Hmm, peculiar, cause I never have any probs engaging Vnav.Maybe there are some others with ideas?/ Leffe

I can remember running into a similar problem. Finally it was a non acknowledged FMC setting. It took a while finding the right page. I still think that this already given hint might be the most promising. Wrong response - and too late as it seems. :(

Edited February 4, 201214 yr by FlyingAxx

Here's the funny thing, all speed/altitude targets are shown and visible on the LEGS page, its just the button that won't turn on.Just a thought, Service based failures are turned OFF

Are you talking about being on the ground or in the air ?Can you see the INDEX prompt on your takeoff page ?Fred.

Are you talking about being on the ground or in the air ?Can you see the INDEX prompt on your takeoff page ?Fred.

Both, and yes, the INDEX prompt on the takeoff page is visible and clickable

The INDEX prompt on the takeoff page lets you know that the FMC has been pre-flighted so no worries there.Will it not let you press VNAV before you press TOGA ?Everything checked on the overhead ?Fred.

Neither before or after I press TOGA can I press VNAV, overhead checked as well.

A screenshot or two maybe !Fred.

Are you getting the v-speeds (v1, etc.) when you attempt to arm Vnav prior to TO? If not, some data is missing, either on the Perf page or on the Route page( hint, check that you have selected a departure runway.....).Thanks, Bruce.

Edited February 5, 201214 yr by brucek

I thought Vnav was available after 400ft on climb out. LNAV is selectable on ground as long as there is a dep. hdg within 5deg of rnwy hdg. Or do I need to read some more? lol.

VNAV should be selectable on the ground provided all the conditions are met.Most common ones to forget are both FDs on and making sure you press EXEC on the PERF INIT page after completing the entries.

Hmm...I was on KIAD 01L direct to the BLUES waypoint, both FD's are on with the left one being the main FD, AT was on, trim set, RTO on, flaps set to 5, I could select V-speeds, but they were not visible on the PFD speed tape, but 01L was confirmed as the departure runway. I thought the EXEC light was the issue, but it was not illuminated.

I could select V-speeds, but they were not visible on the PFD speed tape, but 01L was confirmed as the departure runway. I thought the EXEC light was the issue, but it was not illuminated.

Those are exactly the symptoms of the performance not being initialized. Are you sure you've entered all the required data on the PERF INIT page?

A couple of shots of this. The shots are all of the important pages in the FMC, plus the PFD showing NO VSPDEDIT: Oh man, I messed this one up. I couldn't see the lit EXEC light on the PERF INIT page because I'm using a VRInsight CDU II, which doesn't have a lit EXEC light: I just pressed EXEC on the PERF INIT, and all is well... I apologize for my ignorance, guys...

Edited February 5, 201214 yr by Wendall

If you use a program like FSPassengers, it will change the weight of aircraft. You then have to go back to payload page in CDU and hit the ZFW/CG button to change weight to what FSPax changed it to then go to take off with new CG and VNAV is selectable. This is what happened to me and the fix.

to engage VNAV I did: page CLB, MAX RATE, ECON, EXECUTE and it works well after of course all the other stuff was programmed.

Make sure that on your legs page, next to the upcoming waypoint you altitude is in a magenta colour. Also check for FMC again for any mistakes. Doesn't hurt to do so!