What initiates autobraking during landing?

Autobraking during landing is initiated by the command for ground spoiler extension. When the aircraft touches down, the ground spoilers are deployed to disrupt the lift and increase the downward force on the wheels, which contributes to more effective braking. The extension of the ground spoilers sends a signal to the autobrake system to engage, allowing it to apply braking based on the selected autobrake mode.

While the main landing gear compression, thrust reversers unlocking, and nose gear compression are all parts of the landing process, they do not directly trigger the autobraking mechanism. Main landing gear compression can indicate that the aircraft has landed, but it is not the command that activates the autobrake system. Similarly, thrust reversers play a role in deceleration, but their unlocking does not initiate autobraking. Nose gear compression is a separate aspect of the landing dynamics and does not directly influence the activation of autobraking. Therefore, the correct answer highlights the primary trigger for autobraking during landing as the command for ground spoiler extension.