"THR HLD" and Clamp
 

Hi all.
I'm teaching an ATPL class in instrumentations and I'm trying to find out what actually happens with the thrust levers and autothrottle actuator during take off when the FMA says THR HLD (737) or CLAMP (Md80). I know that it is a mode to prevent undesired thrust adjustments by possibly faulty autothrottle signals, but how is this achieved in regards to the mechanical inner workings of the thrust lever and autothrottle assembly? Unfortunately I've only flown aircraft without autothrottle so my knowledge in this area is limited so trying to make up for it by asking around...

/LnS

Not only that but arguably more importantly, Clamp allows tha pilot to close the throttles (should be thrust levers but MD/Boeing calls them throttles) if a rejected takeoff is desired/required, or, on a flex takeoff, max takeoff thrust is still available simply by pushing the throttles forward. If Autothrottle was still engaged, it would fight these manual thrust selections.

Thust levers are driven by a little electric motor that is connected to the levers via a friction clutch. The clutch gives enough friction so that the motor is able to position the thrust levers when they are "free to move" but the friction is so low that the pilot can easily override the motor driven position manually.

THR HOLD means that the clutch is disengaged, so that faulty behaviour of the motor cannot be transferred to the thrust levers in a critical phase of the take-off and (as stated by Capt Claret) it gives the pilot the option to change thrust lever position without having to fight the motor.

Should the A/T fail to go into THR HOLD mode during T/O, then nothing is amiss as long as the motor does not display quirky behaviour. It may be desired to disengage A/T, just to be sure.

On our jet, clamp is only engaged at 80kias. System ensures the correct thrust is set, even if engine pressure sensors are set, as set thrust is derived from calculated throttle position angle rather than pure epr.

Cheers.
So just to get this right. The autothrottle mechanically disengages from the thrustlever/throttles? No contact with any actuator? After having seen the thrustlevers move with autothrottle input I gather that the autothrottle system achieves its thrust setting through mechanically driving the thrust levers rather than by direct input to some fuel control device further downstream in the system (as I understand the airbus autothust system does)?

/LnS

note: posted at the same time as other informative posts were added. Thanks. I think I got it.