I notice that many switches ( ex Emergency Power,Emergency Light ) indicate an

OFF,ON and ARMED position.

 

What does ARMED indicate as it seems to me a switch can only be on or off.

 

Thanks

 

HUbert Werni

ARMED means that it will automatically activate when the switch's event is needed. For example, if you ARM the spoilers on a 737, the spoilers will automatically deploy on landing.

 

Another example: Have you ever accidentally turned off the engines on a complex plane (e.g the MD11) without having ground power on or the APU running? The EMERGENCY lights come on automatically if you have them armed.