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Treetops45

PMDG 737-600 VNAV holding TOC Altitude

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I got it right the 1st time, but not subsequently.

Setting up a STARS ILS flight via the FMC, I then set a Top of Climb Altitude in the AP that matches the FMC ALT, & check the FMC Legs for discontinuity & so on, & that the Waypoint altitudes are reasonable. Save the flight & Execute Flight Plan ok in the FMC.

Take off & go on to the AP, running VNAV, LNAV.

The 600 climbs to TOC ok, but further into the flight, when it is passing waypoints where it should be losing altitude as part of the descent process, it holds TOC altitude.

I am trying to figure out if I have missed a step in the FMC as regards triggering the descent from TOC?

T45 

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Have lowered the AP ALT ahead of the TOD ?

VNAV won't go below the current AP ALT so you effectively need to authorise the descent by setting the altitude your want it to go down to.

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33 minutes ago, Matchstick said:

Have lowered the AP ALT ahead of the TOD ?

VNAV won't go below the current AP ALT so you effectively need to authorise the descent by setting the altitude your want it to go down to.

And you need to adjust the alt at least 10 nm from TOD. If you are closer to TOD, you need to also hit the ALT INTV button for the ac to begin descend. If you don't adjust the MCP ALT, you should receive a message in the FMC to "Reset MCP ALT".

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20 minutes ago, duckbilled said:

And you need to adjust the alt at least 10 nm from TOD. If you are closer to TOD, you need to also hit the ALT INTV button for the ac to begin descend. If you don't adjust the MCP ALT, you should receive a message in the FMC to "Reset MCP ALT".

1nm is enough too 🙂 Descend will allways start, if ALT will be adjusted anyrywhere before TOD pass. I don't know, if it the same as in the real world, but it works.

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Posted (edited)

I also found that for a Direct Departure, VNAV will not engage if the FMC Legs altitude at the first Waypoint does not match the Altitude in CLB or Cruise.

T45

Edited by Treetops45

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Posted (edited)
3 hours ago, Treetops45 said:

I also found that for a Direct Departure, VNAV will not engage if the FMC Legs altitude at the first Waypoint does not match the Altitude in CLB or Cruise.

T45

Unless I'm misunderstanding what you're saying there's something not right in your setup of FMC and MCP. VNAV will engage even if you don't have any route in the FMC at all and still guide you through acceleration up to the altitude set in the MCP altitude window at the programed climb speed. Whether there is a waypoint after departure or which altitude is calculated for it/which restriction is in place for it on a SID is irrelevant to if VNAV engages.

Since you're talking about both climb and cruise pages, I'm assuming you mean the cruise altitude which is shown on both. The calculated altitude of the first waypoint will rarely match the cruise altitude and that doesn't inhibit VNAV. If you've set up PERF correctly VNAV can be used and armed prior to takeoff and it engages at 400 ft AGL automatically. It will provide climb management with or without a route loaded up until the altitude set in the MCP, at which is point it goes into VNAV ALT and levels off to respect the MCP setting.

Edited by threegreen
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34 minutes ago, threegreen said:

Unless I'm misunderstanding what you're saying there's something not right in your setup of FMC and MCP. VNAV will engage even if you don't have any route in the FMC at all and still guide you through acceleration up to the altitude set in the MCP altitude window at the programed climb speed. ......... It will provide climb management with or without a route loaded up until the altitude set in the MCP, at which is point it goes into VNAV ALT and levels off to respect the MCP setting.

Thanks for this.

I just set up an ILS flight in the Canary islands & couldn't get VNAV or LNAV to engage after T/off & with AP On.

I will have a closer look at my FMC settings & the PERF page.

If VNAV won't engage, what should be checked 1st??

T45

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4 hours ago, Treetops45 said:

VNAV won't engage, what should be checked 1st??

Check the Perf page... maybe your flaps setting.

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