Why must the turntable be locked when stationary?

• A. To allow free rotation while parked.
• B. To ensure hydraulic pressure remains steady.
• C. To unlock the upper structure for repositioning.
• D. To prevent unintended rotation of the upper structure when stationary.

Answer: (D)

Locking the turntable when the aerial is idle keeps the upper structure from turning on its own. Even with the vehicle on the ground, residual hydraulic pressure, wind gusts, or shifts in weight can cause the rotating platform to move if it isn’t held stationary. The lock acts like a brake, securing the turntable bearings and preventing any unintended rotation of the upper structure while parked. This protects nearby people and obstacles, and reduces stress on the drive system during idle periods. It’s not about letting the platform rotate freely, nor about keeping hydraulic pressure steady by itself, and it isn’t used to unlock the upper structure for repositioning—those actions are done with the lock released under operator control.