In most aerial apparatus, hold-down locks are released when:

The hold-down locks on most aerial apparatus are primarily designed to secure the ladder in place during transport and to prevent any unintended movement. They are typically released when the selector valve switch is placed in the aerial device position. This action signals the hydraulic system to prepare for operation, allowing the ladder to be deployed and ensuring that it can function as intended.

When the selector valve is in the aerial device position, it enables the hydraulic system to activate the mechanisms responsible for raising and extending the ladder. This design helps ensure that the hold-down locks are only disengaged when it is safe and appropriate to operate the aerial device. By linking the release of the hold-down locks to the selector valve, manufacturers enhance the safety and effectiveness of the aerial apparatus.

Other options present different scenarios that do not align with the typical operational design of aerial apparatus. For instance, fully extending the ladder or powering down the device would not logically relate to the safety mechanisms in place for releasing the locks, nor would manual removal of the locks be a standard operational protocol, as this could lead to unintended risks and complications during deployment.