markjj

Autothrust Extremely Slow to React on Approach

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The title says it all.  The autothrust while manually flying is extremely slow to react and routinely gets me into the yellow tape.  I can literally fall 10 knots below command speed and only then will AT react, but with ever so slight thust increases.  I will be literally hitting 130 knots when command is 142, and the 777 will react with a mere 45% N1 response which will slow down the deceleration, but never get me to my command speed.  This happens without turbulence as well (forget choppy weather as the thing will literally let me stall).  Here are some photos to show what I mean.  It makes manually flying the 777 with AT on a nearly impossible task.  

 

 

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I have noticed the same behavior yesterday. Was coming in to land in the 77L in P3Dv4, AP off with A/T on, command speed was 140. I dropped all the way down to 130 or so, then the A/T commanded a spool up to get my speed back up. As the speed was decreasing, the engines were spooling up a bit, but not enough to get the speed up to the commanded speed. It wasnt until it dropped by 10 knots or so did it finally try to get back to commanded speed

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True, it can be slow to respond.  Don't forget the very large fans take time to spool up.  Don't get into that situation on final, walk the speeds down.  I'm not sure how you got to the first picture but by then you had already done something wrong.  Never assume the A/T is going to respond quickly to anything, it simply cannot do it.

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28 minutes ago, downscc said:

True, it can be slow to respond.  Don't forget the very large fans take time to spool up.  Don't get into that situation on final, walk the speeds down.  I'm not sure how you got to the first picture but by then you had already done something wrong.  Never assume the A/T is going to respond quickly to anything, it simply cannot do it.

The reason I was in that situation is precisely because I allowed the AT to try and maintain speed.  Minutes before, I was stable on approach with a speed of about 170.  As I entered final landing configuration (gear and flaps 30), the aircraft began its deceleration upon my input of command speed 142.  The issue is that it doesn't "catch" 142.  AT will slowly add thrust to try to "catch" the final approach speed, but it will not be enough.  It will gradually get closer and closer to the yellow tape and I am forced to increase pitch just to maintain the GS (explaining the first picture).  I am forced to disengage AT and add manual thrust just to keep it from stalling.  I can literally be 15 knots under command speed and it will add a meager 55% N1 thrust as I watch the aircraft slowly lose stability.  This is the only aircraft that does this too.  The PMDG 747 and 737 don't seem to have this issue

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32 minutes ago, downscc said:

True, it can be slow to respond.  Don't forget the very large fans take time to spool up.  Don't get into that situation on final, walk the speeds down.  I'm not sure how you got to the first picture but by then you had already done something wrong.  Never assume the A/T is going to respond quickly to anything, it simply cannot do it.

Also, I am aware that the large fans take time to spool up.  The issue here is that it takes a long time before it even initiates a thrust response, and when it does, it does so in tiny increments and even starts reducing thrust long before it reaches command speed.  It makes it unflyable, unfortunately 

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1 hour ago, uhntissbaby111 said:

I have noticed the same behavior yesterday. Was coming in to land in the 77L in P3Dv4, AP off with A/T on, command speed was 140. I dropped all the way down to 130 or so, then the A/T commanded a spool up to get my speed back up. As the speed was decreasing, the engines were spooling up a bit, but not enough to get the speed up to the commanded speed. It wasnt until it dropped by 10 knots or so did it finally try to get back to commanded speed

Exactly...it's very frustrating after a long flight to have an approach ruined by inadequate AT response.  The only way I can keep the aircraft from becoming completely unstable is if I manually control thrust.

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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.

21 minutes ago, markjj said:

Minutes before, I was stable on approach with a speed of about 170.

Why were you at 170?  That happens to be the default speed the FMS throws in there but it is rarely a speed actually used.  A good procedure is to reduce MCP Speed to match flap speeds, and I generally start with F1 for the segment flown at 210, then F5 speed for ILS capture and F15 with gear down then on final landing flaps.  The MCP Speed is set for whatever is the appropriate flaps speed in steps.  The speed is maintained as precisely as you could wish.  In the event of a windshear event where you need thrust now you're only correct option is TOGA.

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1 hour 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.

Why were you at 170?  That happens to be the default speed the FMS throws in there but it is rarely a speed actually used.  A good procedure is to reduce MCP Speed to match flap speeds, and I generally start with F1 for the segment flown at 210, then F5 speed for ILS capture and F15 with gear down then on final landing flaps.  The MCP Speed is set for whatever is the appropriate flaps speed in steps.  The speed is maintained as precisely as you could wish.  In the event of a windshear event where you need thrust now you're only correct option is TOGA.

I'm sorry Dan, but that can't be right. Although the in the real plane the actual speed wobbles around the bug, it only does so in turbulence. And even so it does not deviate more that 5 or 6 knots. If it does, the throttle react to bring it back. In smooth flight, like we have in FS, it shouldn't budge at all. I have sent a ticket regarding this and speed control during go around many moons ago during FSX days. Hopefully it will one day get resolved. I have learned to live with it.

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2 hours ago, downscc said:

True, it can be slow to respond.  Don't forget the very large fans take time to spool up.  Don't get into that situation on final, walk the speeds down.  I'm not sure how you got to the first picture but by then you had already done something wrong.  Never assume the A/T is going to respond quickly to anything, it simply cannot do it.

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

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2 hours ago, markjj said:

It will gradually get closer and closer to the yellow tape and I am forced to increase pitch just to maintain the GS (explaining the first picture).  I am forced to disengage AT and add manual thrust just to keep it from stalling.

Increasing thrust manually should be your first action if you're below GS, definitely not pitch up.

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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.

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1 hour ago, PMDG777 said:

Increasing thrust manually should be your first action if you're below GS, definitely not pitch up.

This is besides the point.  I only adjusted pitch to emphasize the lack of a response from the AT.  I should not have to manually take control of the thrust just to keep the aircraft from falling into the yellow tape

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2 hours ago, B777ER said:

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

That was more to do with the flight crew being in FLCH mode I believe.  I will post a video later to explain the PMDG's behavior

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4 hours ago, downscc said:

True, it can be slow to respond.  Don't forget the very large fans take time to spool up.  Don't get into that situation on final, walk the speeds down.  I'm not sure how you got to the first picture but by then you had already done something wrong.  Never assume the A/T is going to respond quickly to anything, it simply cannot do it.

 

This case is probably only true on the 747-200 Classic certified to CATII ops, where the A/T was very very sluggish. Certainly not on a modern jet like the 777. 

 

The A/T on the 777 is very responsive at approach speed, which was designed to provide enough gust protection and the FCTM did not have a specific bug up requirement in gusty condition, although the usual practice for people on the line is to fly Vref + 10 and even in the most gusty situation with direct cross wind of 20G35kts, the A/T does a very very good job, it almost never allows the speed drops below Vref. 

By design on the 777, the A/T should be superior to Manual thrust, and for that reason, when a pilot decides he/she wants to use manual thrust FCOM 3 specifically said he/she should bug up the speed to half of the head wind + all the gusts with a max speed of Vref+20. 

Yes the GEs are big, but they are very responsive, even more so than the RR Trent 800 with triple spool on the 777-200/300, this one is quite sluggish in relative terms. 

Normally the only A/T overshoot the target speed is when the airplane first reached its cruising altitude, the A/T will reduce the thrust until the speed is 1-2kts below target speed then it will put the thrust back to normal. One of the engineer told me this was to test the thrust level servo motor, but I could not find any reference to this. 

I think most importantly, the A/T in the FSX version does a very good job with manual flying, I have yet to try P3D 77LR with manual thrust, because the last couple of flights I did I used manual thrust.

I am not suggesting this is necessarily a bug, I would recommend the original poster to isolate any potential of the hardware and software setup which may caused such behaviour. 

 

 

 

 

 

 

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13 minutes ago, markjj said:

This is besides the point.  I only adjusted pitch to emphasize the lack of a response from the AT.  I should not have to manually take control of the thrust just to keep the aircraft from falling into the yellow tape

Ah sorry I misunderstood your post.

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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

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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.)

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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.

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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.

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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.

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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. 

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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?

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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

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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. 

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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.

 

 

 

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