Descent FMA on Beta 13

[emerydc8]

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.

[Hardy Heinlin]

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.

[Hardy Heinlin]

VNAV trick is added to Beta 14: http://aerowinx.com/board/index.php?topic=3160.0

[emerydc8]

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?

[Hardy Heinlin]

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

[emerydc8]

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?

[Hardy Heinlin]

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
_____________________________________________________________

[emerydc8]

Cabin 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.

Re 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.

[Hardy Heinlin]

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.

[emerydc8]

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?

[Hardy Heinlin]

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 for this subject?)

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.

Do I need to redo the situ using Beta 14 from the start of flight to get it to take?

No.


Cheers,

|-|ardy

[emerydc8]

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.

[Hardy Heinlin]

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.

[emerydc8]

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.

[Hardy Heinlin]

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.

[emerydc8]

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.

LATE 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.

[Hardy Heinlin]

Hi Jon,

you were confusing me for minute :-) ... with this question ...

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

[emerydc8]

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.

[Hardy Heinlin]

LATE 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

[Hardy Heinlin]

Beta 15 is up:

http://aerowinx.com/board/index.php?topic=3160.0