Difficulty Forcing FMS out of VNAV CLB

[emerydc8]

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?

I think the FMC must copy the missed approach altitude to the CRZ altitude whenever TOGA is pressed and it goes out of DES to CLB.

I'm saying that when you extend from the CF, the new cruise altitude should be whatever is in 1L. If you are presently below that altitude when you extend, you will stay in CLB. If you are at or above that altitude, you will either be in CRZ or, depending on your position, DES mode.

If we do the missed approach from 04L and level off at 1000' with 3000' as the cruise altitude (due to the missed approach altitude), we will be in CLB. If we then extend the CF (ZACHS at 2000'), we will still be in CLB, but the new cruise altitude will be 2000'.

If we leveled off on the missed approach at 2000' and we extended from ZACHS, the new cruise altitude would still be 2000', but we will be in CRZ, or maybe even descent if we are past the T/D boundary.

I have never extended a CF from FL220 and I doubt you would ever need to do this.

[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?

I think the FMC must copy the missed approach altitude to the CRZ altitude whenever TOGA is pressed and it goes out of DES to CLB.

PSX does that. And that's not the question.

I'm saying that when you extend from the CF, the new cruise altitude should be whatever is in 1L.

And I'm saying (and I thought you agreed yesterday) that this feature will cause the FMC to copy 70 ft to your CRZ ALT when 70 is at the runway 1L. What do you want to achieve with a CRZ ALT of 70 ft?

In this context, you misunderstood the word "data source" in my sentence on the previous forum page. I meant FMC data source, not handbook source. I wrote:

"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."

If you are presently below that altitude when you extend, you will stay in CLB.

Yes. And that's also the case in your beta. I designed the model as you told me. It will stay in CLB, and the FMC will not change the CRZ ALT.

If we do the missed approach from 04L and level off at 1000' with 3000' as the cruise altitude (due to the missed approach altitude), we will be in CLB. If we then extend the CF (ZACHS at 2000'), we will still be in CLB, but the new cruise altitude will be 2000'.

If I understand this in bold font correctly, this disagrees with what you said yesterday.

I have never extended a CF from FL220 and I doubt you would ever need to do this.

That may be true, but I have to develop an algorithm that has to take into account conditions and numbers. This machine doesn't understand what a "doubt" is :-)


Regards,

|-|ardy

[Hardy Heinlin]

Please try beta10:

http://aerowinx.com/board/index.php?topic=3685.0

[emerydc8]

Hi Hardy,

I was away from the computer for the past six hours. 

As far as your question, "What do you want to achieve with a CRZ ALT of 70 ft?," I have never checked to see what the new cruise altitude would be when extending from the RW fix. As long as it gets out of CLB when you extend the RW fix and shifts to DES when crossing the T/D boundary, just like you have Beta 9 set up to do, that's fine.

The issue of the new cruise altitude comes into play when you are extending from the CF. In the last situ I posted about 8 hours ago, for the first time, I just happened to try leveling off on a missed approach from 04L at 1000' and noticed that when I did this and extended from the CF (ZACHS), the cruise altitude stays at 3000', which is the missed approach altitude for RW04L. It didn't change to 2000, which was the altitude at ZACHS in 1L. When you try the situ and level off on the missed at 1000', are you getting it to change to 2000' for the new cruise altitude when you extend the CF?

I'll try Beta 10 tonight. Thanks.
Jon

[emerydc8]

Hi Hardy,

I had a chance to test drive Beta 10 and I like it!

When extending from the runway fix, your new cruise altitude is whatever your present altitude is. That's fine because it gets it out of CLB and allows it to go into DES at T/D.

As for extending from the CF, I tried three different scenarios to see how it behaves. On the missed approach for RW 04L, followed by a right downwind for 31L, I extended the CF (ZACHS). The altitude at ZACHS is 2000' in 1L. The missed approach altitude for RW04L is 3000'. So, when TOGA is pressed, PSX sets 3000' as the new cruise altitude, as we would expect.

Level off at 1000' and extend ZACHS = 2000' as the new cruise and it stayed in CLB mode. That's good!
Level off at 2000' and extend ZACHS = 2000' as the new cruise and it goes to CRZ. That's good!
Level off at 3000' and extend ZACHS = 3000' as the new cruise and it goes to CRZ. I think it's supposed to be 2000', but I can live with this because it's not likely to be noticed for training purposes (you are already out of CLB when you hit 3000') and it does shift over to DES when it's ready.

I also tried extending from the FF (MEALS) while level at 1000' in the same scenario. MEALS has a crossing altitude of 1800'. When I extended from MEALS, the new cruise altitude showed 1800'. This is good too.

So Beta 10 does what I was looking for which was mainly:

1. When you extend the RW fix it goes out of CLB right away. On-approach logic is immediately active, allowing it to shift from VNAV ALT to VNAV PTH at the glidepath intercept.

2. When you are lower than the CF altitude and you extend the CF, the new cruise altitude will be the CF altitude and you will stay in CLB until you get to that altitude. During that time, you will see a T/C and T/D on the ND. If you climb up to the new cruise altitude, it will go to VNAV PTH when you get there and shift into CRZ mode. The T/C will disappear.

Thanks again.
Jon D.

[Hardy Heinlin]

Hi Jon.

Level off at 3000' and extend ZACHS = 3000' as the new cruise and it goes to CRZ. I think it's supposed to be 2000' ...

I still don't understand why you think it should be 2000 here. Why do you want to prepare a CRZ DES from 3000 to 2000 shortly before the T/D, despite the fact that the T/D refers to your current altitude of 3000? Why not stay at 3000, keep the CRZ ALT at 3000, and at the T/D start a normal descent from 3000? The T/D refers to 3000, not to 2000. It makes no sense.


Cheers,

|-|ardy

[emerydc8]

Hi Hardy,

It's fine the way it is. I haven't personally observed whether it should make the CF altitude your new cruise altitude. It would really be nice to get into the sim and check some of this stuff out. But having read the training material from the Atlas guys, most of what they have published has been correct and they seem to be the only people in the entire industry since the airplane was built who have taken the time to publish this information for the pilots. Certainly Boeing and Honeywell have no interest in doing so.

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.

[Hardy Heinlin]

But this is what I'm trying to explain all the time :-)

Your quote also says this: "When this altitude is reached, the aircraft is in the CRZ phase"

Jon, you always refer to JFK. But there are places where the CF altitude and missed approach altitude are equal. E.g. at EHAM, at 2500.

CRZ ALT and aircraft altitude may be equal. They may be the same. They may be identical.

E.g. if the author of your quote would refer to EHAM, 2500 would be copied to the CRZ ALT. And the aircarft would be at 2500 already, because that's also the missed approach altitude.

So you would get 2500 in the CRZ ALT field. But how do you know where this 2500 comes from? What is the data source? Is the source the CF altitude? Or ist the source the current aircraft altitude? The author may think it's the CF altitude because it's 2500. But this may be a misunderstanding.

Here's my comment again:

"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."


Regards,

|-|ardy

[emerydc8]

The training at Atlas is done almost exclusively at JFK. For the approaches where this issue of a new cruise altitude would even matter (non-ILS), with the exception of the VOR 13 and the RNAV 31R, the CF altitudes are all 2000' and all the missed approach altitudes are at least 3000'. So, it's likely that you would be level at 3000' or above and setting up for a non-ILS to a runway with a CF at 2000'. I think that's why the distinction is even made that the CF altitude will be your new cruise altitude. Like I said, it sure would be nice to check this out in the sim.

[Hardy Heinlin]

Well, OK, that explains that part.

But -- technically -- I don't understand why it should lower the CRZ ALT. When selecting a STAR while cruising at FL310, it won't lower the CRZ ALT to the first descent constraint either. It would make no sense at all. So why should this be different when selecting an approach while cruising above the CF altitude? (Rhetorical question.)

It only makes sense when the aircraft is below the CF altitude. Maybe the author was thinking of this.

[emerydc8]

I have never seen a waypoint on a STAR that caused the FMC to shift to on-approach logic either. The waypoint on an approach is what triggers it, so maybe the logic is different for approach waypoints?

[emerydc8]

It only makes sense when the aircraft is below the CF altitude. Maybe the author was thinking of this.

Maybe you are right here. The good part is that Beta 10 now causes you to stay in CLB if you extend the CF and are below the CF altitude.

[Hardy Heinlin]

Thanks, Hardy. I remember discussing this a few months back on the re-cruise issue in the context of setting a lower final altitude when you can't get your filed altitude. We all agreed that the way to get to VNAV PTH is to select ALT HOLD and then select VNAV again.

But I also think that if you didn't want to take it out of VNAV ALT, and you simply altitude intervened after setting in your new cruise altitude, it should go to VNAV PTH. Maybe Peter would be able to check this. I found this in some Atlas training material on re-cruise:

• At level off the FMA displays:
|SPD|LNAV|VNAV ALT|

Note: We must re-cruise using the VNAV page

• Enter the desired cruise altitude on the VNAV page.
• Push the altitude selector button on the MCP...
• The FMA will now display:
|SPD|LNAV|VNAV PTH|
• Re-cruising successful

Revisiting this old thread 10 months later ...

So we learned that this push of the MCP ALT knob doesn't work in the BA sim. It does work on Atlas Air aircraft.

And I rediscovered that there are actually two cases of this VNAV ALT situation:

Case 1: MCP ALT captured while FMC in ACT CLB phase.
Case 2: MCP ALT captured while FMC in ACT CRZ phase (cruise-climb or cruise-descent).

In PSX, the MCP ALT knob trick has been introduced for case 1 in version 10.0.9-beta5, and for case 2 in version 10.1.0-beta3.

In other words, there isn't one central function for both cases.

To be precise: In case 1 it's about getting VNAV PTH and the CRZ mode. In case 2 it's only about getting VNAV PTH.

Could it be that BA and Atlas Air disagree because the one company refers to case 1, and the other to case 2?

If this is true I simply need to remove the trick in PSX just for case 1 or just for case 2. If not, I need to add another company option as this seems to be a relevant training subject in both airlines.


Regards,

|-|ardy

[Britjet]

An interesting analysis, Hardy..worth a go..
Whichever is right - I can't believe it's an anomaly/glitch/trick of the system or even an option but rather something more fundamental.
Peter.

[Hardy Heinlin]

19.02. FMC: Recruise execution in ACT CLB while above CF altitude copies current altitude to CRZ ALT as usual, but it will now slightly increase the copied value depending on current climb rate to avoid overshooting.

Hi Hardy. It's been a while since I've thought about this recruise issue, but are you recruising by installing a new approach for a return for landing or how are you recruising? I have to admit my memory has faded on this one.

Jon

I haven't changed the logic (we all agreed to).

When your new CF is far away and you are high above that CF altitude, the FMC will copy the current altitude to the CRZ ALT, and ACT CLB will change to ACT CRZ. You will not instantly go to CRZ DES and then fly level at the CF altitude for 20 minutes. Instead, you will cruise till the T/D and then descend to the CF. I'm not going to change anything in this logic. See discussion this thread. I just added a few feet to the CRZ ALT copy to avoid overshootings.

(If the T/D is located such that a DES is instantly necessary, well then you get the DES to the CF instantly. But not when you're a zillion miles away.)


|-|ardy

[emerydc8]

Sounds good. Thanks, Hardy.

Jon