This is surely to back up in case speedbrake is not manually selected?
Precisely that. The reason why Boeing require the speed brake to be actuated manually is that if the auto speed brake function fails to operate you already have the speed brakes deployed. The speed brake/spoiler combination is vital in a high speed abort to get the weight on the wheels for efficient braking as well as the drag effect at high speed. Interestingly Qantas used to require the reverse thrust action to activate the speed brake rather than first applying manual speed brake. This was in contradiction to the Boeing philosophy which has been in the FCTM since years ago. But Qantas has now reverted back to the original Boeing advice of manual speed brake followed reverse thrust where necessary.
Interestingly in the simulator we have observed on many occasions that some pilots rely totally on the operation of the thrust reverser levers to actuate the auto speed brakes on an abort or landing. But the malfunction section on the simulator instructors panel includes a facility to disable the auto-speed brake function. Clearly the certification of the simulator includes the possibility of the auto speed brake malfunctioning. Otherwise why have that malfunction capability? One thing is for sure and that is pilots who rely on the reverse levers to auto actuate the speed brakes often get a shock when they find out seconds later the speed brakes failed to operate automatically. Valuable efficient braking time is lost before someone spots the fact the speed brakes did not operate.
On another point of technique and this is in regard to disengaging the autothrottle system as part of the abort take off procedure. Once throttle hold is annunciated during the take off run, closure of the throttles can take place without the risk of the throttles opening up again during slow down.
But the FCTM states "After THR HLD is annunciated, the thrust levers, when retarded, remain in idle. For procedural consistency, disengage the autothrottles for all rejected take offs".
But what often happens in the simulator when an abort is initiated at high speed is the pilot first presses the disengage button and then rapidly closes the throttles. There is often a slight delay of maybe one or two seconds as the the pilots thumb hits the disengage button in the throttle handle and then he closes the throttles. That slight delay at 140 knots means the aircraft is still accelerating over 200 feet per second before the throttles are whipped closed. The FCTM also states: "if the takeoff is rejected before the THR HLD annunciation, the autothrottle should be disengaged as the thrust levers are moved to idle. [COLOR="Red"]
It does not say
before the thrust levers are moved to idle. In other words the pilot should close the throttles and as time permits disengage the autothrottles - as Boeing says "for procedural consistency". There is nothing to say you cannot close the throttles fully and then disengage the AT system.