VNAV ALT descending

[Toga]

Hi,

I'm keeping current practising VNAV approaches - legacy FMS - and keep finding the aircraft commands a descent when VNAV ALT is annunciated under these circumstances:

Route in FMS EGSS - EGPH - cruise alt FL300. I fly the SID in LNAV/VNAV climbing to the SID stop alt of 4000ft. During the SID i take a radar heading with HDG SEL and select the RNV22 approach and line select TOTVO - IAF - to 1L with extended centreline. The platform is 2500. ALT cruise in the FMS changes to the current ALT 4000 and VNAV ALT engages. The aircraft then starts to descend with 4000 in the MCP ALT selector. Not sure if its me or the aircraft? Perhaps I need to engage ALT when executing the changes in the FMC then re engage VNAV?

T

[Hardy Heinlin]

Situ file?

[Toga]

Where do I send it?

[Hardy Heinlin]

Code Select Expand

into a code block of a forum post

[Toga]

It must be an issue with that situ I was flying as Ive just loaded up another one and tried to replicate the error and it didn't happen. It remained in VNAV path the whole time which is what I expected. No VNAV ALT this time.

[Hardy Heinlin]

Are you sure you had 4000 in the MCP? As you know, VNAV ALT automatically changes to VNAV PTH when the DES path is captured while the MCP ALT is below the aircraft.

[Toga]

I have been able to replicate it.

No I don't change the MCP ALT it remains at 4000 throughout the entire departure. Ive taken a photo also but not sure how to post it.

Load this situ and then load the RNV22 into the FMC and you'll see VNAV ALT engage and a descent when you execute the arrival. 

There is something weird happening because I have run it several times and only sometimes do I get this problem.

[Jeroen Hoppenbrouwers]

To upload files and illustrations:
http://aerowinx.com/board/index.php?topic=1744.0

[Hardy Heinlin]

Toga, the text content of the situ file should be in the code block, not the file path to your home computer. Luckily, there is no public access from here to your home computer.

Or just post it at Hoppie's link.

[Toga]

Ah ok, thanks guys. Yeah Ive ran this a few times and it keeps happening. I tried pressing the event record just before I executed the approach but when I loaded that situ VNAV ALT was already engaged.

Here's the situ. To replicate what Im seeing simply depart on the sid with LNAV and VNAV, engage A/P at 400ft, clean up and durning the first turn go HDG SEL to something like 040. This gives SPD HDG SEL VNAV PATH. Level off at 4000ft. Then go into the FMC and select the RNV22, TOTVO INTC and execute. You'll see VNAV SPD change to VNAV ALT and it starts to descend at 1200FPM.

[Toga]

Or actually its faster if you use the event record one. Just kick out vnav at the start and re engage it to get PATH. then select the RNV22 and it replicates the descent issue.

http://www.hoppie.nl/forum/var/--Event_record_2020-MAY-25_14-04-47.situ






Hoppie removed the img brackets around the situ

[Will]

Let me just comment before looking at the situ that your screen layout reminds me of PS1.3... ;-)

[Toga]

Its fantastic! Can't wait until im able to get a decent multi monitor setup.

[Hardy Heinlin]

Just kick out vnav at the start and re engage it to get PATH. then select the RNV22 and it replicates the descent issue.

When I load it, VNAV PTH is already engaged.

Please describe every step we need to do. The info "select the RNV22" is insufficient.

Edit: OK, you also mean pushing TOTVO INTC> ... I see the effect now.

[Toga]

Yeah, TOTVO INTC and execute then VNAV ALT engages and the aircraft starts to descend.

[Hardy Heinlin]

There is a data resync timing problem in the moment when EXEC is pushed. This happens in ACT CLB, and after the MOD the next constaint will be 400 ft at the RWY. The FMC, still being in ACT CLB for a nanosecond, takes this 400 as a CLB constraint and thus that same lower 400 will have priority over the higher MCP 4000. A nanosecond later it changes to ACT CRZ, but then it's too late.

I don't know if I can fix it. This is extremely complicated because there are many interdependent chicken-egg-chicken-egg causal chains within a very tight time frame ... plus multitasking and networking ...

(The logic has always three target altitudes to evaluate: CRZ ALT, MCP ALT, waypoint ALT -- in the context of the current flight phase and aircraft altitude. Mode changes have very complex consequences because nothing happens exactly in the same nanosecond.)

[Toga]

Sorry to have stumbled across this madness. Ive found engaging ALT then executing then back to VNAV sorts the problem.

[Jeroen Hoppenbrouwers]

Probably the real FMC also has this kind of issues, just different ones.

[Hardy Heinlin]

Problem should now be eliminated in PSX 10.111:

http://aerowinx.com/board/index.php?topic=4191.0


|-|

[Toga]

Hardy your genius is next level.