Definition
A mode of an automatic flight control system or autopilot in which the approach function has been selected and is ready to engage, but has not yet captured the navigation guidance signal (such as the localizer or glideslope). The system is armed and waiting; once the aircraft intercepts the signal, the mode automatically transitions from armed to active and begins flying the approach.
Plain English
The autopilot is set up and ready to fly the approach, but it hasn't started yet. It's waiting until the aircraft reaches the guidance beam, and then it will take over and follow it.
Context Anchor
Seen on autopilot or flight director mode displays after the pilot selects approach guidance, usually before the airplane captures the final approach path.
Derivation
‘Arm’ here comes from the same sense as arming a switch or a system — making it ready to act. The system is prepared to do something, but the trigger condition (intercepting the signal) hasn’t happened yet.
Why Pilots Care
It ensures the autopilot is primed to capture and follow the precise vertical and lateral guidance required for a safe landing in low visibility.
Grounding Statement
In Approach Arm Mode, the system is waiting at the door; it has not stepped through yet.
Intuition Check
Do not read arm as anything to do with weapons or physical arms. Here, armed means ready to activate later; it does not mean the approach guidance is already active.
Example Sentence 1
After being cleared for the ILS, the pilot selected approach arm mode and watched for the localizer to capture.
Example Sentence 2
After localizer capture the system changed from Approach Arm Mode to active approach tracking.