Which device prevents gear retraction when the aircraft is on the ground?

• A. Squat switch
• B. Altimeter
• C. Speed brake
• D. Gyro

Answer: (A)

The weight-on-wheels sensor, often called the squat switch, is designed to signal when the aircraft is on the ground. It sits in the landing gear circuit and detects that the wheels are bearing the aircraft’s weight. When the switch is closed because weight is on the wheels, the gear control logic keeps the landing gear down and locked, preventing any inadvertent retraction during taxi, the landing roll, or when the aircraft is briefly on the ground. Only after weight is no longer on the wheels does the switch change state, allowing retraction once the aircraft is airborne or when retraction is explicitly commanded. This safety feature stops a gear from retracting while still on the ground, which could lead to a gear-up situation. The other items—altimeter, speed brake, and gyro—measure altitude, drag, and attitude, respectively, and do not provide the ground-condition signal used to inhibit gear retraction.