Autothrust Extremely Slow to React on Approach

[markjj 47]

2 minutes ago, PMDG777 said:

Ah sorry I misunderstood your post.

No problem :)  Like I said, the speed dips to the point that a manual take over is the only option.  Interestingly, on a full autoland, it seems to be fine.  It just seems to have an issue when manual flying is occurring with AT

[Driverab330 74]

1 hour ago, Pilot53 said:

Ive thought this was a problem too on the 777, but I think it is due to the fact that with the 777 it is recomended to leave the a/t on all the time, so they had to build in a "fudge factor" to ensure the aircraft wouldn't get too slow during gusty conditions.  So yes, its slow to respond but that also keeps the engines spooled so that they can reach full thrust quickly if needed.  I found that I was building speed too easily on approach, because I was flying it like I do the 737, but the 777 just has trouble slowing down.  But if you fly the 777 the way the fcom says it works great.  Flaps 5 at loc intercept, flaps 20 and gear down when the g/s is alive, and full flaps on the g/s.  The ensures enough drag and that you are configured before you start down.

 

The 777 is hard to slow down but not that hard compared to the A330/340. The way you flew will be too conservative and will be commented on a annual line check. You are able to intercept the G/S at 250kts and still be stabilised by 1000ft AAL, just go full speed brake and gear down straight away. 

Nowadays, most of the approaches into major airports will have speed control of something like 180kts till 7nm then 160kts till 4nm (i.e. Heathrow, Hong Kong, Milan, Frankfurt etc etc) 

So the recommended practice is to intercept both the LOC and G/S with at least F5 and 180kts, sometimes with weights >233tons, F5 speed will be higher than 180kts, in that case, you will need to use F15. 

Approaching 8nm select F20, at 7nm Gear Down. 

For F30, at the latest 5nm or if you use F25 the latest 4nm, you will need to lower the flap in order to meet the 1000ft stabilised approach criteria*. 

However, sometimes the workloads are very high, and it is highly recommended to configure early, so this is not a strict rule.

*Stabilised approach criteria for most airlines is to have the airplane fully configure + speed reducing towards target approach speed + fully established on a vertical and lateral approach path before decent below 1000ft AAL or to some airline 1500ft AAL.  (For visual approach, to achieve wings level by 300ft AAL.)

[PMDG777 231]

1 minute ago, markjj said:

No problem :)  Like I said, the speed dips to the point that a manual take over is the only option.  Interestingly, on a full autoland, it seems to be fine.  It just seems to have an issue when manual flying is occurring with AT

Do you get the same behaviour with no weather engine running? Just want to rule out some possibilities before we claim bug and waste PMDG's time looking into it.

[achutchison 26]

3 hours ago, B777ER said:

Dan is right on point. See the Asiana 214 KSFO incident as a real world example. 

The asiana 214 crash happened with the autothrottles disconnected. I don't know about the real thing, but in the sim I can get much better engine response manually than with the auto throttle by setting the target n1 much higher than the current n1. The autothrottles do not advance very quickly leaving a small difference between the target and the actual n1. I think this is the difference. In the sim, the greater the difference between the target n1 and the actual n1 then the faster the engine will accelerate.

[Pilot53 543]

2 hours ago, Driverab330 said:

 

The 777 is hard to slow down but not that hard compared to the A330/340. The way you flew will be too conservative and will be commented on a annual line check. You are able to intercept the G/S at 250kts and still be stabilised by 1000ft AAL, just go full speed brake and gear down straight away. 

Nowadays, most of the approaches into major airports will have speed control of something like 180kts till 7nm then 160kts till 4nm (i.e. Heathrow, Hong Kong, Milan, Frankfurt etc etc) 

So the recommended practice is to intercept both the LOC and G/S with at least F5 and 180kts, sometimes with weights >233tons, F5 speed will be higher than 180kts, in that case, you will need to use F15. 

Approaching 8nm select F20, at 7nm Gear Down. 

For F30, at the latest 5nm or if you use F25 the latest 4nm, you will need to lower the flap in order to meet the 1000ft stabilised approach criteria*. 

However, sometimes the workloads are very high, and it is highly recommended to configure early, so this is not a strict rule.

*Stabilised approach criteria for most airlines is to have the airplane fully configure + speed reducing towards target approach speed + fully established on a vertical and lateral approach path before decent below 1000ft AAL or to some airline 1500ft AAL.  (For visual approach, to achieve wings level by 300ft AAL.)

Thanks for the real world insight.  I just flew into KSFO flying the approach exactly as you described and it flew like a dream.  In my opinion the A/T does a great job as long as you fly a normal stabilized approach.

[Driverab330 74]

56 minutes ago, Pilot53 said:

Thanks for the real world insight.  I just flew into KSFO flying the approach exactly as you described and it flew like a dream.  In my opinion the A/T does a great job as long as you fly a normal stabilized approach.

I am glad it works out nicely for you. 

I was once told by Chicago approach to maintain 210kts till 8nm on ILS 10C.

in rare cases like this. Use F5 (or at least F1) with the gear. Otherwise the airplane will not be able to maintain the speed with idle thrust and will keep on accelerating. 

the use of speed brake in this scenario is also fine. But the way I see this is, I will need to lower the gear a minute or two later anyway, might as well just do it now to reduce workload. 

Flying a big jet is all about workload management. 

[PMDG777 231]

28 minutes ago, Driverab330 said:

I am glad it works out nicely for you. 

I was once told by Chicago approach to maintain 210kts till 8nm on ILS 10C.

in rare cases like this. Use F5 (or at least F1) with the gear. Otherwise the airplane will not be able to maintain the speed with idle thrust and will keep on accelerating. 

the use of speed brake in this scenario is also fine. But the way I see this is, I will need to lower the gear a minute or two later anyway, might as well just do it now to reduce workload. 

Flying a big jet is all about workload management. 

Quick question for a real world pilot, I see that you often use the gear for drag to help slow the aircraft down or maintain speed, is it generally okay to do this providing you're below VLE? Is there anything else to consider before doing something like this?

[spesimen 135]

there must be some additional circumstance that is affecting things and causing the original poster's behavior. i have no idea what, but i don't really think i've ever seen the drop below the speed like it appears in the screenshot. i just did a hand fly ILS in the 777F from about 13nm out and the autothrottles kept me within 2-3 knots of the commanded speed all the way in. and that is with some surly winds too ( 06015G23KT)

 i don't think i did anything unusual just kept the loc and glideslope needles centered (with some glances at the PAPI too since i could easily see the runway from around 9nm out in this case..)

are you positive that your thottle's hardware position wasn't overriding the autothrottle somehow? or speedbrake extension didn't cause the initial drop below the commanded speed? is this with P3D or FSX (i use fsx:se still)? very curious

cheers,- andy crosby

[markjj 47]

Update:  First, thanks for the replies. I have unchecked the "enable turbulence" setting in the weather page of Prepar3d's menu and have so far been able to execute two stable and well maintained (in terms of speed) approaches.  AT maintained my command speed (VREF + 5) within +/- 2 knots I always had this unchecked, but as I have recently migrated to V4, I must have forgotten to turn it off. Keep in mind however that this could be coincidental, as my previous flights where the AT did not maintain thrust weren't particularly turbulent, if at all. I'll do some more flights tomorrow in different conditions to see if that did the trick. 

[Driverab330 74]

3 hours ago, PMDG777 said:

Quick question for a real world pilot, I see that you often use the gear for drag to help slow the aircraft down or maintain speed, is it generally okay to do this providing you're below VLE? Is there anything else to consider before doing something like this?

Yes as long as it is below Vle (270/.82), it is ok. 

But in practice we normally do it below F1 speed and we try to tight it with at least one flap setting to keep it operation standard.  

However, on the airbus, the fctm (what it used to be called), suggests pilots to lower the gear below ~210-230kts (don't remember the exact figure) to minimise the stress on the gear doors. Although not a limitation, but as professional pilots we would try to observe the requirement. 

I dont recall I have seen such requirement on the 777. But primarily we try to use speed brake which is designed to slow the airplane down unless it is very close to configuring the airplane for landing.

 

 

 

[kevinh 409]

12 hours ago, downscc said:

Neither the B744 nor B777 A/T will respond to decaying airspeed until about 10-15 kts below target airspeed.  This brought some discussion during beta testing and the developers were confident they had it right.

That can't be right. I've never come across an A/T control law that does not react until speed is 10-15 knots low. Even on the 747 Classic a 10 knot error would bring up an amber speed caution light.

Recent posts linking the A/T sluggishness to FSX/P3D turbulence sound more likely.

[longrangecruise 57]

I agree I flew the 777 for 4 years as a captain. The a/t of the 777 reacts very quick compared to a 767. That's why you are allowed to keep it on for landing.

I noticed the same slugish behavior of the PMDG 777's compared to the real aircraft.

Also note during approach the FADEC sets higher "approach idle" for quicker spool-up.

The 15-20 kts tolerance till the a/t reacts is only applicable when using VNAV and while staying on path in idle and letting the speed drift.

Some adjustments would be appreciated for the a/t on final approach.

[downscc 1,327]

5 hours ago, longrangecruise said:

The 15-20 kts tolerance till the a/t reacts is only applicable when using VNAV and while staying on path in idle and letting the speed drift.

That is probably the correct context and I stand slightly corrected.  However, it looks like most of us agree the A/T works very well if you have a stabilized approach.  I just cannot imagine allowing the speed to decay 10-15 kts below target speed on final approach, I've personally have not seen this with the PMDG product.  Probably cause I'm very obsessive when it comes to my approach speeds.

[BlueStar 323]

It's all about energy management.  It does not matter how good the A/T is, if the pilot does a poor job of energy management the A/T will not keep up.  As a note if I had an F/O that let the A/S get 5 or 6 knots slow on approach the next thing the FO would hear from me is "I've got the aircraft." Two potential CLM's are slow on final and landing short. 

blaustern

[Driverab330 74]

8 hours ago, Bluestar said:

It's all about energy management.  It does not matter how good the A/T is, if the pilot does a poor job of energy management the A/T will not keep up.  As a note if I had an F/O that let the A/S get 5 or 6 knots slow on approach the next thing the FO would hear from me is "I've got the aircraft." Two potential CLM's are slow on final and landing short. 

blaustern

 

Personally I reckon a "check speed" call would have suffice in this case as per the FCOM3 normally call out during the approach phrase. 

I tend to give people a bit more room, or if he / she is quite slow to keep up with the speed trend, I would suggest bug up the speed for another 5kts. That way the A/T is able to coupe with almost the worst case. 

This way everyone is happy, and it helps to give him/her some confidence as well. 

Just my two cents.