747-8F does not correcty respect speed constraints

[Jetlinker 13]

17 minutes ago, downscc said:

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.

I do see what you are referencing. Perhaps I’m wrong, I just don’t remember ever having to speed intervene on an arrival because the airplane wanted to fly the min flap speeds instead of what was in the FMC. Arrival speeds are not necessarily at or below by default, as I said speeds are either notated as MAX MIN or AT on a STAR. If it says AT, you are expected to maintain that speed. 

I have nothing however to back this up aside from my own recollection of the real aircraft. If the FAA ever gets around to approving my medical paperwork I’ll have to test it out sometime and let you know for sure. 😁

Edited November 18, 2018 by Jetlinker

[downscc 1,327]

1 hour ago, Jetlinker said:

Arrival speeds are not necessarily at or below by default, as I said speeds are either notated as MAX MIN or AT on a STAR. If it says AT, you are expected to maintain that speed. 

True... and for example the FYTTE into KORD I just flew does in fact have in bold red font AT 280 or AT 250 and those are not at or below speeds.  But if the there is no AT MAX or MIN then it is an at or below..... I don't think I've seen a MIN charted... gotta think about that.

Regardless, if you have a Boeing technical reference that contradicts how the simulation performs please put that on a support ticket.  I've reviewed the NG FMS Guide for the B748 and the FCOM and FCTM and cannot find anything in this regard that is contradictory.

Not only does the speed drop as in your example the SPD becomes 208 instead of 210 with flaps.... if you play with it and say insert a 190 constraint at say the IAF of the approach because it is 10 nm from touchdown, then the FMC will look for a solution where there is a deceleration ramp to allow speed to decay from for example 240 to 190.  Usually this deceleration is within the 2 nm before the constraint.  However, if there is no space for such a ramp due to the lack of room (because of a descent where the aircraft cannot maintain 240 and descend early enough to make room for the ramp) then the FMC will set the SPD of 190 at the 240 constraint.  You can play with this arriving KONT from the high desert and put a 190 at the ILS27L IAF.  Your descent from 8000-10000 becomes 190 instead of 240.  It's unexpected but it's actually nice... no more descent path unachievable messages and a lot less need for speed brake.... and due to the very clean design of this big machine this is very desirable.

Edited November 18, 2018 by downscc

[BlueStar 323]

52 minutes ago, downscc said:

Your descent from 8000-10000 becomes 190 instead of 240.

Dan,

Are you saying have the aircraft slowed to 190 KIAS at "8000-10000" in your example?

Grace and Peace, 

[Jetlinker 13]

15 hours ago, downscc said:

Regardless, if you have a Boeing technical reference that contradicts how the simulation performs please put that on a support ticket.  I've reviewed the NG FMS Guide for the B748 and the FCOM and FCTM and cannot find anything in this regard that is contradictory  

No worries. I understand you all have to design the aircraft in accordance with the manuals. Sometimes Boeing manuals can be a bit ambiguous, or as we are finding out in the case of the Lion Air accident things are left out of the manuals completely. Either way, it’s not really that big of a deal.

Edited November 18, 2018 by Jetlinker

[BlueStar 323]

18 minutes ago, Jetlinker said:

as we are finding out in the case of the Lion Air accident things are left out of the manuals completely

Are you sure that runaway trim is not covered in the Boeing manuals?  

Grace and Peace, 

[Jetlinker 13]

Just now, Bluestar said:

Are you sure that runaway trim is not covered in the Boeing manuals?  

Grace and Peace, 

You know what I’m referring to. Boy, there are some folks on this forum that sure love to pick every post apart to create an argument. 🙄

[BlueStar 323]

7 hours ago, Jetlinker said:

You know what I’m referring to. Boy, there are some folks on this forum that sure love to pick every post apart to create an argument. 🙄

Well no not really. Telepathy was never a strong suit of mine.  🤣

Grace and Peace,

[Sekkha 15]

On 11/18/2018 at 3:01 AM, downscc said:

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.

For  the 747-400  the FCOM states the same on page 11.41.8:  "Speed constraint is assumed to be at or below the displayed speed" ...But the 744 seems to behave differently...

 

15 hours ago, Jetlinker said:

 Sometimes Boeing manuals can be a bit ambiguous

 

I imagine this is what Jetlinker possibly meant.... 

 

 

best regards
 

[scandinavian13 5,192]

17 hours ago, Jetlinker said:

You know what I’m referring to. Boy, there are some folks on this forum that sure love to pick every post apart to create an argument. 🙄

No need to be rude.

You made a strong statement, which has some pretty serious implications. He questioned it, and rightly so.

If you make a statement, stand behind it and support it if questioned. If you can't support a statement, or worse yet, stand behind a personal shot, then you shouldn't make the statement in the first place.

[windshearDK 52]

On 11/18/2018 at 12:54 AM, Jetlinker said:

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. 

PMDG replicated the 748 very well. I can ask a pilot friend who flies both the 748 and 744 if you want, but its the same behavior in the 777. The speed constraints in STARs are no issue for the PMDG version, but if you are flying a fully loaded 748F the weight is very different form a 77W or 744F, so in this case you, the pilot, has to be on top of that. Meaning either you ask for min flap speed with the controllers or you use FL CHG or V/S.

Very Seldom you might call it, and frankly I use it 1/10 times I would think, but listen its there so you can use it when you deem it necessary, that's why Boeing puts their planes into the hands of the qualified crew. Its up to you what you do to perform a good and safe approach.

[Jetlinker 13]

4 hours ago, scandinavian13 said:

No need to be rude.

You made a strong statement, which has some pretty serious implications. He questioned it, and rightly so.

If you make a statement, stand behind it and support it if questioned. If you can't support a statement, or worse yet, stand behind a personal shot, then you shouldn't make the statement in the first place.

A strong statement? I stated a fact, the Lion Air accident investigation has revealed a stall protection system that was never put into Boeing manuals. I used that as an example that the manuals are not necessarily the holy grail when it comes to how the real airplane actually does things. 

[scandinavian13 5,192]

3 minutes ago, Jetlinker said:

A strong statement? I stated a fact, the Lion Air accident investigation has revealed a stall protection system that was never put into Boeing manuals. I used that as an example that the manuals are not necessarily the holy grail when it comes to how the real airplane actually does things. 

I understand. All the same, your delivery was a bit disrespectful, and your response did not correctly address the question posed. There was an implicit understanding that the manuals may not have included info about the system, but the assertion is that runaway trim is actually addressed, which could have helped avert the disaster.

If someone addresses your post, defend it, but leave the ad hom out.

 

All the same, we're off on a tangent - let's get back.

[Jetlinker 13]

6 minutes ago, scandinavian13 said:

I understand. All the same, your delivery was a bit disrespectful, and your response did not correctly address the question posed. There was an implicit understanding that the manuals may not have included info about the system, but the assertion is that runaway trim is actually addressed, which could have helped avert the disaster.

If someone addresses your post, defend it, but leave the ad hom out.

 

All the same, we're off on a tangent - let's get back.

My apologies to him if I indeed misinterpreted his tone. I didn’t want to go into a debate about the specifics of the accident, just as debating automation use from his earlier post wasn’t really what this thread was about either. Regardless, I appreciate the useful info here about the product and from here on out I will keep my posting to a minimum. 

Edited November 19, 2018 by Jetlinker

[Jetlinker 13]

1 hour ago, windshearDK said:

PMDG replicated the 748 very well. I can ask a pilot friend who flies both the 748 and 744 if you want, but its the same behavior in the 777. The speed constraints in STARs are no issue for the PMDG version, but if you are flying a fully loaded 748F the weight is very different form a 77W or 744F, so in this case you, the pilot, has to be on top of that. Meaning either you ask for min flap speed with the controllers or you use FL CHG or V/S.

Very Seldom you might call it, and frankly I use it 1/10 times I would think, but listen its there so you can use it when you deem it necessary, that's why Boeing puts their planes into the hands of the qualified crew. Its up to you what you do to perform a good and safe approach.

Yes, I agree they have done a fantastic job with the aircraft. Again, my intent was only to back up the experience of the original poster. The problem with switching to FLCH or VS on an RNAV STAR is that you are leaving the VNAV profile, and unless you continue to set floor altitudes along the way you also lose altitude constraint protections as you descend. The easiest solution to get around the issue that was posed here is simply to speed intervene. Which in the -8 particularly isn’t a big deal since you can intervene and remain on the VNAV path unlike in the 400. I mean, to each his own, if you want to set floor altitudes along the way and fly in FLCH, you can certainly do that. If I attempted that with a check airman on board I believe it would cause some backlash and a hefty debrief though. 😁

[windshearDK 52]

1 hour ago, Jetlinker said:

A strong statement? I stated a fact, the Lion Air accident investigation has revealed a stall protection system that was never put into Boeing manuals. I used that as an example that the manuals are not necessarily the holy grail when it comes to how the real airplane actually does things. 

The investigation is far from over dude...

For reference here's the FAA's bulletin

This AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer. We are issuing this AD to address this potential resulting nose-down trim, which could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.

http://rgl.faa.gov