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Texas Hood

VNAV Issue It's most likely me.

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I can't seem to climb with the 777 with VNAV pushed and on, I'm sure I have missed something but just can't find it please help. I have a CRZ alt set (360) it's purple there are altitudes set on my legs page. VNAV is green on the CDU and AP is green as well. I climb if I push the VS button but it's a slow climb because the aircraft will slowly low down. It wont increase speed as I climb. I think it is related to the VNAV issue though. I don't have the 737 but I do have the MD-11 so I can't really figure out what I'm missing. Please help stranded at 20,000'.

 

Ryan Gann

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Please help stranded at 20,000'.

 

Are you using online weather, particularly FSInn?  There's an altitude bug where the program increases temperature with altitude, which can cause performance problems.

 

Outside of that, the tutorial that came with the aircraft could help you to understand the function of VNAV a little more.  It's different from what you'd expect from the MD-11.

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Just a thought: Are you pushing the altitude button after selecting a new flight level. If I remember correctly you don't need to do this in the MD11 but you do in the 777 and in the 737 you need to press the alt intervene button to the right of the altitude selection knob. Once the button is pressed the plane should climb in VNAV

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I can't seem to climb with the 777 with VNAV pushed and on, I'm sure I have missed something but just can't find it please help. I have a CRZ alt set (360) it's purple there are altitudes set on my legs page. VNAV is green on the CDU and AP is green as well. I climb if I push the VS button but it's a slow climb because the aircraft will slowly low down. It wont increase speed as I climb. I think it is related to the VNAV issue though. I don't have the 737 but I do have the MD-11 so I can't really figure out what I'm missing. Please help stranded at 20,000'.

 

Cruize altitude set where?

In the FMC/CDU (thats the computer you use to programm your route) or on the Autopilot Panel (MCP)?

 

For Vnav to climb you need to set your cruize FL both in the FMC/CDU as well as on the AP panel.

This is only so for Vnav!

For VS and FLCH is does not matter what is in the FMC/CDU.

 

Then if you are already flying in Vnav SPD mode (Green VNAV SPD indication on your primary flight display (PFD)) then changing for example from 10.000ft to 36.000ft on the MCP will just continue the climb to the newly set FL360.

 

If you were in another mode (like VS) then you have to press the Vnav switch again to engage Vnav Spd again (you dont have to, you can also continue climb with VS or FLCH).

 

And if you see "Vnav Alt" green on your PFD then it means the aircraft has leveled off at an altitude set on the MCP that is below the cruize altitude set in the FMC/CDU. To contiue climb in this case, you do not need to press the Vnav switch again (because you are in Vnav mode already) but you do have to press the grey round altitude selector. This is called altitude intervention and will resume the climb.

(Vnav Alt (green) changes to Vnav Spd).

 

If you are climbing in Vnav Spd and on the LEGS page in the CDU/FMC you have entered and altitude restriction to pas point XXX at (or at/below) a certain altitude then Vnav Spd will change to Vnav Pth and the aircraft will stop the climb untill you have passed this XXX point or untill you remove the altitude restriction.

 

 

 

"VNAV is green on the CDU and AP is green as well"

That is not the CDU but the PFD!

The PFD (primary flight display) flight mode annunciations show what mode is active....so what mode the AP or the FD is following.

 

What do you mean with this?

"I climb if I push the VS button but it's a slow climb because the aircraft will slowly low down."

slow VS or slow IAS?

and

slowly low down??

 

and

"I think it is related to the VNAV issue though."

huh?

Vnav issues can not have anything to do with VS issues.

 

VS is set on the MCP (autopilot panel).

You are directly controlling the AP now (or FD if the AP is not engaged!!!)

You are telling the AP directly to climb with a certain VS (which you have to set with the round wheel on the MCP) at a certain speed (which you have to set with the speed selector on the MCP).

The whole CDU/FMC and Vnav are not used in this mode and have no infuence on anything at all.

(if Lnav is engaged while in VS then you are still using the CDU/FMC for lateral navigation, but not for vertical navigation)

 

when you use Vnav and Lnav you are using the CDU/FMC to tell the AP what to do.

So you are not in control of the AP (or FD) directly.

The FMC/CDU is telling the AP what to do according to how you programmed the FMC/CDU.

 

Hope this helps.

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