and the battery has less capacity available for cranking.
The switch should not be treated as a universal surge protector; protection and bus architecture vary between aircraft. Accidentally starting with it on does not mean the avionics have been damaged, but routinely doing so ignores the intended load-management procedure.
Can I turn the avionics on before starting the engine?
You may power the avionics before start when the aircraft checklist calls for a test, clearance, route entry or flight-deck setup. Use external power or an APU where prescribed, monitor battery use, and return the switch to off before cranking if required by the checklist.
Some integrated flight decks power essential displays as soon as the battery master is switched on. Large transport aircraft may not have a single avionics master switch at all; their equipment is managed through multiple electrical buses and automatic load-shedding systems.
Do both halves of a split avionics master need to be on?
Both halves are normally switched on for flight, but each half controls a different avionics bus. This arrangement permits selective load shedding and prevents one bus fault from removing every radio or navigation unit.
Do not assume that side one always powers COM1 or that side two always powers COM2; the allocation depends on the aircraft. In simulators, a hardware assignment may operate only one bus, so check both cockpit switch positions if some equipment works while other units remain dark.
Why are the avionics still blank after switching it on?
An avionics master cannot power its buses when the electrical source is unavailable, a split switch remains off or an individual unit is switched off.
- Check the main electrical supply. Confirm the battery master is on and, after engine start, that the alternator or generator is online without a low-voltage warning.
- Inspect both avionics bus switches. A split master with only one side on produces a convincing partial failure.
- Check individual controls. Some radios and displays have their own power knobs, brightness controls or standby modes.
- Inspect circuit breakers. In a real aircraft, do not repeatedly reset a tripped breaker unless the approved checklist permits it.
- Check simulator assignments. Third-party aircraft may use custom electrical logic that does not respond correctly to a generic avionics-master binding; operate the virtual cockpit switch to confirm.
If the buses appear powered but the displays remain dark, use our step-by-step checks for blank avionics screens in MSFS to separate an electrical-state problem from brightness, binding or aircraft add-on issues.