Descent FMA on Beta 13

[Kieber]

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

 

[emerydc8]

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?

[United744]

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!

[Hardy Heinlin]

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.


|-|

[emerydc8]

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.

[Hardy Heinlin]

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.

[emerydc8]

Initiation 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

[emerydc8]

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.

[Hardy Heinlin]

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

[emerydc8]

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?

[Hardy Heinlin]

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.

[emerydc8]

I see. Thanks, Hardy. I'll try out Beta 22.

[emerydc8]

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.

[Hardy Heinlin]

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

[Hardy Heinlin]

I guess I can set IDLE when coming in with VNAV SPD, but not when VNAV PTH is running with SPD.

[emerydc8]

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:

The 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

[emerydc8]

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.

[Hardy Heinlin]

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.

[emerydc8]

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.

[Hardy Heinlin]

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