IAS/Mach descent with Autothrottle

[Ecosse_821 24]

If I am cruising along using mach or speed hold mode with the Autothrottle system engaged, and I wish to descend using IAS mode with AT still engaged, what should be done to get the AT to get it to switch to some sort of idle mode?  Right now, it seems like it stays at its current setting after I have engaged the IAS descend mode.   If I move the (CH Yoke) throttle axis all the way down, the AT goes to idle for the descent.  Is that normally how the pilot commands the AT to go to idle to allow an IAS descent?

Thanks!

[ahsmatt7 1,199]

Is it going into CLMP mode? If it is....then you have manual control over the thrust levers and the system won't move them at all.

[Omaniac 3]

6 hours ago, Ecosse_821 said:

If I am cruising along using mach or speed hold mode with the Autothrottle system engaged, and I wish to descend using IAS mode with AT still engaged, what should be done to get the AT to get it to switch to some sort of idle mode?  Right now, it seems like it stays at its current setting after I have engaged the IAS descend mode.   If I move the (CH Yoke) throttle axis all the way down, the AT goes to idle for the descent.  Is that normally how the pilot commands the AT to go to idle to allow an IAS descent?

Thanks!

Normal. Do not confuse IAS/Mach hold in this aircraft with LVL CHG in the 737. When you press IAS Hold your autothrottle will go to CLAMP. All signals to the autothrottle are removed and you must control your throttles manually .A nice way to start a descent is to use VS and set around 2500 ft/m, let your throttles come back to idle and press IAS. Adjust throttles to make sure they are at idle. You now control your speed with the thumb wheel. Remember to set your speed window at the same speed as your throttles will come alive when you level off. This to avoid your throttles to "overreact" when you level off.

[Ecosse_821 24]

Thanks guys.   When I press IAS it does go to CLMP.  I then pull back my throttle lever on the CH Yoke and it goes to idle.

 

[thibodba57 462]

On 3/20/2018 at 1:59 PM, Ecosse_821 said:

Thanks guys.   When I press IAS it does go to CLMP.  I then pull back my throttle lever on the CH Yoke and it goes to idle.

 

There is no "idle mode". Four ways to descend. VNAC, FMS OVRD, VS, or IAS/MACH.  If you want a non VANV Idle descent with the Autothrottle still active then VS is your option.  If you want to descend in IAS/MACH there is no way except for CLMP (Boeing term is HOLD).  The Autothrottle should re-engage upon reaching ALT HLD. Make sure you have the correct speed set in the speed window when it does though or things might get interesting.