I recall a Boeing bulletin or similar document. In effect it said there is a tolerance of three seconds allowed from the rate of spool up between idle N1 of 22% to 40%. Thus if both throttles are pushed evenly (same rate of movement) from 22 % to 40% and one of the engines decided to take up its 3% tolerance then a lag and a possible strong yaw will occur.
However, from 40% to rated thrust for both throttles at the same rate of movement, the tolerance is only one second. Using normal autothrottle procedure of manually pushing the throttles to approximately the 40% position then selecting TOGA, the throttles are programmed to advance to within 8% N1 of the selected take off thrust - then a slight hesitation - then they advance to the selected take off power.
The Bulletin cautions not to try to hurry the throttle levers by manually pushing them as TOGA moves them. This can damage clutch motors in the throttles. Damaging the clutch motors can cause erratic autothrottle operation.
In the simulator we have often seen the PM "backing up" the throttles and pushing them under the captain's hand and not allowing the autothrottles system to do its thing. This is what Boeing warns about. It is also irritating to have the PM "riding" the throttles under the PF's hand.
There is no need for the PM to have his hand anywhere near the throttles even when the pilot says "Set take off thrust" or whatever terminology is SOP for the operator. A visual observation of the N1 needles is sufficient to confirm correct power is set. According to the FCTM, once take off thrust is set automatically by the A/T system, only then the PM should, if need be, adjust the
final setting to -0% +1% target N1.
The habit of some pilots to "back up" by placing their hand behind the throttles in the 737 and no doubt other types, may stem from previous general aviation aeroplanes where hand operated throttle friction nuts were worn or not effective and throttles were apt to slip back once the hand as released from the throttles.
The big danger of unnecessary "backing up" is if a sudden decision to reject the take off happens and the captain whips the throttles closed, trapping the PM's hand between the aft edge of the levers and the start levers directly behind the throttles. Serious injury can occur.
Of course, the captain is always entitled to politely warn his PM to "kindly remove your hairy mitt from behind the throttles as I wish to make a rapid abort decision.."
