747-8F does not correcty respect speed constraints

[Burkhard Bickmann 1]

an example:

• Lets say the minimum speed for Flaps up is 230kn, Minimum Speed for Flaps 1 is 180kn.
• I'm in descend phase below 10000ft at 250kn and the next waypoint has a speed constraint of 200kn.
• The Aircraft is now decelerating until it reaches 230 and correctly stays at this speed to avoid stalling.
• And now I deploy Flaps 1 and the aircraft continues to decelerate, but it NOT keeps the 200kn constraint. It decelerates to 180kn. And so on.
• Each time, I deploy the next level of Flaps, it decelerates to the flaps minimum speed and does not respect the speed-constraint in the CDU Flightplan.
• This is really annoying, because each time i have to intervent by manual speed selection.

And I cannot imagine, this is like the real one. I remember, at one time this problem was not there!

Can you please have a look at that?

Edited November 16, 2018 by Burkhard Bickmann

[scandinavian13 5,192]

26 minutes ago, Burkhard Bickmann said:

Can you please have a look at that?

We'll need a much more specific example to look at. While your post has a decent narrative of what you're seeing, I need something very specific to test in the sim: an exact STAR, an exact configuration of speeds versus flap limits.

Sounds more like a misunderstanding of autopilot modes versus flap speeds, at least at first glance. Without a specific scenario to test, that's what I have to rest on.

[Jetlinker 13]

I have noticed this as well. I can try to dialogue a specific example the next time I use the sim, but just like the original poster I have noticed that the VNAV will ignore an inputted speed constraint from the FMC and instead slows to the minimum flap speed as soon as a new flap setting is selected. This occurs even if the inputted speed is within the flaps structural limits. The only way I’ve found to get around it is to speed intervene.

Jeffery Williams

[windshearDK 52]

13 hours ago, Jetlinker said:

but just like the original poster I have noticed that the VNAV will ignore an inputted speed constraint from the FMC and instead slows to the minimum flap speed as soon as a new flap setting is selected. This occurs even if the inputted speed is within the flaps structural limits. The only way I’ve found to get around it is to speed intervene.

As far as I know, this is true to the original Boeing logic. That you can slow down the aircraft, but once you put flaps on, VNAV will slow to that flap min spd. Lets say the min flap 0 spd is 200, can you slow down to this speed? if not which speed will it slow to?

[windshearDK 52]

20 hours ago, Burkhard Bickmann said:

• And now I deploy Flaps 1 and the aircraft continues to decelerate, but it NOT keeps the 200kn constraint. It decelerates to 180kn. And so on.
• Each time, I deploy the next level of Flaps, it decelerates to the flaps minimum speed and does not respect the speed-constraint in the CDU Flightplan.

I think this is normal Boeing logic. If you are in VNAV and you put flaps it will slow down to the min flap speed. I always use FL CHG or Vert SPD on these final 15NM to the approach fix. Its better to handle the speed manually etc and comply with ATC. I think this is the most common thing in real life as well. Very rarely are the entire approach flown fully in VNAV

[Jetlinker 13]

3 hours ago, windshearDK said:

I think this is normal Boeing logic. If you are in VNAV and you put flaps it will slow down to the min flap speed. I always use FL CHG or Vert SPD on these final 15NM to the approach fix. Its better to handle the speed manually etc and comply with ATC. I think this is the most common thing in real life as well. Very rarely are the entire approach flown fully in VNAV

Well, yes, you’re correct for the final approach segment. I believe what the initial post was referring to as well as my own experience is on a STAR where there is, for example, a 230kt speed over a fix. At heavier weights that will require at least flaps 1, however selecting flaps 1 would cause the VNAV to fly the min flaps 1 speed and ignore the speed instruction on the STAR. The 400 doesn’t appear to do this, only the -8. 

Ive been out of the real aircraft on medical leave for a while, so I can’t say for certain... but I’m 99% sure the real aircraft doesn’t behave this way as it would cause all sorts of problems while on a STAR with several speed constraints. I will try to dig through the real world manuals and see what I can find though. I’ve got a couple of coworkers I can check with as well who are better gurus with this stuff than I am. 

Edited November 17, 2018 by Jetlinker

[BlueStar 323]

47 minutes ago, Jetlinker said:

ve been out of the real aircraft on medical leave for a while, so I can’t say for certain... but I’m 99% sure the real aircraft doesn’t behave this way as it would cause all sorts of problems while on a STAR with several speed constraints.

It is my belief that it is not the aircraft's responsible for complying with the speed and altitude restrictions. 🙂

This is one of the reasons that I hard load all speed and altitude restrictions on SIDS and STARS.  As an example if I had an altitude and speed restrictions on a STAR of 290 KIA an 240A280B for altitude, then I would have 290 in the speed window and 240 in the altitude window.  If the aircraft does not make the speed and altitude restrictions then it's not the airplanes fault, it's mine.   I also always use FLCH below 10,000. 

I do not ever trust the magic. 🙂

Grace and Peace, 

[scandinavian13 5,192]

1 hour ago, Bluestar said:

It is my belief that it is not the aircraft's responsible for complying with the speed and altitude restrictions. 🙂

Agreed. The automation is there to help, and not do.

1 hour ago, Bluestar said:

I do not ever trust the magic. 🙂

Precisely.

[Jetlinker 13]

3 hours ago, Bluestar said:

It is my belief that it is not the aircraft's responsible for complying with the speed and altitude restrictions. 🙂

This is one of the reasons that I hard load all speed and altitude restrictions on SIDS and STARS.  As an example if I had an altitude and speed restrictions on a STAR of 290 KIA an 240A280B for altitude, then I would have 290 in the speed window and 240 in the altitude window.  If the aircraft does not make the speed and altitude restrictions then it's not the airplanes fault, it's mine.   I also always use FLCH below 10,000. 

I do not ever trust the magic. 🙂

Grace and Peace, 

Different pilots have different techniques. Obviously it is still the responsibility of the PIC to monitor automation and comply with restrictions, if an intervention becomes necessary then it’s obvious that is what you would do. In a 20 or 30 year old aircraft such as an MD11 or even a 747 400 one has to be aware that the automation was never designed to manage the type of complex airspace that now exists, and trust in automation should be minimal. However, in a modern aircraft like a 747-8, or a 787, the philosophy is to let the automation do the work as most of the time it will do it quite accurately and precisely. In the unlikely event that it doesn’t, you intervene. 

In any case I wasn’t really looking to rehash the old debate of overuse or underuse of automation. I’m merely stating that I have experienced the same behavior with the sim as the original poster. 

Edited November 17, 2018 by Jetlinker

[downscc 1,327]

OP hasn't responded with clarification.  Could be that there is a misunderstanding of how the FMC responds to speed constraints as "at or below?"  It correctly slows to 180 instead of the 200 constraint in his narrative.

[windshearDK 52]

7 hours ago, Jetlinker said:

Well, yes, you’re correct for the final approach segment. I believe what the initial post was referring to as well as my own experience is on a STAR where there is, for example, a 230kt speed over a fix. At heavier weights that will require at least flaps 1, however selecting flaps 1 would cause the VNAV to fly the min flaps 1 speed and ignore the speed instruction on the STAR. The 400 doesn’t appear to do this, only the -8. 

Selecting flaps in this scenario has the same result. It doesn't matter when you put flaps 1, the logic is the same. A pilot needs to think ahead of the aircraft. If he needs to put flaps 1 to keep 230kts, and he's not on downwind or base, he should do this in FL CHG or VERT SPD... In the real world you could ask for your min. 0 flap spd instead of the 230KTS contraint also.

I know that BA flies 744 into Heathrow at min. clean speed or 190 kts. until base at least.

[Jetlinker 13]

37 minutes ago, windshearDK said:

Selecting flaps in this scenario has the same result. It doesn't matter when you put flaps 1, the logic is the same. A pilot needs to think ahead of the aircraft. If he needs to put flaps 1 to keep 230kts, and he's not on downwind or base, he should do this in FL CHG or VERT SPD... In the real world you could ask for your min. 0 flap spd instead of the 230KTS contraint also.

I know that BA flies 744 into Heathrow at min. clean speed or 190 kts. until base at least.

That’s fine, but again all of that is technique. The question is regarding the behavior of the real aircraft versus the sim in this particular scenario. You can discuss technique with 5 different pilots and get 5 different answers. In the scenario that I provided I do not believe that the aircraft would ignore the 230 knot input into the FMC simply because flaps have been selected as you are suggesting. As long as the inputted speed is within the acceptable flap limits it should fly that speed.

Also, many modern STAR procedures will give altitude and speeds as well as lateral navigation sometimes all the way onto final. What you are suggesting is that one should have to switch to FLCH to comply with restrictions that are already in the FMC and I respectfully will have to disagree with you.  As for VS that is rarely ever used in the real world as it has no low speed protection which is provided by other modes. When I was typed in the 400 I was taught that VS stands for “Very Seldom”. It’s use is discouraged unless necessary. 

downscc does bring up a valid point as to whether these were max speeds or “hard” speeds. Some speed constraints on a STAR will state “AT” and others “MAX”. To my knowledge though, and I could be wrong, the FMC does not recognize at or below or at or above when it comes to speed as it does with altitude. 

Edited November 18, 2018 by Jetlinker

[downscc 1,327]

39 minutes ago, Jetlinker said:

the FMC does not recognize at or below or at or above when it comes to speed as it does with altitude. 

Partly true.  You cannot enter as an example 220B/ as a constraint as you can in the NG; however, the VNAV solution will always treat 220 as at or below constraint.  This was hammered into us by the developers during testing when we noted -8 slowing to 190 at 10000 on specific arrivals instead of 240 (good example is arriving KONT Ontario over the San Bernardino mountains landing West).  It is different from the B744, it is more like the B787.

[Jetlinker 13]

Ok guys, so I just did a couple of quick flights from DFW-IAH to try to better illustrate this. I used the DRLLR 5 arrival runway 26R transition as this is a fairly complex STAR with several speed constraints.  First I did it in the 400 fairly close to MGLW. The first two restrictions are 250kts at DRLLR then 240 at DOMNO. Since 240 was still slightly above min flaps up speed, no flaps were required. Then the final waypoint, SKYLR, has a 210kt restriction. As I approached SKYLR the VNAV commanded a speed reduction to 236kts. This is the min flap up speed. This is correct and to be expected as a speed slower than 236 would be unsafe. I select flaps 1. The VNAV then commands 210kts, as is consistent with the specified speed on the arrival. The instruction at SKYLR is “AT 210kts”, not max. The airplane continues to fly at 210. All is well. 

Then I repeated this exact scenario in the -8F. Just as before the first restriction of 250kts was fine without any flaps, however the 240kt restriction was slightly below min flaps up speed. The speed commanded the min flaps up speed of 243, just like the 400. However when I selected flaps 1 the speed then command 226 knots, as this was the min flaps 1 speed. Rather than intervening I continued on the arrival. As I approached SKYLR where there is the 210kt restriction, I select Flaps to 5. VNAV speed reduces to 208 knots, as this was the min speed for flaps 5. From the point that I began selecting flaps, the speed constraints within the FMC were ignored, requiring speed intervention to be compliant with the STAR.  

The question is whether or not the real aircraft does this, and I don’t believe that it does. We often fly in VNAV until we are vectored off of an arrival, or given instructions that conflict with the VNAV profile. That is how we are trained at least where I work. Based on this it would be required for the pilot to speed intervene in order to comply with the published speed requirements as following the VNAV speeds would not work. 

I’m not trying to split hairs, it’s a wonderful aircraft for a home simulation. But since someone else brought this up I just wanted to back up his experience. 

Edited November 18, 2018 by Jetlinker

[downscc 1,327]

FCOM 11.31.22  All speed constraints are considered by the FMC as at or below constraints.

Waypoint speed constraints on arrival and approach charts are also at or below constraints unless otherwise noted.  The PMDG aircraft performs with regard to speed constraints as described in the FCOM and as tested by the technical team.

If you have information to the contrary please provide a reference.