A/T Problems

[sedacajr 17]

Hi everyone.

I just want to know how to deal with A/T in 777 because I feel the reaction of autopilot very slow to maintain the desired airspeed. For exampe, I am in approach phase, keeping 170kts before intercepting the glide slope with autopilot and A/T on. Then the airplane intercepts the GS and begins to descent in order to keep the glide path, but, obviusly, when it start to descent the plane starts to increase the airspeed, and the A/T takes a lot of time to reduce throttle enough to maintain 170 kts, so the airspeed increases reaching 190kts! 

The problem is worst in windy conditions. I am starting to use manual throttle in approach phase to avoid this situations.

Any tip?

 

Senén Castillo.

 

 

[Nighthwk 566]

Yeah, it’s the same for me as well. Im not quite sure if it’s the same on the real aircraft or a problem with the A/T. I’m going with the former as I’d assume it would have been fixed by now if it truly was a problem with their product.

[xkoote 57]

This has been a recurring topic. It has never been addressed to my liking and I still feel the 777's autothrottle to lag behind actual conditions. The 747 is way better at this. I have just learned to cope with it by helping with F2/F3 keys. But the system is by far not the way the real one handles.

 

cheers,

[BlueStar 323]

On ‎11‎/‎17‎/‎2017 at 8:51 AM, sedacajr said:

I just want to know how to deal with A/T in 777 because I feel the reaction of autopilot very slow to maintain the desired airspeed.

Or you could do what I do, adjust the thrust manually.  There is a reason my hand is on the throttles during approach. 

blaustern

[CaptainLars 21]

I also think that the A/T-system is less than perfect. Often, the thrust goes up, then it tells me "ding dong, Speedbrakes extended". Why doesn't it leave the throttle just where it is...

[Budbud 227]

14 hours ago, CaptainLars said:

I also think that the A/T-system is less than perfect. Often, the thrust goes up, then it tells me "ding dong, Speedbrakes extended". Why doesn't it leave the throttle just where it is...

Hi Lars,

If AT kicks in, I bet that your current speed is decaying below the target speed or your speed is still above the target speed but it is dropping so the system anticipates to avoid slowing below target speed. In both cases, the FMC commands some thrust to maintain the speed and advises you that the speedbrake should be retracted since it is a nonsense to have the speedbrakes extended in a situation where thrust is required to maintain the speed.

 

[CaptainLars 21]

4 hours ago, Budbud said:

... that the speedbrake should be retracted since it is a nonsense to have the speedbrakes extended in a situation where thrust is required to maintain the speed.

 

I think that the A/T and me oftentimes have different opinions on how much thrust is required to reach the target speed.

In many occasions, that typically occur when levelling off in a descent, I perceive that the A/T as giving too much thrust. If I would leave the speedbrakes down, we would be much farther away from target speed. On other occasions, typically before final approach, it gives so little thrust that we fall 5 or even 10 kts below the target speed, and I switch A/T off and manually increase power.

[Budbud 227]

That's weird, I almost never have to manually interfere with the thrust with AT on in normal conditions. But it's true that I leave a bit of slack in descent regarding the speed. I comply strictly with the speed restrictions when there are but in free segment, if the speed is off by 5-10kt, I don't try to stick with the target speed to the knot. 

In the final approach, I never got any issue with the AT except in really strong winds where I sometimes disconnect the AT and manage the thrust manually if the winds variations are significant.

[downscc 1,327]

The A/T works best when the aircraft is flown as intended, where everything is smooth and not over controlled.  The engines will not respond to fast changes in thrust and the A/T is designed to not ask for that.  Two examples cited:  Level off from a descent, not sure what thrust mode you were using in descent or if you are below the 250 KIAS restriction so it is hard to guess why you had a problem, or why you were using speedbrakes.  If the airspeed decays below target about 15 kts then the A/T will add thrust but you will be providing a smoother response by adding a little thrust on your own.  No need to cancel A/T in this case as i assume you were in a HOLD or ARM mode.

It is a significant consideration that you have to give to engines that have fans 128 in. in  diameter.... the fans diameter is larger than the body of a B737.  Your entire concept of thrust commands has to accommodate this reality.

[andreh 125]

As I've understood it, it's not uncommon in the B777 to have use speedbreaks on decent due to the airframe being so aerodynamic. The sleek airframe probably contributes to the speed creeping up, not just thrust

[Budbud 227]

Also the winds in the descent page have a big influence in the calculation of the descent path and therefore with the energy management.

[CaptainLars 21]

On 5.12.2017 at 0:00 AM, downscc said:

The A/T works best when the aircraft is flown as intended, where everything is smooth and not over controlled.  The engines will not respond to fast changes in thrust and the A/T is designed to not ask for that.  Two examples cited:  Level off from a descent, not sure what thrust mode you were using in descent or if you are below the 250 KIAS restriction so it is hard to guess why you had a problem, or why you were using speedbrakes.  If the airspeed decays below target about 15 kts then the A/T will add thrust but you will be providing a smoother response by adding a little thrust on your own.  No need to cancel A/T in this case as i assume you were in a HOLD or ARM mode.

It is a significant consideration that you have to give to engines that have fans 128 in. in  diameter.... the fans diameter is larger than the body of a B737.  Your entire concept of thrust commands has to accommodate this reality.

I almost always use VNAV in descent, sometimes FLCH, sometimes V/S when I want more control of the descent path. In VNAV, the thrust mode goes into IDLE and then in HOLD. I have to use speedbrakes regularly because the aircraft is accumulating too much speed quite easily. Even when the aircraft is above the target speed, especially at the 10,000ft border where you have to decelerate below 250kts, it's seldomly on idle (when it's not in HOLD). Then I deploy speedbrakes, and especially when the next level-off-altitude is reached, e. g., target speed 240kts, IAS being 245kts, the throttle increases nonetheless and I get a speedbrake warning. I have the impression that the automatic gives quite abrupt thrust commands.

 

11 hours ago, andreh said:

As I've understood it, it's not uncommon in the B777 to have use speedbreaks on decent due to the airframe being so aerodynamic. The sleek airframe probably contributes to the speed creeping up, not just thrust

I've heard this, too.

 

6 hours ago, Budbud said:

Also the winds in the descent page have a big influence in the calculation of the descent path and therefore with the energy management.

I didn't know that their influence was so big, thanks.

[BlueStar 323]

Use FLCH below 10,000, it's how the big boys (and girls) do it.

blaustern

[downscc 1,327]

13 hours ago, Bluestar said:

Use FLCH below 10,000, it's how the big boys (and girls) do it.

blaustern

Agree, but I often get a pitch oscillation with the 777 as it seems to hunt for the pitch required for the MCP SPD commanded.  This is why I have started using FPA but only for reasons associated with this specific product, she will hold a rock solid FPA with A/T in SPD mode.