Check Alt Tgt / VNAV ALT

[srce 57]

Hi,

I'm using the 777-200 in P3D V4 with latest Navigraph data. I'm taking off from KSFO 28L on the AFIVA1 SID. I think I've been through the FMC setup procedure OK, have the FDs on, and LNAV/VNAV selected, A/T on and A/P off. The MCP has altitude set to 10000. First waypoint is at 520 then 4000A.

Just after takeoff at a few hundred feet TOGA changes to VNAV ALT instead of VNAV SPD and I get an FMC message saying 'CHECK ALT TGT'. Any idea why?

If I run through the same setup procedure at another airport, it seems fine.

Also, when in VNAV ALT, is the altitude target displayed anywhere (other than by the horizontal FD bar)? I can't see it anywhere in the PDF/CDU as a numeric value.

Thanks, Jon

 

 

[Budbud 227]

Hi,

The ALT TGT is the one you set on the MCP. Getting the message "CHECK ALT TGT" means that the FMC detected a conflict between the altitude set on the MCP and the next altitude constraint preventing the FMC from meeting that constraint.

Please don't take it the wrong way but are you sure that you enter 10000 on the MCP and not 1000?

The only reason I can see for which the FMC would change to VNAV ALT instead of VNAV SPD is if it reaches the altitude set on the MCP while that altitude is not the one of the constraint of a waypoint not yet reached.

As for the second question, once in VNAV ALT there is no ALT TGT anymore per se. The FMC has stopped the climb/descent to the altitude set on the MCP (which was the target).

[srce 57]

11 hours ago, Budbud said:

Please don't take it the wrong way but are you sure that you enter 10000 on the MCP and not 1000?

 

Yes - that's the altitude the MCP starts up with. I've also tried 4000 and 5000 and the same thing happens.

Screenshot

It appears to be related to the first way point, displayed as (520)    174/520 on the LEGS page. If I remove that, with the next waypoint at 4000A, it goes in to VNAV SPD after TOGA.

11 hours ago, Budbud said:

As for the second question, once in VNAV ALT there is no ALT TGT anymore per se. The FMC has stopped the climb/descent to the altitude set on the MCP (which was the target).

Is the altitude it has 'stopped at' displayed though? As the FD bars seem to direct you towards that.

[Budbud 227]

Hi,

3 hours ago, srce said:

Is the altitude it has 'stopped at' displayed though? As the FD bars seem to direct you towards that.

It is the one set on the MCP if you didn't change it after the VNAV ALT has activated.

[downscc 1,327]

I noticed that once too.  The sidstar file is correct in it's syntax "HDG 284 UNTIL 520" then it issues a direct to a fix.  The LEGs page should not have an altitude constraint at that waypoint.  This might be a bug. I think you should document this and put it on a trouble ticket at Product Support to get it into the bug tracking system.  The deal is, we want to hold that heading until 520 but 520 is not an altitude constraint so it should appear in parenthesis on the left side of the legs page.  I haven't played with it, but does changing it to 520A/ correct the behavior?

[srce 57]

2 hours ago, downscc said:

The deal is, we want to hold that heading until 520 but 520 is not an altitude constraint so it should appear in parenthesis on the left side of the legs page.  I haven't played with it, but does changing it to 520A/ correct the behavior?

It does have parenthesis on the left hand side:

https://www.dropbox.com/s/6y935ed0b0i3eu1/CDU_520.PNG

I can't seem to change it though. If I enter: 172/520A (or any other value) and press LSK 1R, it says: INVALID ENTRY. I can only seem to change subsequent waypoints.

FCOM2 suggests conditional waypoints (i.e. those in parenthesis) can't be entered manually.

 

[downscc 1,327]

Correct, conditional waypoint can only be entered via the database. I think the VNAV ALT behavior is wrong and should be reported as a bug.

[Budbud 227]

I don't think the altitude restriction should be the cause of the VNAV ALT. If you have a hard constraint of 520 then the FMC will the level up at 520 but remain in VNAV SPD not revert to the VNAV ALT. The only case where the FMC reverts to VNAV ALT is when the altitude set on the MCP is prevent the FMC from reaching the next constraint:

_ In climb if you altitude on the MCP is below the next XXXXA or XXXX altitude or below the entered cruise level

_ In descent if the altitude set on the MCP is above the next XXXX or XXXXB or any altitude set on the MCP if not constraint on the leg page.

[srce 57]

Well, the MCP is definitely set high enough. Looks like it does the same thing in the 747.

[Budbud 227]

As Dan suggests, it may a good idea to report it to the support team by submitting a ticket. The reversion to VNAV ALT doesn't make sense to me in that situation.