Difficulty Forcing FMS out of VNAV CLB

[emerydc8]

If it would start only at the path interception, you won't see the deviation indicator before and how the needle is coming down. There is no deviation indicator in CRZ.

So there is no way to put it in DES mode and keep it in VNAV ALT until the path intercept, like the real plane? If given a choice of seeing the VPP early or seeing it shift from VNAV ALT to VNAV PTH on path intercept, I would take the shift to VNAV PTH, so at least the FMA logic is correct.

[Hardy Heinlin]

There is a way but it would take 5 years to retest all consequences. This part alone of the whole VNAV computer consists of thousands of lines of highly interdependent code.

[emerydc8]

Okay. Well, I guess extending the runway fix is just something that can't be modeled in PSX. If you want to change it back to where it was so the changes made don't affect the rest of the VNAV logic, that is okay with me.

[Hardy Heinlin]

It's just the mode change that occurs a few seconds earlier.

[emerydc8]

The mode changed to VNAV PTH when I was 1620' below the glidepath. The one modification you made in your last Beta where it shifts to CRZ as soon as you extend from an approach fix at 1L might transfer across to extending from the CF or FF and would probably be good to keep (presuming it doesn't interfere with the rest of VNAV) because it would model the shift out of CLB whenever the approach fix is extended and you are at or above the altitude in 1L. It is correct to have the shift occur without having to altitude intervene.

[Hardy Heinlin]

That's still there. I'm not going to revert anything.

[emerydc8]

Okay, I was concerned that the changes you have already made for this (mostly on my account) might affect other parts that we aren't even aware of yet. Thanks, Hardy.

[Hardy Heinlin]

I can keep VNAV ALT until path capture, but the DES phase will activate a bit earlier and thus show the deviation indicator before path capture. Can you live with that?

[emerydc8]

That's good. I don't mind the deviation indicator showing prior to path capture and VNAV PTH. In fact, I want the deviaiton indicator to show early. It's the shift to VNAV PTH that should happen upon path capture. Maybe I didn't express that well enough before.

[Hardy Heinlin]

Beta 9 is up.

[emerydc8]

Thanks, Hardy. You either stay up late or get up early. Or maybe you don't sleep at all

[emerydc8]

Way to go, Hardy! You got it to work right. If you take it out on a downwind far enough and turn in, it seems to display the VPP starting at just over 1000' fly-up. As it counts down, it correctly stays in VNAV ALT until intercepting, then goes to VNAV PTH. That's what I was looking for!

On a missed approach from 04L, I leveled off at 2000 and extended the CF from 31L (ZACHS) as I turned onto a right downwind. When I extended, it correctly went from CLB to CRZ and the new cruise altitude was 2000. The FMC altitude at ZACHS is 2000 too, so when I did the LVSA procedure on a right base, I was happy to see that VNAV correctly went to VNAV PTH and stayed there until intercepting the path between ZACHS and MEALS, crossing MEALS right at 1800' and following the path perfectly to the runway. I like it. I just hope the changes didn't adversely affect anything else. Thanks again.

Jon D.

[Panos Bilios]

Jon,

Please describe the LVSA procedure

Panos

[Hardy Heinlin]

In a new thread please :-)

[emerydc8]

Okay. Thanks, Hardy.

[emerydc8]

Hi Hardy,

I was experimenting with Beta 9 and discovered something that slipped by me yesterday. When I did the missed approach off 04L and came around for a right downwind for 31L, I extended from the CF (ZACHS) (see above), and I noted that it went to the new cruise altitude of 2000' when I extended. 2000' also happens to be the altitude at ZACHS in 1L. I had thought PSX was looking at what was in 1L for ZACHS (2000), not my actual altitude (2000). Now I can see that VNAV is making the new cruise altitude whatever my actual altitude is at the time I extend the approach fix.

On a missed approach, when you install and extend another approach fix, whatever altitude is in 1L should be your new cruise altitude. If that new cruise altitude is above your present altitude, it will stay in CLB. For instance, if you installed the CRI VOR approach and extended from ASALT (3000'), you would not get out of CLB at 2000' because the altitude at ASALT is higher than your present altitude. This comports with a recent comment from an Atlas check airman:

When we depart on a normal takeoff with a cruise altitude of say FL 220 for example, the vnav mode will only change to cruise if we put in an approach at our return airport and execute it and our current altitude is at or above the highest approach altitude. So for example in JFK, the highest approach altitude is 3000 feet [ASALT for CRI VOR 13L]. If I have not obtained 3000 feet, vnav will remain in CLB mode. This is why I do not allow pilots to level off below 3000' during training after a takeoff from JFK. They usually try to request this after a V1 cut. If we have obtained 3000 feet and put in another approach, VNAV will change to CRZ mode and a T/D should be displayed on the ND.

It is hard to detect on the situ I used above to test the other issues, so I made a situ that should make it more apparent. https://drive.google.com/file/d/0B-WRh0Hf7VdZR1htTkhET1U3cm8/view?usp=sharing

In this situ, I am 18 miles south of JFK at 6000'. My VNAV cruise altitude is FL310. When the situ opens, install the ILS31L and extend the CF (ZACHS). It should show a new cruise altitude of 2000' -- not 6000' -- because the altitude in 1L at ZACHS is 2000'.

This training excerpt below deals with missed approaches, but I don't think there is a difference in the logic between missed approaches and a regular departure as far as leveling off, installing an approach, and getting out of CLB mode. I think the only difference would be that on a normal takeoff, you set the cruise altitude in the PERF INIT page on preflight, and on a missed approach, VNAV automatically sets the missed approach altitude to the higher of your missed approach altitude or your MCP level-off altitude.

Would you consider having PSX make the approach fix altitude in 1L the new cruise altitude?

 Missed approach.
 Selecting TO/GA takes the aircraft from DES phase into CLB phase.
 The CRZ altitude is the highest missed approach altitude on the LEGS page, or, the missed approach altitude that was set into the MCP, whichever is higher. This altitude is displayed on the ND as the top of climb (T/C). When this altitude is reached, the aircraft is in the CRZ phase and the T/C is removed from the ND.
 Entering a new approach will allow the aircraft to transition from CRZ to the DES mode. When the new approach is entered, the altitude at the final approach course fix (CF) fix becomes the new CRZ altitude. A top of descent point (T/D) is displayed on the ND. If the T/C and T/D merge, the T/C will go away.
 If you are flying below the CF fix altitude and both the T/C and the T/D are displayed, you are in the CLB mode. You cannot get into the DES mode without first getting into the CRZ mode.

[ADDENDUM]: In this situ, I also noticed that when you are level at 6000' and you re-cruise to a lower altitude (say 4000'), it will insert your present altitude (6000') for the new cruise altitude. I remember going through initial, I always thought you had to insert a lower altitude than your present to re-cruise, so that's what I always did. In the sim, it would always accept the lower altitude as the new cruise altitude. I think PSX is also using the current altitude for the new cruise altitude on a re-cruise, like it is using the current altitude for the new cruise altitude when you extend from the CF or FF.

[Hardy Heinlin]

Now I can see that VNAV is making the new cruise altitude whatever my actual altitude is at the time I extend the approach fix.

Yes, that's intentional.

Would you consider having PSX make the approach fix altitude in 1L the new cruise altitude?

No. I don't want to get the runway's altitude constraint of 50 ft copied to my CRZ ALT. And I don't want to copy the missed approach altitude to my CRZ ALT when I'm below that altitude and unable to climb to it with 2 E/O; I want to get out of CLB mode at whatever altitude, and this may require a lower CRZ ALT than the programmed missed approach altitude. Your manual does not state the data source of the CRZ ALT copy; if the aircraft is at the CF altitude constraint, and that altitude is copied to the CRZ ALT, you don't know if the data source is the CF altitude or the aircraft altitude, as both are the same.

In the sim, it would always accept the lower altitude as the new cruise altitude.

I' adding the condition that the copy action is inhibited when the manually modified CRZ ALT is lower than the current aircraft altitude.


Jon, when the CRZ ALT is 2000, would you keep cruising at 3000 while waiting for vertical path capture? Or would you start a CRZ DES to 2000 and then wait for vertical path capture?


In PSX, and I guess in real life too, the T/D is based on the active CRZ ALT, not on the current aircraft altitude. Otherwise, the T/D may move back and forth while you're climbing above or descending to your active CRZ ALT. This would produce unstable predictions and may even trigger a premature DES phase activation.

[emerydc8]

I don't want to get the runway's altitude constraint of 50 ft copied to my CRZ ALT. And I don't want to copy the missed approach altitude to my CRZ ALT when I'm below that altitude and unable to climb to it with 2 E/O; I want to get out of CLB mode at whatever altitude, and this may require a lower CRZ ALT than the programmed missed approach altitude.

The missed approach altitude is copied to cruise only until you install another approach. Then, whatever altitude is at the fix you extend from (CF or FF) will be your new cruise altitude. If you are below that, it will stay in CLB. You shouldn't be able to extend from ZACHS (2000') while only at 1000' and have the new cruise altitude (1000') replace the 2000' constraint at ZACHS on the LEGS page. [See infra].

Extending from the runway may be a special case, just as extending from the RW causes on-approach logic to be active immediately (within 25nm), whereas it normally only goes active at the approach fix you extend from (CF or FF).

Your manual does not state the data source of the CRZ ALT copy

Very little of what we're talking about here can be found in any manual, so all we have to go by is our actual experiences in the sim and the experiences of others. It's frustrating to try to make sense of this when there is no data source.

Jon, when the CRZ ALT is 2000, would you keep cruising at 3000 while waiting for vertical path capture? Or would you start a CRZ DES to 2000 and then wait for vertical path capture?

I don't understand what you are asking.

[emerydc8]

I think I may understand part of your reasoning. In this situ https://drive.google.com/file/d/0B-WRh0Hf7VdZWi1ZRVlZUEh0d1k/view?usp=sharing if you do a missed approach from RW04L at JFK when it opens and you level off at 1000', it will correctly show the new cruise altitude as 3000' (that's your missed approach altitude). Now, if you install the ILS31L and extend from ZACHS, it will continue to say 3000' for your cruise altitude (I personally think it should say 2000').

Conversely, if you extend from the RW fix, the new cruise altitude is 1000', which I'm sure is because you are actually at 1000'. So, for a 2-engine when you extend from the runway fix (pretty much the only time you would ever extend from the RW fix), this makes sense to have your new cruise altitude be the altitude you are presently at when you extend. I like that.

But when you extend from ZACHS, I think the new cruise altitude should be 2000'.

Right now, Beta 9 has a separate condition for extending from the RW fix versus extending from the CF. If you extend from the RW fix the new cruise altitude defaults to your actual altitude. If you extend from the CF, it seems that there is no new cruise altitude -- it just keeps the missed approach altitude. In this case, that would actually be higher than the CF altitude (3000' v. 2000').

Since there is no data source that addresses this, all we have to go by is secondary source material like the training manual I cited. The Atlas check airman thinks the new cruise altitude should be whatever is in 1L when you extend from the CF. If you are below that it should stay in CLB. Sure wish we could test this in the real sim.

[Hardy Heinlin]

Are you saying, whenever an approach is selected from the DEP/ARR pages and executed, the FMC should copy the missed approach altitude of that approach to the CRZ ALT?

Imagine you are at FL200 in CRZ and you do the above. Your CRZ ALT will become, say, 3000 and all you can do now is not a normal descent but a CRZ DES from FL200 down to your new CRZ ALT of 3000.



Jon, you misunderstood the word "data source" in my sentence. I meant FMC data source, not handbook source:

"Your manual does not state the data source of the CRZ ALT copy; if the aircraft is at the CF altitude constraint, and that altitude is copied to the CRZ ALT, you don't know if the data source is the CF altitude or the aircraft altitude, as both are the same."