Oops, probably am Matt.

This surprised me -> • NG aircraft equipped with Collins P4 FCC or later; or Honeywell -710FCC or later; and CDS BP06 software. Im at work now and I don't remember the model , that's the one with the display numbers in cyan or grey not the yellow-orange ones. I thought it was a cosmetic feature only :(

You're probably thinking of LNAV.

I ran into this too a few times. I think (not 100% sure....) that: VNAV won't arm unless LNAV is.LNAV won't arm unless you have a "good" (non-acute?) angle to the first track. Too bad the FMC doesn't give us helpful feedback when we press buttons and nothing happens. I suppose it expects we're trained pilots! Sherm

It might be useful to remember the following:a. VNAV and LNAV arming on the ground, let the FDs use calculated flight path output to drive their displays. Useful after liftoff to determine accurate pitch angle for V2 plus 10, and manually fly the planneds vertical and lateral departure procedure profile while the autopilot isn't yet engaged.b. Airborne, once height constraints are met, pressing CMD connects active FD output to the autopilot selected channels and modes, for hands off autoflight.

I ran into this too a few times. I think (not 100% sure....) that: VNAV won't arm unless LNAV is.LNAV won't arm unless you have a "good" (non-acute?) angle to the first track. Too bad the FMC doesn't give us helpful feedback when we press buttons and nothing happens. I suppose it expects we're trained pilots! Sherm

Nope. You can take off without selecting a roll mode. Just like honey badger, VNAV don't care.

As a recent newbie to this aircraft I have been following the 'Tutorial #1' but cannot seem to arm VNAV mode prior to takeoff as instructed. I press the VNAV button on the MCP, but it does not illuminate green like the LNAV button does after pressing. Has anybody else experienced this? Can someone please help. Regards Darren.

Not only does the aircraft have to be setup properly, but to arm VNAV on the ground, you need the Collins MCP. From what I've read, arming VNAV on the ground is also a new feature of the 10.8 FMC software. If you search for "VNAV takeoff", you will find a couple of threads that have already gone over this. I arm it on the ground for every flight, and have never had a problem. - John

It might be useful to remember the following:a. VNAV and LNAV arming on the ground, let the FDs use calculated flight path output to drive their displays. Useful after liftoff to determine accurate pitch angle for V2 plus 10, and manually fly the planneds vertical and lateral departure procedure profile while the autopilot isn't yet engaged.b. Airborne, once height constraints are met, pressing CMD connects active FD output to the autopilot selected channels and modes, for hands off autoflight.

This is basically my approach. LNAV at 400 agl and then hand fly based on the FD prompts to 10K feet, VNAV when flaps are up. For me it makes take off and departure more challenging and enjoyable.

what in your opinion could be the cause of the VNAV not arming until few seconds or 400R after takeoff ?

Well, as I said, there is no particular "reason". You can arm VNAV on ground, yes, but 400R is the minimum for it to engage, just like 50 is the minimum for LNAV, and 400 is also the minimum for the APs. As Matt explained above, there isn't exactly a reason why one would need VNAV at 400, but all I'm saying it's the technical minimum height for the system to operate. Just how it was designed. Having VNAV on after flap retraction is however very sufficient. And anyway, manually selecting bug up or having to use SPD INTV in VNAV, where's the great difference. It's all mostly pitch-for-speed modes by then anyways.

Not only does the aircraft have to be setup properly, but to arm VNAV on the ground, you need the Collins MCP. From what I've read, arming VNAV on the ground is also a new feature of the 10.8 FMC software. If you search for "VNAV takeoff", you will find a couple of threads that have already gone over this. I arm it on the ground for every flight, and have never had a problem. - John

vnav will arm on the ground if your using the honeywell mcp also