From my understanding of the Auto-Brake logic, it activates when the spoilers extend. On Medium there is a 2 second delay before pressure is sent to the brakes and 4 seconds on low.
I believe the reason for selecting reverse is a protection so if the spoilers have inadvertently not been armed they will deploy as soon as reverse is selected on one engine, provided the other is still at idle.
Note: The RTO situation is slightly different
If ground spoilers are armed and GS exeeds 72 Kts, the spoilers will extend when the thrust levers are set to idle.
If the spoilers are not armed and the speed exceeds 72 Kts, the spoilers will extend as soon as reverse is selected.
If the RTO is executed at less than 72 Kts the spoilers will not extend.
Note: As the autobrake looks for a signal from the spoilers, it will not activate in the 3rd case as the spoilers do not deploy!
Last edited by Bill Smith; 10th Feb 2008 at 02:45.
Reason: RTO Case