Which signals does the MAU use to determine ground or air status?

• A. Proximity switches on the gear doors, Airspeed, and Radar altimeter
• B. Gear door proximity switches only
• C. Airspeed alone
• D. Radar altimeter only

Answer: (A)

The signals used to determine ground or air status come from multiple, complementary sources so the system can reliably know where the airplane is in its flight regime. Proximity switches on the gear doors give a direct mechanical indication of whether the gear is up (air) or down (ground). Airspeed helps distinguish between moving through the air versus being stationary on the ground, since airspeed patterns differ dramatically between takeoff/landing phases and while parked or taxiing. Radar altimeter provides height above terrain, which is especially valuable in the low-altitude regime near the ground to confirm you’re not airborne.

Together, these signals create a robust ground/air determination: when gear is down, airspeed is low, and radar altitude is near the surface, the system can confidently label the aircraft as on the ground. When gear is up, airspeed is above the threshold, and radar altitude is high enough, it’s in the air. Relying on any single sensor could lead to misclassification in edge cases or during transitions, so the combination of all three ensures accurate status and proper system behavior during taxis, takeoff, and landing.