ALT INTV kicks me out of VNAV

[rfresh737 36]

I was wondering what might cause pressing ALT INTV to kick me out of VNAV mode?

 

I'm in the final stages of my descent, starting my initial downwind leg and I have a way point altitude constraint coming up I want to remove from the Legs page. On the MCP I set my lower altitude and then I press ALT INTV and it does remove the constraint but it also immediately kicks me out of VNAV.

 

My setup: I'm on my down wind leg, on my way to CHIPZ way point (8 miles to go) at 8000 feet. My Legs page shows:

 

CHIPZ 8000 (active way point, 8 miles to go)

POKRR 7300

PRINO 6500

HAWKO 5400

 

I've been cleared by ATC to descend to 6500 feet.

So I set the MCP altitude to down to 6500 (I want to remove the POKRR 7300' constraint).

Then I press ALT INTV and that's when VNAV kicks off.

FMA pitch section blanks out (when VNAV PATH annunicator goes away).

Amber "CWS P" annunicates to confirm no pitch mode (lost VNAV).

 

I must be doing something wrong or have something else set wrong?

 

Thanks...

 

[BRIAN1158 1]

Click on PRINO 6500 in the FMC and insert it where POKRR 7300 is and just carry on with the decent..no need to touch ALT INV......I Think...Anyway try It, see what happens............

                                                                                                    Brian.

[Michael Moe 497]

Happens alot on my flights as well . Never understand why.

 

Michael

[rfresh737 36]

Click on PRINO 6500 in the FMC and insert it where POKRR 7300 is and just carry on with the decent..no need to touch ALT INV......I Think...Anyway try It, see what happens............

                                                                                                    Brian.

 

Yes, I'm aware I can do that but I'm trying to use ALT INTV to do it. I wonder if there is a lower altitude limitation whereby it kicks off VNAV for some reason. I'll try it at cruise and compare the behavior.

[Spin737 102]

I don't think it should do this, but I have seen something similar, RW. Going in to Seoul, there was a STAR that would go to CWS-P. I never figured out why.

 

Were there any FMC alerts?

[rfresh737 36]

Yes, there is a CDU message when I press the ALT INTV button: "VNAV DISCONNECT".

 

Maybe there is a lower altitude limitation that will not keep VNAV engaged when using ALT INTV , I don't know for sure, yet. I have a formal technical question logged with the PMDG folks.

[Spin737 102]

Did you have a runway selected?

[rfresh737 36]

Yes, I have a runway set in the CDU...

[rfresh737 36]

I got a response from the PMDG folks:

 

"This is probably a bug honestly - there was a similar issue with the 777 that got fixed during development. I assume the same thing probably exists in the NGX and will get fixed when we update the NGX again (no ETA on that though, the focus is on the 777 service pack and 300ER right now here)."

 

So, there you have it. A bug.

[ark 1,296]

I got a response from the PMDG folks:

 

"This is probably a bug honestly - there was a similar issue with the 777 that got fixed during development. I assume the same thing probably exists in the NGX and will get fixed when we update the NGX again (no ETA on that though, the focus is on the 777 service pack and 300ER right now here)."

 

So, there you have it. A bug.

Good to know -- thanks for looking into the problem.

Al

[Richy 1]

Click on PRINO 6500 in the FMC and insert it where POKRR 7300 is and just carry on with the decent..no need to touch ALT INV......I Think...Anyway try It, see what happens............

Brian.

You know you'll make a direct to PRINO then? It will only work if CHIPZ to PRINO is a straight line ;-)

 

Edit: or did you mean the right buttons instead of the left ones?

[rfresh737 36]

I got another support email tonight from PMDG.

 

Note: and I am a subscriber to the FMS database update cycles from Navigraph.

 

"Ralph,
Just tried this. The current real world Jepp chart for the STAR does not show any restrictions at POKRR or PRINO - just the 170/8000 at CHIPZ. That's exactly what I'm seeing here using the latest Aerosoft Navdata Pro cycle. I went ahead and added the restrictions you stated you had though. At 8 miles from CHIPS, I dialed 6500 and pressed ALT INTV and I do see the issue you mentioned. Something very much like this was fixed during the 777 development and it'll probably be fixed when we roll that code backwards into the NGX later on (no ETA on that sorry).  Work around was just to press VNAV again to reengage it.

Ryan Maziarz

Technical Support"