News:

Precision Simulator update 10.173 (24 February 2024) is now available.
Navburo update 13 (23 November 2022) is now available.
NG FMC and More is released.

Main Menu

"On-Approach" Logic on STAR?

Started by emerydc8, Sat, 1 Aug 2015 10:56

Britjet

I don't know, Jon, but the quote above makes sense to me...
Peter

emerydc8

#41
Morning, Peter. I have been researching this issue for the past ten hours. I think the DRAG REQUIRED message comes on even if you are on the path but just fast.

Example:

You are level at 15,000' doing 300 knots in |SPD|LNAV|VNAV PTH|. You are 5 miles from a fix that has no speed restriction associated with it. Then, ATC instructs you to cross that fix at 220 knots.

If you insert that 220 knot speed restriction into the LEGS page and execute the throttles will go to idle and I will bet money that you will get a DRAG REQUIRED message.

Hardy Heinlin

If a 100-page document says ...

"DRAG REQUIRED appears when X is true"

... then such a sentence just mentions one condition. You cannot know if X is the only condition, or whether there are additional conditions A, B, C, D ... mentioned somewhere else on those 100 pages, in different words or in a less vague language, or even with contradictory details.

There are zillions of function triggers in the avionics of this aircraft. Nearly every function is not just triggered when a target value is exceeded; a trigger typically also involves:
- a time delay
- a hysterisis
- a trend

Without these additional interactive conditions, you would get lots of nuisance messages or flip-flop effects.

For example, I bet that the DRAG REQUIRED message will not be triggered only when a speed is exceeded; there must also be a time delay that allows short-term fluctuations and/or a hysterisis to inhibit flip-flops, and a trend away from the target.


Cheers,

|-|ardy

emerydc8

Hi Hardy. I agree that there is obviously more that will set off the DRAG REQUIRED message than just being unable to stay on a VNAV PTH descent at 349 knots. I'm just trying to get something in writing on it, but Boeing seems to have decided that no one needs to have that information.

I can find only two places where DRAG REQUIRED is mentioned in my 744 manuals. One is the two-page document I presented above. The other is under FMC scratchpad messages, where it defines the DRAG REQUIRED message as "VNAV active and additional drag required or autothrottles off and less thrust required to maintain descent path."

It must make it difficult to write an accurate simulator program without access to it. Do you have access to this information?

Jon D.

emerydc8

#44
Quote from: Hardy HeinlinIf a 100-page document says ...

"DRAG REQUIRED appears when X is true"

... then such a sentence just mentions one condition. You cannot know if X is the only condition, or whether there are additional conditions A, B, C, D ... mentioned somewhere else on those 100 pages, in different words or in a less vague language, or even with contradictory details.

True, but when we see RESET MCP ALT on the scratchpad, is there any other place -- other than 2 minutes prior to top of descent in VNAV -- that we should expect to see that? I don't think so. Somewhere, there is documentation of what other conditions should trigger the DRAG REQUIRED message. I'd like to find it.

[ADDENDUM]:

The two pages I provided above came directly from my company's FCOM, which I presume they got from Boeing. I did a word search of the FCOM for DRAG REQUIRED and other words that might turn up any discussion of this message. So, if Boeing is trying to hide it somewhere using different words, they are doing a good job of it!

cavaricooper

According to "the Man" Bill Bulfer there are only 3 scenarios that result in the DRAG REQUIRED message...

DRAG REQUIRED (AL/4)
VNAV engaged and additional drag required or A/T off and less thrust required to maintain descent path.

DRAG REQUIRED (AL/7/g)
Airplane is unable to maintain the precomputed nominal descent path and stay within speed tolerances

DRAG REQUIRED (AL/n/p)
VNAV engaged and additional drag required to track descent path and maintain command speed.

I quote from page 257 of his Big Boeing FMC User's Guide.... so Instance 2 or 3 would seem to indicate that you WOULD get this message even when on path, but fast....

HTH- C
Carl Avari-Cooper, KTPA

emerydc8

#46
The problem here is that only the first instance (al/4) clearly applies to the 744. It looks like the second (al/7/g) applies to the 777 Pegasus; and the third (al/n/p) applies to the 757/767 and non-PIP aircraft. I don't know whether the third would apply to the 744, but even if it did, it's still only covering the descent -- not level flight. It doesn't shed any more light on when exactly the DRAG REQUIRED message will appear -- specifically what triggers it.

KEY
Alerting            AL
Advisory          AD
Entry Error       EE
Comm              C
all                    all versions of software
n                      Non-PIP
P                      PIP 757/767
g                      Pegasus
4                      747
7                      777
AAA                  VOR identification
AAAA                LOC identification
NNNN               Numerical value
XXXX                Alphabetical value
These codes are not 100% accurate.

cavaricooper

#47
Jon-

I've copied Bill and asked for his thoughts... will advise as soon I hear back.

Best- C
Carl Avari-Cooper, KTPA

emerydc8

#48
Thanks. Not to beat a dead horse, but it seems that everyone has an opinion about when the DRAG REQUIRED message is triggered; and I haven't seen anything in writing other than the 349 knots (VMO/MMO-16), which I presume came directly from Boeing. I know there's got to me more because I think everyone who's flown the airplane recalls seeing it when they were not even close to 349 knots.

Hardy Heinlin

#49
Quote from: emerydc8 on Sun,  2 Aug 2015 10:22
I agree with Peter that the only time you can speed intervene and stay in VNAV PTH is when flying level at an FMC altitude constraint, or when in on-approach logic.

I did, however, find three situations on a descent where you can see |SPD|LNAV|VNAV PTH| with the speed window closed:

1) If after the first altitude constraint waypoint, the second VNAV path segment is too shallow to be flown in IDLE, then you will see |SPD|LNAV|VNAV PTH| while descending on that segment.

Hi Jon,

if your item 1) is a real-world feature and not a PSX problem report, I actually see no significant reason to modify PSX in this area. PSX operates in SPD | | VNAV PTH when the first constraint is sequenced and the path deviation is small.

When the first descent constraint after T/D is sequenced, i.e. when the idle path segment is passed, the following path angles are almost always shallower than the path angle of the idle descent.

The FMC shifts the T/D so that the predicted idle path leads directly to the first altitude constraint. After the first altitude constraint, it is not possible to shift any waypoints in order to build an idle descent.

If an idle descent works thereafter anyway, then it's pure coincidence, I would say. I think the STAR designers take into account a certain groundspeed reserve for tailwind, anti-ice high idle, and high QNH (high geographic flight levels) etc. In other words, to get these reserves, these following, geographic paths must be shallower than the initial, typical idle descent path. So in an average situation, you would always fly these segments with the thrust out of idle as you wrote in your item 1).

And that's what PSX does as well.

Also, PSX changes to HOLD | | VNAV SPD when the aircraft is too high above the path.

The only problem I see on this subject is that PSX keeps SPD | | VNAV PTH when descending with the MCP SPD window open before the approach logic is on. I'll try to correct this.

But changing SPD | | VNAV PTH to THR | | VNAV PTH after the idle descent is not trivial at all. This is not at all a cosmetical job of just changing a word on the FMA. That would be a fake modification. Changing this feature would mean a massive modification in the inner mode logic, and this affects thousands of lines of code, and requires a very long testing phase. Also the mode itself is different: it would be an airspeed-on-elevator mode instead of an airspeed-by-thrust mode. It's not just a word on the FMA :-)


Cheers,

|-|ardy

emerydc8

Hi Hardy and thanks for looking at this subject again.

With regard to the speed intervention point, I think you are on top of that and hopefully it won't be too hard to fix.

As for the FMA indication, the good news is that there is no need to change it from SPD||VNAV PATH to THR||VNAV PTH because you should never see THR||VNAV PTH on a VNAV descent. The path is +/- 150'. If you are within that envelope (on path) and you encounter an unforecast headwind, and slowing 15 knots below FMC target is still not enough to maintain the path, you should see SPD||VNAV PTH as the power is applied. I think PSX also does this part correctly.

If for whatever reason you find yourself below the path (>150') and the speed window is closed, the FMS will enter the path capture mode (just like on an early descent) and you should see THR/HOLD||VNAV SPD as the aircraft attempts to recapture the path with a 1250'/min descent rate. Once it recaptures the path, it should go back to IDLE/HOLD||VNAV PTH.

So, when power is applied while on the path you should see SPD||VNAV PTH. When power is applied while under the path you should see THR||VNAV SPD. On a VNAV descent, you should never see THR||VNAV PATH, so there is no need to change that.

Of course, with the speed window closed, you should never get under the path to begin with because eventually power will be added to prevent this. But just for illustration purposes, if you were on a normal VNAV descent and you speed intervened with say 340 knots, this would take you under the path. The act of speed intervention would shift the FMA out of VNAV PTH and into VNAV SPD (even if you were within 150' of the path). So, let's say you did this until you were 4,000' below the path and then closed the speed window. What would happen? The pitch mode would stay in VNAV SPD and the nose would pitch up to target the FMC speed. This would shallow the descent rate. If this slower speed was not enough to shallow the descent rate to 1250'/min, then power would be applied and you should see THR/HOLD||VNAV SPD as you converge back to the path. When you recapture the path (within 150') the FMA should shift to IDLE/HOLD||VNAV PTH.

With regard to getting high on the path, prior to the first speed constraint, based on all my research, it would be very unusual to lose the path because the aircraft will accelerate up to VMO-11 (354 knots) to maintain it. If you did lose the path, it would shift to HOLD||VNAV SPD (speed reversion) when it gets more than 150' high.. I have observed PSX correctly shifting into VNAV SPD when it was high, but it usually occurs when much higher than 150'. If you could change that parameter, it would be helpful.

The reason 354 knots sounds like an unusually high speed to maintain the path is that to be in VNAV PTH, you would (by definition) always be within 150' of the path; so some speed -- usually much less than 354 knots -- would be enough to keep it on path. If you were more than 150' high, you would be in VNAV SPD and no attempt would be made by the FMS to recapture the path. Presuming you have entered the descent winds prior to ToD, getting close to 354 knots, or worse yet completely losing the path, would be unusual. Maybe if you flew through the core of a jetstream and picked up a significant unforecast tailwind it might occur. But absent significant unforecast tailwinds, you should never drop out of VNAV PTH on a descent prior to the first speed constraint. If you did get more than 150' high, that's where you should see speed reversion occur and it should shift from VNAV PATH to VNAV SPD.

I agree with your point about the segments after the first speed constraint usually being shallower and I think PSX correctly shows SPD||VNAV PTH as it adds power to stay on path in these segments.

Thanks,
Jon






Hardy Heinlin

#51
Hi Jon.

Quote from: emerydc8 on Sun, 20 Sep 2015 22:09
... thanks for looking at this subject again.

I've never stopped looking at this subject :-) There's always a lot of work in the background.

Quote from: emerydc8 on Sun, 20 Sep 2015 22:09
As for the FMA indication, the good news is that there is no need to change it from SPD||VNAV PATH to THR||VNAV PTH because you should never see THR||VNAV PTH on a VNAV descent. The path is +/- 150'. If you are within that envelope (on path) and you encounter an unforecast headwind, and slowing 15 knots below FMC target is still not enough to maintain the path, you should see SPD||VNAV PTH as the power is applied. I think PSX also does this part correctly.

If for whatever reason you find yourself below the path (>150') and the speed window is closed, the FMS will enter the path capture mode (just like on an early descent) and you should see THR/HOLD||VNAV SPD as the aircraft attempts to recapture the path with a 1250'/min descent rate. Once it recaptures the path, it should go back to IDLE/HOLD||VNAV PTH.

So, when power is applied while on the path you should see SPD||VNAV PTH. When power is applied while under the path you should see THR||VNAV SPD. On a VNAV descent, you should never see THR||VNAV PATH, so there is no need to change that.

I think your system description is not quite correct. I cannot believe that there should never be a THR | | VNAV PTH mode pair. Maybe you are generalizing two different features here.

My understanding is this:

Yes, when you are on an early descent, started before the T/D, you will be below the path until recapturing the path from below. During this early descent VNAV SPD remains engaged, and the A/T mode will go to THR until -1250 fpm are reached, then HOLD.

No, the minimum allowed negative IAS deviation is -10 kt, not -15 kt. E.g. if the bug sits on 280 KIAS, the command won't go below 270 KIAS.

No, the whole idle descent segment is an airspeed-on-elevator operation, that is, the A/T mode will never be SPD. The pitch mode will be VNAV SPD when the aircraft is high above the path and unable to recapture it. Otherwise, it will be VNAV PTH, even if the aircraft is slightly below the path and the command speed is -10 kt below the bug. Along the entire idle descent segment, VNAV PTH is an airspeed-on-elevator function, and the descent rate is controlled just by modulating the command speed by -10 to +15 kt around the bug (+10 below FL100). During this idle descent with VNAV PTH, the A/T goes to IDLE and then HOLD; and if thrust is required to maintain the path, it goes to THR, with VNAV PTH still engaged. VNAV PTH on the airspeed, HOLD or THR on the descent rate. Not SPD | | VNAV PTH, that mode pair would be VNAV PTH on the descent rate, SPD on the airspeed. If the A/T would be disconnected in this case, the FMC message wouldn't read THRUST REQUIRED, but SPEED REQUIRED -- and that wouldn't make sense.

To make it short: I think you are confusing the "early descent" function with the "thrust required" function. Both functions refer to the aircraft being below the path. But the first one is a capture function from far below the path, while the second function is a "maintain" function used when the trend goes below the path. This "maintain" function maintains the VNAV PTH mode, and varies the A/T mode with HOLD and THR.

Correct me if I'm wrong.

Edit: I'm wrong, see: http://aerowinx.com/board/index.php?topic=3078.msg32035#msg32035


Cheers,

|-|ardy

emerydc8

#52
Hi Hardy,

I am just quoting from the FCOM Flight Management Navigation Descent chapter, but I would like to get Peter's take on this too.

The FCOM says, "If the VNAV path segment is too shallow to be flown satisfactorily at IDLE thrust, the FMC commands speed on thrust levers (SPD). Elevators control the shallow descent path." Don't you  think it would be SPD||VNAV PTH at this point? I know it may be referring to a position when you have passed the first speed constraint (shallow segment), but are you saying that the SPD||VNAV PTH  versus THR||VNAV SPD pairing does not apply prior to the first speed constraint?

The up to 15 knots below target speed to maintain the path while above the first speed constrant is coming from the FCOM as well. It says "with greater than 15 knots below the target speed, the autothrottle changes from IDLE/HOLD to SPD to provide thrust to accelerate to the target speed. If the autothrottle is not active, the scratchpad message THRUST REQUIRED displays." In this case, wouldn't you also see SPD||VNAV PTH? As long as you are within 150' of the path in a VNAV descent with the speed window closed, you will always see VNAV PTH.

Jon






Hardy Heinlin

In my entire comment above I just refer to the idle descent segment which starts at T/D and ends at the first altitude constraint (a speed constraint is always combined with an altitude constraint, so I just mention the altitude constraint).

I guess your documentation refers to the segment after the first altitude constraint, and before the approach segment. In an earlier post you requested that in this descent segment (after the first altitude constraint, before the approach) the mode pair should never be SPD | | VNAV PTH, unless there is a level-off. I don't call this segment an "idle descent segment".

In my previous comment at http://aerowinx.com/board/index.php?topic=3078.msg32011#msg32011 I refer to  the segment after the first altitude constraint. I say that after the first altitude constraint the paths are almost always shallower anyway, hence PSX goes to SPD | | VNAV PTH. In an older post you said this is wrong.

Very confusing discussion! :-)


Regards,

|-|ardy

emerydc8

QuoteIn an earlier post you requested that in this descent segment (after the first altitude constraint, before the approach) the mode pair should never be SPD | | VNAV PTH, unless there is a level-off. I don't call this segment an "idle descent segment".

I think this was in reference to speed intervention on a VNAV PTH descent prior to getting on-approach logic.

Quote
In my previous comment at http://aerowinx.com/board/index.php?topic=3078.msg32011#msg32011 I refer to  the segment after the first altitude constraint. I say that after the first altitude constraint the paths are almost always shallower anyway, hence PSX goes to SPD | | VNAV PTH. In an older post you said this is wrong.

I admitted that I was wrong on this in the subsequent post. You would see SPD||VNAV PTH on a STAR descent if power was required to stay on the path. http://aerowinx.com/board/index.php?topic=3078.msg31095#msg31095

[EDITED WITH CORRECT LINK]






Hardy Heinlin

#55
Quote from: Hardy Heinlin on Mon, 21 Sep 2015 03:28
No, the minimum allowed negative IAS deviation is -10 kt, not -15 kt. E.g. if the bug sits on 280 KIAS, the command won't go below 270 KIAS.

I'm not quite correct either. I wished I could find the source of this (probably wrong) information.

At least it looks like it is neither a fixed -10 nor a fixed -15 value. In an old engineering book from 1989 (when the 744 FMC was just introduced) I just found this:


Speed Reversion Case 1

When the airplane is above the first speed at altitude constraint and not in speed intervention:

-- Reversion to VNAV SPD occurs at:

VDEPARTHI
(Speed Reversion Speed - High Side) = VGMAX

or:
VDEPARTLO
(Speed Reversion Speed - Low Side) = The Lower of:
the FMC Speed Target - 15KTS CAS or the Maximum L/D speed (best hold speed),
unless VGMIN is higher, in which case it is used.



From this I'm learning two new things: The allowed negative speed deviation is limited by the best hold speed, hence it may vary, and may be higher than -15, or even higher than -10 knots.***  Bill Bulfer also mentions "L/D max" in his guide on page 201. -- Secondly, when THR engages, VNAV PTH indeed should go to VNAV SPD. In this detail PSX is wrong, and you're right, Jon. Until someone proves the opposite :-)


Regards,

|-|ardy


*** It says the "Lower of", i.e. it may be even lower than -15 if the best hold speed is lower, unless Vgmin is higher.

emerydc8

Hi Hardy,

That's funny -- I was looking at page 211 of the Big Boeing Book too where it discusses Vgmax, but I didn't want to complicate matters further. I don't mind if I'm wrong here -- just like you, the important thing is that we both want to get it right and I think we are heading in that direction. I would still like to get Peter's input on this.

In the mean time, I did engage in a related post on pprune earlier this year and I think CCA seems to feel pretty confident that the mode will be THR||VNAV SPD when below the path (speed window closed) versus SPD||VNAV PTH to maintain speed while descending on the path and compensating for an unforecast headwind.  http://www.pprune.org/tech-log/553693-b777-vnav-question.html#post8895416


Hardy Heinlin

Sigh. Half a page later in that 1989 engineering book there is a note that describes something completely different. On that page it says, when below -15 kt etc., the A/T engages in SPD mode. On the previous page it says the pitch goes to VNAV SPD.

But, as we know, SPD and VNAV SPD are never coupled.

So it has to be either THR | | VNAV SPD or SPD | | VNAV PTH. Nobody knows which description is correct. Not even this highly detailed engineering book is consistent.

I'll chose SPD | | VNAV PTH and the fish is done. Roger over, Victor.

Like before, THR | | VNAV SPD is used in PSX when starting a new capture flight below the path, but not when trying to compensate a down drift from an already captured path. That would lead to hard 2-mode oscillations anyway.


Regards,

|-|ardy

emerydc8

Yes, the Boeing manuals sure leave a lot to the imagination. It helped me to boil it down to two regimes where power is applied on a VNAV descent -- one where you are on the path and another where you are below the path. Of course, both regimes would be with the speed window closed.

If you are on the path you will see SPD||VNAV PTH until it returns to the FMC target speed, then IDLE/HOLD||VNAV PTH to continue the descent.

If you are below the path (>150'), you will see THR/HOLD||VNAV SPD until it captures the path, then it will go to IDLE/HOLD||VNAV PTH to continue the descent. If you are going really fast with the speed window open and you are below the path, but not too far below it (1000'), I think it is possible that no power would be needed to recapture the path once the speed window is closed because the aircraft will pitch for the FMC target speed and in doing so will probably capture the path without any power required.


Hardy Heinlin

Quote from: emerydc8 on Mon, 21 Sep 2015 08:41
If you are below the path (>150'), you will see THR/HOLD||VNAV SPD until it captures the path, ...

Yes, but when SPD engages to assist in maintaining the path, the chances are very low that you will ever sink below -150 ft.