Re-engaging 747 autothrottle

[IMAGRANDPA2 0]

Hi agiain guys,Well here's another on of those issues that's beyond me and I don't think the manual covers it. If I get a message "NEXT ALTITUDE UNABLE" and I decide to take manual control to get down, I don't seem to be able to re-engage autothrottle after making the altitude and hitting the waypoint. What am I missing?Eric Olson

[Turbine777 13]

Hi agiain guys,Well here's another on of those issues that's beyond me and I don't think the manual covers it. If I get a message "NEXT ALTITUDE UNABLE" and I decide to take manual control to get down, I don't seem to be able to re-engage autothrottle after making the altitude and hitting the waypoint. What am I missing?Eric Olson

Normally in those situations I just use VS and crank it to about -3500 and hit the spead brakes haha. It has been a while since I touched the queen (sort of speak), but I think it will re-engage if you double click Vnav. Not sure why, but I'm pretty sure that's what I've done (had to do).

[downscc 1,327]

If there is a tail wind, you need to enter those for the descent forecast. Also, slowing the airspeed down a little helps a lot on steep descents. STARs seldom require more than 250ft/nm, which is very easy to obtain unless your ground speed is very fast.

[IMAGRANDPA2 0]

Thanks as always.Eric Olson

[Flamin_Squirrel 71]

If there is a tail wind, you need to enter those for the descent forecast. Also, slowing the airspeed down a little helps a lot on steep descents. STARs seldom require more than 250ft/nm, which is very easy to obtain unless your ground speed is very fast.

Pretty sure that's wrong. The faster you go, the faster you should decend.

[downscc 1,327]

Care to add your name to that thought?The higher your groundspeed, the higher your required rate of decent to maintain a given angle of descent. Simple math. Another way of putting it to borrow your words is the faster you go, the greater your descent rate needs to be. If your ground speed is 350 kts, your vertical speed for a 250 ft/nm descent is 1458 ft/min but if you are going 300 kts it is only 1250 ft/min. This is related to the reason descent rates are much higher at higher altitude... because a given IAS results in a much higher groundspeed.

[qnh 70]

..... but I think it will re-engage if you double click Vnav. Not sure why, but I'm pretty sure that's what I've done (had to do).

Thanks for this, saved my bacon. Now where on earth is this in the manual - I have been searching high and low for it!

[jon b 2,047]

There are 3 modes which re-engage the auto throttle, if you look at the speed selector there are 3 lines coming of it pointing to these modes as a reminder, they are VNAV,FLCH and SPD. SPD will operate the AT if you are in ALT V/S or G/S verticle modes. the other 2 options are to cycle the AT to off then back to arm or pressing TOGA will also bring it back,if TOGA is armed.CheersJon B