Hi hardy,
I've been enjoying some of the many changes you made up to Beta 13. During a normal descent it does a pretty good job now of staying within 200' of the path.
One issue I did come across is the FMA indication after entering a new altitude crossing restriction during a descent. I don't know how hard this would be to change, but if you load this situ you will see what I mean.
https://drive.google.com/file/d/0B-WRh0Hf7VdZYkdrNzNyVk8xcVk/view?usp=sharing
As soon as it starts the descent, insert /12000 at SKUNK. It was originally planning to cross SKUNK at around 17,000', so the new path will be over 5000 below the actual aircraft altitude. I noticed it stays in HOLD||VNAV PTH and accelerates up to 348 knots to capture the path.
I am open to any opinions or documentation otherwise, but I am pretty sure that when you enter a new crossing restriction like this, and the airplane is more than 150' above the newly calculated path, it should go to HOLD||VNAV SPD and the speed maintained should be the FMC target speed. It will not attempt to dive for the path when it is 5000' high.
On a different subject, check out the duct and cabin info on the upper EICAS while the lower EICAS is closed.
Thanks again.
Jon D.
Hi Jon,
I'm trying to modify this VNAV SPD thing ...
But what's wrong with the EICAS?
Regards,
|-|ardy
The EICAS cabin stuff display, once it has been activated for whatever reason, will not disappear on its own.
VNAV trick is added to Beta 14: http://aerowinx.com/board/index.php?topic=3160.0
That was fast. I'll check it out.
The upper EICAS is showing the duct and cabin information while the lower EICAS is closed. Is there something I'm not seeing that would cause this information to be up there if no issues are present with the duct or cabin?
It was displayed in CRZ because a parameter was above the limit.
Not only amber limits will show it:
http://aerowinx.com/board/index.php?topic=3194.msg32195#msg32195
That's strange. It was a normal flight from LAX to SFO and I didn't throw in any malfunctions. Is it normal for PSX to exceed a limit like this? Does the data continue to display on the upper EICAS even if everything is back within parameters?
The EICAS cabin stuff display, once it has been activated for whatever reason, will not disappear on its own.
Not only amber limits will show it. There are also white limits.
PSX manual page 118:
_____________________________________________________________
Duct Pressure and Cabin Altitude Indications:
Displayed if at least one of the following conditions is true:
• ECS or ENG synoptic is selected
• Any indication is amber or red *
• CABIN ALT AUTO caution message is displayed
• BLD DUCT LEAK caution message is displayed
• Cabin altitude is above 7400 ft
• Cabin altitude rate is above 700 fpm or below -500 fpm
_____________________________________________________________
QuoteCabin altitude rate is above 700 fpm or below -500 fpm
Thanks, Hardy. It must have been the cabin altitude rate because the cabin never went much more than about 5000'. I asked a friend over at UPS on the -400 and this was his reply. Despite what the manual says, it still remains a mystery to some of us.
QuoteRe the cabin and duct info. This is a great question. I've looked in the past, and I just went through our systems manual and can't find anything about it in writing. In practice, I can tell you this is something that does drive some of us nuts! I would like to know (like you) how this is supposed to work, but can't find any info. On some planes, and it could be all, but sometimes on the taxi out after we finish our before takeoff checklist we hit the ENG button to get rid of the lower EICAS and most of time the duct and pressurization stuff blanks from the upper EICAS as well, but sometimes it doesn't. And to get rid of it you have to reselect ENG and deselect ENG, and it goes away, or like you mentioned sometimes I'll select ECS and deselect ECS and then it will go away. I'm always wanting it to go away. I don't know if there is a way to make it appear without the lower EICAS info or not. I suspect this might be a bug or something.
I don't think the real EICAS has a bug. The function is clearly described in engineering books (see my description above; that's the logic).
Maybe there was a software update 10 or 20 years ago, and they forgot to update the crew manuals.
Maybe so, but I do seem to recall seeing this behavior on a number of our airplanes too.
FYI, I tried Beta 14 with the SFO ToD situ and noted that while the FMA now correctly goes to VNAV SPD when you insert the /12000 at SKUNK, the actual airspeed still increases to 328 knots when it should seek the FMC target speed of 284. Do I need to redo the situ using Beta 14 from the start of flight to get it to take?
Quote from: emerydc8 on Wed, 14 Oct 2015 23:13
Maybe so, but I do seem to recall seeing this behavior on a number of our airplanes too.
Why "but"? As opposed to what? (Would you mind using that other thread (http://aerowinx.com/board/index.php?topic=3194.msg32195#msg32195) for this subject?)
Quote from: emerydc8 on Wed, 14 Oct 2015 23:13
FYI, I tried Beta 14 with the SFO ToD situ and noted that while the FMA now correctly goes to VNAV SPD when you insert the /12000 at SKUNK, the actual airspeed still increases to 328 knots when it should seek the FMC target speed of 284.
It should start decelerating to 284 kt only shortly before SKUNK. But in your situ, SKUNK is over 50 nm away and 20000 ft below. Why should it set 284 so early? That would be like applying 240/10000 SPD TRANS at FL300 already.
Also, you just told me to enter 12000, not 284/12000. So there is no speed constraint at SKUNK, and no reason to decelerate even when crossing SKUNK.
Quote from: emerydc8 on Wed, 14 Oct 2015 23:13
Do I need to redo the situ using Beta 14 from the start of flight to get it to take?
No.
Cheers,
|-|ardy
The FMC target speed was 284. That's the speed it should go to when you enter the new restriction and it drops out of VNAV PTH into VNAV SPD.
When you enter the /12000 at SKUNK you are asking the FMC to compute a new path based on an idle power descent from SKUNK at 12000 and working back towards the airplane. This new path would be about 5,000 below the aircraft when you execute it. Once it drops the path it should also revert to the FMC target speed. In my situ, it accelerates to 328 for no reason.
[ADDENDUM]:
If you are descending in VNAV SPD with the window closed, what speed should the aircraft maintain? It shouldn't dive past the target to capture the new path. You correctly stated this in your Beta 14 update - 0.6.0102. VNAV PTH descent: When a modification is executed that causes the path deviation to exceed 150 ft, the FMC will now promptly change to VNAV SPD without trying to acquire the new path by VNAV PTH.
There is no mention in the manuals that VNAV SPD should maintain command speed when the first speed constraint is not passed yet.
In your situ, with beta14, the reasons why it may accelerate to VMO-16 are these:
• The first altitude constraint is not sequenced yet (still on initial idle path).
• The first speed constraint (or speed restriction) altitude is not passed yet.
According to the LH OM, which has a precise description of this subject, VNAV SPD aims at command speed only when passing the first speed constraint.
Here's what Beta 14 applies:
0.6.0053. VNAV maximum target speed is now VMO - 16 kt (349) if in ECON mode, else VMO - 11 kt (354).
0.6.0054. VNAV PTH idle descent ABOVE first speed constraint altitude: DRAG REQUIRED now appears when reaching maximum target speed; if path still cannot be maintained within 150 ft, DRAG REQUIRED reappears and VNAV PTH changes to VNAV SPD which maintains maximum target speed.
0.6.0055. VNAV PTH idle descent BELOW first speed constraint altitude: DRAG REQUIRED now appears when exceeding command speed + 10 kt; VNAV PTH may accelerate to command speed + 15 kt, or to flap placard - 5 kt, whichever is lower; if path still cannot be maintained within 150 ft, DRAG REQUIRED reappears and VNAV PTH changes to VNAV SPD which maintains SEL SPD, e.g. 240 kt (SEL SPD) when below 250/10000 transition.
If you do a late descent and don't reset the MCP altitude until you are 5000' above the path (say you fly 15 miles past ToD), once you do push the MCP knob, at what speed should it descend?
I see what you are saying about the VMO-11, but that is a situation where it was on the path and an unforecast tailwind pushed it high and caused speed reversion. I think that situation is very different from when you enter a new restriction, causing an entirely new path to be computed, and the airplane was never trying to stay on the new path to begin with.
On Instructor > Analysis > Miscellaneous there is a new indicator:
1st DES SPD restr passed: Yes (or No)
When you start a descent from CRZ ALT, be it late or early, the indicator will be saying "No" until you pass the first speed limit waypoint or altitude.
All the logic depends on this single flag: Yes/No. Nothing else.
If No: VNAV SPD may reach maximum target speed (VMO-11).
If Yes: VNAV SPD maintains SEL SPD.
Hi Hardy,
I'm not following what you have posted above. For purposes of a descent in VNAV SPD, I don't think it matters whether you have passed the first speed constraint. The aircraft is going to target the FMC speed on VNAV 3/3 (taking into account any flap limit speed) when the speed window is closed.
My personal experience and what I see in the Big Boeing book doesn't appear to point to the speed or altitude constraint as a determinative factor if you are descending in VNAV SPD (VNAV PTH is a different story). If it did, there would be no need to deploy the speed brakes or "dial in some drag" by opening the speed window and running the speed up to 340 knots to try and manually catch the path. The reason for having to do this is because VNAV SPD is not going to descend any faster than the FMC target speed and you won't get back down to the path if you don't take action to steepen your descent path.
QuoteLATE DESCENT
If clearance for descent is not granted prior to top-of-descent, a late descent must be accomplished and a steeper path must be flown. . . . The vertical deviation indicator (VDI) on the HSI Map will show your position above the path. Since the path is below our position, the FMC engages in VNAV SPD instead of VNAV PATH. The airplane will descend at the descent target speed and may remain above the path unless the pilot intervenes with a steeper descent rate and/or drag.
Hi Jon,
you were confusing me for minute :-) ... with this question ...
Quote from: emerydc8 on Thu, 15 Oct 2015 09:34
If you do a late descent and don't reset the MCP altitude until you are 5000' above the path (say you fly 15 miles past ToD), once you do push the MCP knob, at what speed should it descend?
... because in the previous post the question was whether VNAV PTH or VNAV SPD should engage when too high above the path. For a minute I thought you were referring to the assumption that at a late descent PSX would engage VNAV PTH instead of VNAV SPD, but it doesn't.
The current question is, what speed VNAV SPD should aim at.
I use the logic from the LH OM:
Above first speed limit altitude: VNAV SPD may reach maximum target speed (VMO-11).
Below first speed limit altitude: VNAV SPD maintains SEL SPD.
Do you think this is wrong? If so, can you prove it is wrong?
Cheers,
|-|ardy
I can offer my personal experience and what I posted above from the Big Boeing Book. There is other training material that supports this position too, but the material is not officially sanctioned by the airline. Descending in VNAV SPD is essentially dive-and-drive where it descends in idle and targets the FMC-target speed when the window is closed. Could it be that LH has a custom feature that operates differently than other aircraft?
I know Peter has been busy with the big event this weekend, but I would be curious to hear his opinion on this.
Quote from: emerydc8 on Thu, 15 Oct 2015 19:59
QuoteLATE DESCENT
If clearance for descent is not granted prior to top-of-descent, a late descent must be accomplished and a steeper path must be flown. . . . The vertical deviation indicator (VDI) on the HSI Map will show your position above the path. Since the path is below our position, the FMC engages in VNAV SPD instead of VNAV PATH. The airplane will descend at the descent target speed and may remain above the path unless the pilot intervenes with a steeper descent rate and/or drag.
By
target speed I understand the
command speed + additive.
VNAV always maintains the target speed. The quote above says nothing about the value.
Command speed: Magenta bug
Target speed: Magenta bug + additive
That's how I understand these terms.
And ... speedbrakes
may be required. It may also capture the path without them.
Cheers,
|-|ardy
Beta 15 is up:
http://aerowinx.com/board/index.php?topic=3160.0
Hi,
if I may interrupt for a moment.....the behavior of the FMS for descent which hardy here describes confirms exactly my state of knowledge and the LH OM.
The descent may be planned at economy Mach/CAS (based on Cost Index) or a manually entered Mach/CAS. VNAV will not command an economy target speed greater than 349 knots (VMO/MMO minus 16 knots) or a pilot entered speed greater than 354 knots (VMO/MMO minus 11 knots).
Speed Reversion:
If flight plan modifications or unknown winds occur when above the first speed constraint, VNAV varies speed to maintain the path up to the following limits:
With greater than 349 knots (VMO/MMO minus 16 knots), the scratchpad message DRAG REQUIRED displays. The airplane may accelerate up to 354 knots (VMO/MMO minus 11 knots) to maintain the path. If further correction is required, VNAV may allow the airplane to rise up to 150 feet above the path. If VNAV can no longer maintain the airplane within 150 feet of the path without further acceleration, speed reversion occurs, the pitch mode annunciation changes from VNAV PTH to VNAV SPD, VNAV resets the target speed to 349 knots (VMO/MMO minus 16 knots), and the scratchpad message DRAG REQUIRED displays again.
I don't know, if there are different custom features, sorry.....
HTH
Cheers
Walter
Hi Walter,
thanks for the confirmation :-)
Cheers,
|-|ardy
Hi Walter,
The flight plan modifications Boeing refers to are modifications that would still keep you within +/-150' of the path--not a modification that puts you 5,000 above the path. If you modify the flight plan on the legs page but you are still within the path limits then, yes, it will attempt to pitch to get you exactly back on path.
However, if a modification takes you outside the path, like in my situ, then you should see IDLE/HOLD||VNAV SPD and the airspeed it will maintain is the mach/speed on VNAV 3/3. It's not going to exceed the FMC speed with the window closed to try to get back on path when in VNAV SPD. In VNAV SPD, it has given up trying to maintain the path.
It will also do this when you fly past your ToD point and set a lower altitude in the MCP. When you press the altitude knob, you will see IDLE/HOLD||VNAV SPD but the aircraft will not dive and accelerate to 349 knots in an attempt to catch the path at this point. It will just do a normal descent and maintain the descent speed in VNAV 3/3. So, unless LH has modified their software, there is no path capture feature in VNAV SPD when you are above the path. If there was, then all the discussion in the Big Boeing Book about "dialing in some drag" (speed intervening to a higher speed and using speed brakes) would be moot. If the aircraft would do this on its own (increase to 349 knots), then why would you have to speed intervene at all?
Jon D.
Hi Jon,
whether speedbrakes are required or not, depends on the situation. If you are 5000 ft too high, you may capture the path in less than 3 minutes without speedbrakes. If that lies well before the next constraint, you keep it clean, if not you extend the brakes.
|-|ardy
Hi Hardy,
I agree that it is possible that you could capture the path if you are close enough without using speedbrakes. But the most common way to get back down to the path when you are descending in VNAV SPD would be to speed intervene and dial in a higher speed (the FMC is not going to do this automatically). You can usually get away without deploying the brakes if you have enough room and you started out with a low enough descent speed. When you get back on path you just push the speed button in, the speed window will close and you will go from VNAV SPD to VNAV PTH and resume an idle descent.
I just don't think the speed is going to increase to VMO-11 or even FMC target + 15 if you wind up above the path in VNAV SPD after a modification. Of course, it will if the modification is small enough that you can remain in VNAV PTH, but once you are in VNAV SPD it's going to descend at the speed in VNAV 3/3. Maybe LH has modified its software to do this, but this isn't my understanding of how the system works on our planes.
I am checking with some other pilots to verify this.
So do you agree that VNAV SPD is allowed to reach VMO-16 once it was on idle path with VNAV PTH, within 150 ft, and above the first speed limit altitude? If it never was under this condition before (e.g. late descent at CRZ, or route mod/exec during descent), VNAV SPD must hold command speed?
If that is true, the LH OM is not wrong, it is just incomplete because it doesn't explicitely mention these exceptions.
I agree that in VNAV PTH the speed can go up to VMO-11 to hold the path and if it can't hold it within 150' then speed reversion occurs, the command speed is reset to VMO-16 and the FMA will go to VNAV SPD.
But I don't think it will trigger a speed reversion condition if you do an altitude or speed modification and the new path is 5,000' below you. In this case, I think it will go to VNAV SPD and descend at the ECON speed on VNAV 3/3 (presuming ECON is being used). At that point, it is up to you to either speed intervene to a higher speed to get down or use speed brakes, or both.
Quote from: emerydc8 on Fri, 16 Oct 2015 23:00
I agree that in VNAV PTH the speed can go up to VMO-11 to hold the path and if it can't hold it within 150' then speed reversion occurs, the command speed is reset to VMO-16 and the FMA will go to VNAV SPD.
You mean the magenta bug will jump to VMO-16?
You know, my definition of
command speed is the speed displayed on the FMC pages and marked by the speed bug.
Target speed, on the other hand, e.g. command+15, has no indication.
Right. Sorry for not being clear on the nomenclature. The command speed is the magenta speed on the top of the speed tape. The target speed is what the FMC is targeting.
If you are descending above the path in VNAV SPD with the speed window closed, the command speed will be the same as the target speed.
By "right", you mean the magenta bug will jump to VMO-16? :-)
This would make sense. It would maintain the rule of VNAV SPD always aiming at the bug. To get VNAV SPD aiming at VMO-16 instead of the bug, the bug has to jump to VMO-16. If you don't want to go to the mountain, have the mountain come to you.
If this is true, the FMC will forget what your previous SEL SPD was. The only chance to get back to the original speed (when the path is recaptured) is to check the ECON speed of the current cost index. But then the FMC will also have forgotten whether the ECON mode was selected at all prior to the VMO-16 speed reset. It cannot just chose ECON on its own once it has changed to SEL SPD.
No. In a VNAV SPD descent with the window closed, there shouldn't be any difference between the magenta command speed and the speed the FMC is internally targeting.
Also, even in VNAV PTH, I am pretty sure that you will not see the command speed change as the aircraft increases or decreases speed to stay on path. It is hard to test this with an unforecast tailwind, but if you are descending in VNAV PTH with the speed window closed and you deploy the speedbrakes, the speed will be allowed to decay up to 15 knots below the ECON descent speed before power is added; but you will not see this speed decrease reflected in the magenta command speed. It's still going to match what is in 2L on VNAV 3/3.
I am also pretty sure that when Boeing is discussing the speed reversion scenario where it hits VMO-11, goes into VNAV SPD and resets the target speed to VMO-16, you are not going to see this reflected in the magenta command speed. This all goes on internally.
Boeing and Honeywell do an absolutely terrible job of describing the function of the FMS. Whoever wrote these manuals should have been fired. What's even worse is that decades later they haven't even acknowledged it nor have they made an attempt to clarify their initial piece of work.
Another possibility (instead of automatically setting SEL SPD to VMO-16):
When VNAV SPD engages at VMO-16, the MCP SPD window automatically opens and sets the current speed (VMO-16). That would be the most elegant solution.
That could very definitely be a possibility; but since I have never seen speed reversion in the real airplane (and probably most other pilots haven't either), I don't know. And Boeing sure doesn't feel it is necessary to let us in on it. I think you may have something with the speed window opening at VMO-16.
Quote from: emerydc8 on Sat, 17 Oct 2015 00:18
No. In a VNAV SPD descent with the window closed, there shouldn't be any difference between the magenta command speed and the speed the FMC is internally targeting.
I just realized you may have misunderstood my question.
"By "right", you mean the magenta bug will jump to VMO-16?"If the bug (command speed) will jump to VMO-16, there will be no differences whatsoever because everything is reset to VMO-16. As you say, there shouldn't be any difference. And this reset won't break the rule then.
There is another case of VNAV SPD not aiming at command speed:
When below the path and above the first speed limit altitude, VNAV SPD maintains best hold speed or minimum maneuver speed, whichever is higher, plus 5.
That's in PSX and in LH.
Of course, you can test this only if you disconnect the A/T.
I think I was referring to two different situations. One is where speed reversion occurs because VNAV PTH couldn't maintian the path even at VMO-11, so it reverts to VNAV SPD and resets the target speed to VMO-16.
The other is where you either start down late or modify the route so that you are well above the path and descending in VNAV SPD. In this case, you will not see a magenta command speed more than what is in VNAV 3/3 at 2L and it will not dive to capture the path.
Good point. In that case, you will not see the best hold speed/min maneuver speed commanded in magenta.
Quote from: emerydc8 on Sat, 17 Oct 2015 00:45
I think I was referring to two different situations. One is where speed reversion occurs because VNAV PTH couldn't maintian the path even at VMO-11, so it reverts to VNAV SPD and resets the target speed to VMO-16.
The other is where you ...
We have clarified this already :-)
It's now about situation #1. What does "reset to VMO-16" mean?
Will the command speed (speed bug and SEL SPD) jump to VMO-16?
Or will the MCP SPD window open and set VMO-16?
Or will nothing change?
That's the $64,000 question. I like your idea of the MCP speed window opening at VMO-16, but unless we get into a sim and try it, I don't how we get the answer. I will ask some people I know. It's just not something you would intentionally do in the real plane.
OK, so if everyone agrees, I'll modify my code and will never let VNAV SPD aim at a speed higher than FMC command speed. The only way to exceed it is to open the MCP window. And that will happen automatically when reaching VMO-16, when VNAV PTH changes to VNAV SPD.
Does anyone have any objections?
Sounds good to me. We will probably never see speed reversion in the sim anyway.
I just tested the MCP window opener.
It looks strange. Non-intuitive. I don't like it. I'll remove it.
Instead, I'll implement a reminder that is set when the path is captured. When set, it allows VNAV SPD to go to VMO-16. The reminder is cleared at CRZ or when pushing EXEC.
I haven't been able to get PSX to get anywhere close to VMO-11 while descending in VNAV PTH. Are you inserting an artifical wind to push it high on the path when you test it? I think you mentioned last week that when you removed the FMC+15 restriction that it was able to maintain path within ECON + 20ish.
I set max thrust. Then shortly before VMO-16 I set idle.
I guess that would do it. Thanks for working on this Hardy. I don't know when you sleep.
Beta 17 is uploaded.
And I download myself now into the pillow. Good night ...
Hi Hardy,
I had a chance to test-drive Beta 17 into SFO on the Big Sir Arrival. I was impressed. First, I flew it without entering the 12,000' at SKUNK to see how it would do without throwing it any curves. It was very nice the whole way down. The Econ command descent speed was 284 knots. It seemed to get pushed a bit low and it slowed at one point to just under 269 knots and, predictably, the power came in and the FMA correctly indicated SPD||VNAV PTH. Back to the path and then the power goes back to idle and back to HOLD||VNAV PTH. Right on.
The second time, I threw a curve at it right after the top of descent. I entered /12000 at SKUNK. The FMA correctly went to VNAV SPD and the new path was over 4000' below the aircraft. It descended right at 284 knots, just as I believe it should.
This is a good scenario if anyone wants to see how to get down without using the speed brakes and just using speed intervention. Load the scenario and as you are descending through FL345, insert the /12000 at SKUNK.
https://drive.google.com/file/d/0B-WRh0Hf7VdZMUk4OTAwTHR3VmM/view?usp=sharing
Note that it correctly drops out of VNAV PTH and into VNAV SPD since you are no longer within 150' of path. The speed stabilizes at the command speed of 284 knots. Now, speed intervene (open the speed window) and select 330 knots. You can watch it converge on the path from above as you descend. You will get the DRAG REQUIRED message but just ignore it and watch. When you intercept the path from above, close the speed window and observe it will go back to VNAV PTH. It will cross SKUNK at exactly 12,000 and 284 knots! I took it all the way down to OAK and selected flaps 1 as it slowed towards 210 knots -- Very nice! Once past the first speed constraint the VTK ERROR is virtually zero the rest of the way.
As for the speed reversion, I tried to add power as I descended to simulate a severe tailwind, but had some trouble keeping it in the descent while accelerating. The nose kept wanting to pitch up as I fed in the power and it decreased the descent rate to about 1500 FPM. I decided to stop it at 70% N1, which was enough to eventually push it up to 349 knots. At that point I was 3,000' high and still in VNAV PTH (maybe this is due to trying to create the unforecast tailwind with thrust which is not something you would ever do in the plane). At 349 knots it reverted to VNAV SPD just as you said it would.
I'll have some fun playing with this more over the weekend. Thanks again for making all the revisions. I think it's coming together very well. Maybe it can be tested over the weekend during the flights.
Hi Jon,
thanks. I just tried your situ.
Quote from: emerydc8 on Sat, 17 Oct 2015 07:10
This is a good scenario if anyone wants to see how to get down without using the speed brakes and just using speed intervention. Load the scenario and as you are descending through FL345, insert the /12000 at SKUNK.
https://drive.google.com/file/d/0B-WRh0Hf7VdZMUk4OTAwTHR3VmM/view?usp=sharing
Note that it correctly drops out of VNAV PTH and into VNAV SPD since you are no longer within 150' of path. The speed stabilizes at the command speed of 284 knots. Now, speed intervene (open the speed window) and select 330 knots. You can watch it converge on the path from above as you descend. You will get the DRAG REQUIRED message but just ignore it and watch.
To be precise, on my system, -- and that's intentional --, the DRAG REQUIRED message occured only after VNAV PTH was re-engaged, not when VNAV SPD was already engaged. I hope you were seeing the same on your system.
Quote from: emerydc8 on Sat, 17 Oct 2015 07:10
When you intercept the path from above, close the speed window and observe it will go back to VNAV PTH.
That's the point where I got DRAG REQUIRED (as expected). I would close the window a bit later to prevent a premature deceleration. It can't follow the path during deceleration because the path is designed for command speed related descent rates. During deceleration the aircraft is not at the command speed yet and thusly will fly a shallower path, causing the deviation to increase again for some seconds.
How do you control this mode transit on the real aircraft? I would slowly decrease the MCP SPD. If not that, I would open the MCP window when the aircraft is ca. 400 ft below the path.
QuoteAs for the speed reversion, I tried to add power as I descended to simulate a severe tailwind, but had some trouble keeping it in the descent while accelerating.
I intentionally want it to
climb, and expect a climb. It's just for creating a path deviation :-)
You can also create a tailwind using the tropopause wind on the Planet weather page, but the path error will not be that big because the FMC uses the currently sensed IRS wind data for the next 5000 vertical feet down the path and mixes this with forecast wind.
Cheers,
|-|ardy
I think that's where I got the DRAG REQUIRED message too -- right when closing the speed window.
I experimented and tried letting it get 100' below the path before closing the speed window, so it had more room to decelerate back to 284 knots. I think it resulted in the actual airspeed settling in five or ten knots lower than when I closed the window right on the path.
In the real plane, I do just as you suggested and just slowly reduce the speed with the MCP knob to make the transition smoother. Some guys will pull the speed brakes out in addition to speed intervention, then close the window when right on the path and slowly take the speedbrakes out as it slows back to the command speed.
Actually, I'm not a fan of using speed brakes in anything other than V/S. You have to be careful in VNAV SPD or FLCH mode because the nose will pitch over to attempt to maintain the MCP speed when you pull the speed brakes out. If you pulled them fast enough, everything not strapped down would probably be in the air, so it's kind of a balancing act. In V/S, you don't have to worry about that.
On the real aircraft, with HOLD | | VNAV PTH, and 10 knots or so below command speed due to being below the path, and not much acceleration in sight, do you add some thrust manually or do you wait until SPD engages?
I just rediscovered a public Boeing document that confirms the Lufthansa quotes re maximum target speeds:
Quote from: Boeing's AERO_Q207_article5.pdf
Vertical Navigation (VNAV) limits the
maximum target speed as follows:
747-400: 349 knots (VMO/MMO minus
16 knots) or a pilot-entered speed greater
than 354 (VMO/MMO minus 11 knots).
Source:
http://www.boeing.com/commercial/aeromagazine/articles/qtr_2_07/AERO_Q207_article5.pdf
Cheers,
|-|ardy
Hi,
Sorry answering late....but the family.....
Although you are now headed in the right direction, I try to put it straight and send my message. May be it's to late now and just an overseeable roundup for our hobby-pilots.
VNAV PATH descent in connection with speed reversion or VNAV SPD descent are two very different animals.
1. When descending in VNAV PATH the system behavior is exactly as discussed above and represented in the OM, involving above or below the first speed constraint. After speed reversion VNAV PATH changes to VNAV SPD, VNAV resets the target speed to 349 knots (VMO/MMO minus 16 knots). This high speed remains constant unless you like to change the speed with speed intervention by MCP or CDU-VNAV-page and the first speed constraint is not anymore relevant. (I think when back on the path and closing the MCP speed window VNAV SPD changes again to VNAV PATH).
2. When descending in VNAV SPD caused by different modifications like starting descent after cruising beyond the ToD, changing/entering altitude/speed constraints and much else, the system behavior is not like described in the OM and it doesn't matter whether the airplane is above or below the first speed constraint. The speed is always the speed from the FMC calculated or pilot entered. Speed changes are now possible with speed intervention by MCP or CDU-VNAV-page.
HTH
Cheers,
Walter
Walters point number two is what I observed in the big sim last night.
FMC Descent speed was 290 kts if I remember, and I deliberately didn't reset the ALTSEL at TOD until the aircraft was 7000ft above profile, then I selected the first constraint altitude and pressed the ALTSEL.
The aircraft went into a descent but the speed stayed at 290 kts and I didn't get a DRAG REQUIRED message.
It never tried to regain the path.
Peter
Beta 17 behaves like that.
|-|ardy
P.S.: Just an operational question:
On the real aircraft, with HOLD | | VNAV PTH, and 10 knots or so below command speed due to being below the path, and not much acceleration in sight, do you add some thrust manually or do you wait until SPD engages?
Thank's Peter...... confirming my very simple explanation.
Walter
Super Hardy.........great job!
Walter
Reg. your speed question. In the early days of the 747-400 (1990 - 1997) we mostly used FLCH or VNAV with speed intervention.
Hardy as in Peter's example would you use the Offpath Des function? Is this function used by pilots?
Quote from: Kieber on Sat, 17 Oct 2015 15:562. When descending in VNAV SPD caused by different modifications like starting descent after cruising beyond the ToD, changing/entering altitude/speed constraints and much else, the system behavior is not like described in the OM and it doesn't matter whether the airplane is above or below the first speed constraint. The speed is always the speed from the FMC calculated or pilot entered. Speed changes are now possible with speed intervention by MCP or CDU-VNAV-page.
Hi Walter,
Thanks for confirming my understanding. When you say FMC calculated, are you referring to what is actually on VNAV 3/3 at 2L (the command speed that appears in magenta on the speed tape)? I think Hardy has it right in Beta 17. Thanks.
Jon D.
Quote from: jtsjc1 on Sat, 17 Oct 2015 19:21
Hardy as in Peter's example would you use the Offpath Des function? Is this function used by pilots?
The word "offpath" in that context refers to lateral offpath, not vertical offpath. The energy management circles are not needed when you are on the route. For more details I suggest to start a separate thread :-)
Cheers,
|-|ardy
Thanks Hardy I still got ALOT to learn about this airplane.
Hi Jon,
regarding the speed from the FMC calculated or pilot entered. Yes, that's the way I see it like you and it is the speed shown on the VNAV page 3/3 2L which is cloned in magenta into the PFD.
I think slowly we are close to success and we can say many hands make light work!
Cheers
Walter
Thanks, Walter. So it looks like Hardy's Beta 17 is correct regarding the issues previously discussed:
1. Descent in VNAV PTH -- Prior to the first speed constraint, it correctly adds power when more than 15 knots slow (pushed low on path). It correctly increases speed up to VMO-16 (pushed high on path) to stay in VNAV PTH, but reverts to VNAV SPD and resets the target to VMO-16 when it can no longer hold the path.
2. Modification or Late Descent -- If the modification causes you to be well above the path, upon execution the FMA indicates VNAV SPD and aircraft descends at the magenta command speed (VNAV 3/3 2L speed) without increasing speed to capture the path. If desired, speed intervention will allow you to increase speed to capture the new path, or speed brakes may be used to increase the descent rate and capture the path.
I've noticed the bracketing limits for the VNAV PTH is around +/- 350' instead of the 150' found in the Boeing manual, but I believe this is acceptable for the following three reasons:
First, although I don't have a hard copy of the LH manual (I wish I did), Hardy found that the 150' is not an absolute condition for reverting to VNAV SPD. The LH manual says that if there is still speed to use up getting back on path, it will stay in VNAV PTH even if it is more than 150' off.
Second, the VDI on the ND has a +/- 400' scale, so unless you were actually on PROGS 2/3 looking at the VTK ERROR you wouldn't even know you were more than 150' off.
Third, most of the guys who fly the airplane don't even know about the 150' path width.
Does everyone agree with this?
You've nailed a point that I couldn't figure out forever with VNAV! I was beginning to think I was going nuts. "Something" didn't seem right, but I can't remember enough to know what it was.
Awesome work by Hardy as always! :D
Jon, I agree. And here's #3:
3. If the modification causes you to be well below the path, upon execution VNAV SPD engages and the aircraft descends at best hold speed or Vmin, whichever is higher, plus 5, provided the aircraft is above the first speed limit altitude (speed constraint, speed restriction, or speed transition), otherwise the aircraft descends at FMC command speed which is in SEL SPD mode when below speed limit altitude.
|-|
Is that where it does the 1250 FPM descent to capture from below? We all know that above the path in VNAV SPD (prior to first constraint) there is no attempt to get back to the path by incresing above command speed; but in VNAV SPD and below the path, the path capture feature shallows the descent rate to 1250 FPM until the path is intercepted. I would presume you would see THR/HOLD||VNAV SPD if power was required at that point.
I haven't tried my situ where I speed intervene to 330 just after ToD and go 4000' below the path, then close the speed window. I'm presuing it will target best hold speed or Vmin, whichever is higher, plus 5, and set the power to attain a 1250 FPM descent? The FMA will be in THR/HOLD||VNAV SPD. When it intercepts the path it will go to IDLE/HOLD||VNAV PTH.
In PSX, the THR mode engages only when you start an early descent from CRZ.
The THR mode will slowly reduce the thrust, and when -1250 fpm are reached, it changes to HOLD.
Are you sure HOLD will change to THR when a below-path modification is executed during descent? That would require the THR mode to increase the thrust until -1250 fpm are reached.
My problem is that -1250 fpm is still pretty steep when you are 5000 ft below the path. It works well from CRZ. But for that other modification -500 fpm would be better.
Actually, upon execution, PSX goes to SPD | | VNAV PTH and aims at -500 fpm. And maintains command speed.
Strange, in a previous test it changed to HOLD | | VNAV SPD.
For example, when I set 22000 at SKUNK in your situ, a minute after leaving CRZ, -1250 fpm will never capture the path before SKUNK. And I'm 4000 ft below the path.
QuoteInitiation of a late descent from cruise altitude or resumption of the descent when the airplane is above the descent path, results in descent guidance of VNAV SPD and A/T IDLE followed by HOLD, until VNAV can capture the descent path. Resumption of the descent when the airplane is below the descent path results in descent guidance of VNAV SPD and an initial thrust setting (THR) to achieve a flight path angle that is half the angle for the descent path. A/T HOLD occurs after the initial thrust target is reached.
Big Boeing 10.3
I shouldn't say 1250 FPM, but it is pretty much what it works out to be when you cut the original path descent angle in half.
If you are doing 330 knots with the speed window open (in VNAV SPD) and below the path, when you close the window, the speed is going to go back to the command speed and this will automatically cause the nose to pitch up and slow the descent rate to half the angle of the path. If you are far enough below the path that idle thrust cannot maintain half the descent rate, then it will add thrust. If you are only 4,000' below the path in VNAV SPD doing 330 knots and you close the speed window, you're probably not going to need any thrust, so it will stay in HOLD||VNAV SPD until it gets to the path, then it will go IDLE/HOLD||VNAV PTH.
[ADDENDUM]:
This string may shed some more light on the issue.
http://www.pprune.org/tech-log/553693-b777-vnav-question.html#post8895416
Hi Hardy,
I'm not sure where to post this, but since it is directly related to my above post on the behavior while below the descent path, I'm posting it here.
On this situ at ToD into SFO https://drive.google.com/file/d/0B-WRh0Hf7VdZMUk4OTAwTHR3VmM/view?usp=sharing (also posted above), I used Beta 21 and as it was descending out of FL340, I speed intervened to 330 knots until it went 3,000' below the path (I didn't insert the /12,000 at SKUNK for this example).
When I was 3,000' low, I closed the speed window and noticed that, while it did add thrust and target the FMC command speed, the FMA showed SPD||VNAV PTH instead of THR/HOLD||VNAV SPD until getting back to the path. Thanks.
Jon
[ADDENDUM]:
Just to be clear, I'm not complaining about this. I am just thinking that you may have made a change to another part of the descent logic leading to Beta 21 and it might have affected this without your having a chance to catch it. Thanks.
Hi Jon,
that was intentional (different to EXEC). But I can change it so that closing the window has the same effect as pushing EXEC.
Cheers,
|-|ardy
Ah! I think I see what happened. So, you had it where it would go to THR/HOLD||VNAV SPD if the modification that put it below the path was done with the FMC. For instance, if you put SKUNK at FL200 and executed it, it would actually show THR/HOLD||VNAV SPD until it got to the path?
Closing the window performs validation X.
Pushing EXEC performs validation Y.
Before Beta 22, validation X engaged VNAV PTH if the deviation was below +155 ft.
In Beta 22, validation X engages VNAV PTH if the deviation is below +155 ft and above -155 ft.
Please note that there are actual modes behind the curtain, and each mode performs its dedicated FCC program. There's not just a "show of words on the FMA". The FMA just annunciates which modes are engaged. The FMA itself is not a program. It's just an indicator. The big programs run inside the AFDS.
I see. Thanks, Hardy. I'll try out Beta 22.
Hi Hardy,
Beta 22 is looking good. The only issue I saw with it -- and it's only for a second or two -- is that when you are capturing the path from below after the speed intervention exercise, when it does recapture the path it goes to SPD||VNAV PTH momentarily (less than 2 seconds), then HOLD||VNAV PTH. Since there was some thrust being used to control the descent rate, it should go IDLE||VNAV PTH and then HOLD||VNAV PTH when it captures. It's a minor issue but I wanted to bring it to your attention.
It must be hard to program this scenario because you have two things that are going to reduce your descent rate when you are below the path at 330 knots and you close the speed window -- (1) thrust, and (2) pitching for the FMC command speed (284 knots in this case). I like how you programmed the thrust to come in after it gets back to 284 knots and it becomes apparent that it will need some thrust to capture the path.
Hi Jon,
this is intentional.
In the beta, IDLE can engage with VNAV PTH only on an early descent.
In your case, the normal path deviation program is used; this adds thrust with SPD when below the path, and reduces thrust with SPD when the path is recaptured (and then it goes to HOLD). During this process the SPD mode may vary the thrust more than once. I can't engage the IDLE mode just because the thrust is decreasing. It could be increasing again in the next second if there is a disturbance. And then it may go IDLE, SPD, IDLE, SPD, IDLE, HOLD. -- SPD does the idling job just as well.
Are you sure it should be IDLE in this case as well?
Cheers,
|-|ardy
I guess I can set IDLE when coming in with VNAV SPD, but not when VNAV PTH is running with SPD.
Hi Hardy,
I know the logic must be hard to program on this.
I'm pretty sure it will go to IDLE/HOLD||VNAV PTH, just as it does during a cruise descent or early descent. This is from the Big Boeing Book:
QuoteThe cruise descent guidance law - IDLE to 1200 fpm then THR HLD - is used in the following situations:
1 When descending from one cruise altitude to another.
2 When making an early descent before the end of cruise.
3 When engaging VNAV when below the descent path.
4 When the airplane is flying a descent path and, after a modification, ends up below the reconstructed descent
path
Also, check out the bottom of the second page https://drive.google.com/file/d/0B-WRh0Hf7VdZYzZpS0x5eTdoWGs/view?usp=sharing
Really, it's close enough right now that I wouldn't even worry about it unless it's an easy fix.
99% of the guys flying this airplane probably wouldn't even notice it.
I'll try to implement the IDLE thing, but this quote above from the book refers to VNAV SPD. Also, I don't think the mode will be IDLE when the target is not idle but -1250 fpm. It's THR that goes to -1250 fpm. IDLE goes to idle and ignores the target descent rate.
I agree that the quote in the book refers to VNAV SPD while it maintains the 1250 FM descent. But there has to be some thrust that is being commanded to do that. When it captures the path it has to go to IDLE from that thrust position in order to continue an idle descent on the path. That's when the A/T would have to go from HOLD (prior to capturing) to IDLE/HOLD (once captured).
The SPD FMA only happens for a second or two and then it goes to HOLD, so it's not a big deal.
In PSX, the sequence on an early descent is this:
1. Push MCP ALT knob
2. THR | | VNAV SPD
3. Descent rate goes to -1250 fpm
4. HOLD | | VNAV SPD
5. Path deviation approaches zero
6. IDLE | | VNAV PTH
7. Thrust goes to idle
8. HOLD | | VNAV PTH
Do you agree?
In other processes with THR | | VNAV SPD, the previous betas applied SPD | | VNAV PTH at step #6.
Beta 23 will apply IDLE | | VNAV PTH at step #6, the same effect as with early descent.
PSX will never apply IDLE | | VNAV PTH when working with SPD to maintain the path. With SPD, thrust is modulated, and may be get very close to idle, and may rise again. When it maintains the speed while the throttles are at idle, and the aircraft is on the idle descent path, SPD simply changes to HOLD, because there is no IDLE job to do.
|-|ardy
I agree. So, once you are back on path, it will go to IDLE||VNAV PTH and if no thrust is required it will go to HOLD||VNAV PTH? If thrust is required it will say SPD||VNAV PTH?
In other words Beta 23 logic for the path capture feature (below the path and close the speed window) will be the same eight steps you lsted above when you commence an early descent and eventually intercept the path from below at 1250'/min?
Quote from: emerydc8 on Thu, 22 Oct 2015 06:23
I agree. So, once you are back on path, it will go to IDLE||VNAV PTH and if no thrust is required it will go to HOLD||VNAV PTH? If thrust is required it will say SPD||VNAV PTH?
IDLE engages whenever VNAV SPD changes to VNAV PTH for an idle descent while the throttles are out of idle.
QuoteIn other words Beta 23 logic for the path capture feature (below the path and close the speed window) will be the same eight steps you lsted above when you commence an early descent and eventually intercept the path from below at 1250'/min?
Yes, except for the 1250 figure; that may be shallower to get the half path angle.
|-|ardy
If I could figure out how to post two thumbs up I'd do it! Thanks, Hardy.
Beta 23 Review:
Beta 23 seemed to be working correctly under the scenarios that I tried out. I was using my descent situ going into SFO starting at FL350. The FMC command speed for the descent was ECON at 284.
Scenario 1: Regular Descent (speed window closed)
Upon glide path intercept, it went IDLE/HOLD||VNAV PTH
Bracketed the path within the VPP (+/-400'). Most of the time it was within +/-300'. I pulled the speed brakes at one point when it was about 300' low and it correctly came in with power as it got to 268 knots (more than 15 knots slow). The FMA then indicated SPD||VNAV PTH. As speed returned to 284 knots and back on path, it went to HOLD||VNAV PTH.
Scenario 2: Speed Intervention at FL340 to 330 Knots.
On speed intervention, it went to HOLD||VNAV SPD and pitched down to capture the 330 knots that I dialed in. When it was 3000' low, I closed the speed window. Observed THR||VNAV SPD as aircraft pitched for 284 knots. Descent rate was around 600 FPM. It added power to around 65% N1, then went to HOLD||VNAV SPD. Captured the path, then went back to IDLE/HOLD||VNAV PTH.
Scenario 3: Modification -- /12000" at SKUNK (this creates a new path well below the aircraft)
When leaving FL340, executed the modification. It went to HOLD||VNAV SPD and pitched for 284 knots. I speed intervened to 330 knots and added speed brakes. As it converged on the path from above, I closed the speed window. It went to SPD||VNAV PTH and pitched for 284 knots. Then went to HOLD||VNAV PTH.
Scenario 4: Late Descent (late to reset MC altitude until 2000' high)
When flying past the ToD it went to SPD||VNAV ALT. Then, when I reset the MCP altitude to 7000' and altitude intervened (pushed the alt knob), it went to IDLE/HOLD||VNAV SPD and pitched for 284 knots. I pulled the speed brakes out and descended for the path from above. On path capture, it went HOLD|VNAV PTH.
From what I can tell, the descent logic and FMA's are working correctly. Thanks, Hardy!
Thank you for the test :-)
Cheers,
|-|ardy
Thank you!