Final answer:
The unit that prevents the gear from being retracted while on the ground is called the landing gear lock.
Step-by-step explanation:
In the context of your question, it appears you are referring to an airplane's landing gear. The unit that prevents the gear from being retracted while on the ground is called the landing gear lock. This safety mechanism is designed to prevent accidental gear retraction while the aircraft is on the ground.
When the airplane is ready for takeoff, the pilot must engage the landing gear lock, ensuring that the gear remains in the extended position. Once the aircraft is safely airborne, the lock is disengaged, allowing the pilot to retract the landing gear.
By preventing gear retraction on the ground, the landing gear lock helps maintain the stability and safety of the aircraft during takeoff and landing.