I embarrassed myself only yesterday when I was showing a friend a take off using PMDG 737 and I also could not engage VNAV.

I discovered that I had not activated the flight plan and then pressed the EXEC button on the FMS. Silly me!

When you activate it you should still have a blue dotted line showing proposed flight path on the navigation display and when you press the EXEC button on the FMC you will have a solid pink line.

Then the jolly ole VNAV worked!!

Hope this will solve your problem?

 

Kind regards

Richard

In that case, LNAV would not work either. That is why I suggested the PERF INI page. The OP has not come back yet.

In the off chance other people reading this, for me it is always the PERF page that i keep forgetting activating :) After that usually VNav starts to work.

Hallo,

I had the same prblem.
The reason was:

I forgot an entry in the field "FLAPS"
IN FMC - capitel "TAKEOFF".

All necessary data must be entered

I also noticed that VNAV does not engage if you do not stay close to the "V2+20 knots" climb speed immediately after take off. This happened to me many times in the 737 NGX until I accidentally discovered the source of the problem! Having said that, I can quite happily fly the aircraft using LVL CHG instead.

Just now, Christopher Low said:

I also noticed that VNAV does not engage if you do not stay close to the "V2+20 knots" climb speed immediately after take off. This happened to me many times in the 737 NGX until I accidentally discovered the source of the problem! Having said that, I can quite happily fly the aircraft using LVL CHG instead.

I think the actual issue is more that you need to be in trim, and you're likely out of trim if you're off speed.

Good point, Kyle. I hadn't thought of it like that.

One thing I can tell you is it is very important you program the FMC in a particular order

If you look at the first tutorial it will list the order that you need to input information into the FMC

The other major thing is do you have both Flight directors turned on? if you are using the 2d panels you will not see the copilot Flight director you have to get used to using the 3d panels

 

 

7 hours ago, Christopher Low said:

I also noticed that VNAV does not engage if you do not stay close to the "V2+20 knots" climb speed immediately after take off. This happened to me many times in the 737 NGX until I accidentally discovered the source of the problem! Having said that, I can quite happily fly the aircraft using LVL CHG instead.

That sounds odd to me. What modes are you seeing then? Are you entering LVL CHG by selection? Are you arming VNAV before T/O?

I always arm VNAV and LNAV before take off, but VNAV seems to "switch off" when I am above 400 feet if the climb speed is not reasonably close to "V2+20". When this happens, I see ALT HLD on the FMA when I reach cruise altitude (which in my case is only 5000 feet at the moment; I will need to check what is displayed when I am climbing). I can reactivate VNAV if I want to, but sometimes it is just easier to use LVL CHG on descent. I am not bothered about following real world restrictions to the letter, so I tend to give myself plenty of time to reduce speed/altitude when approaching my destination.

What do you mean VNAV "switches off" when you are at 400 ft ? You engage the roll mode and pitch mode on the ground for flight director commands only on takeoff and up to a certain altitude above 400 feet. VNAV will only be armed for takeoff and early climb until reaching acceleration altitude, then you will see the pitch FMA switch from white armed to green active. Still, as long as you do not engage the autopilot this will be for flight director commands only.

Your autopilot will do what your flight director commands it to do depending on the modes that you have selected. The flight directors will provide the clues for hand flying also as long as they are engaged. If you are not doing your CDU preflight correctly and have skipped on the takeoff trim settings, N! climb power, or other stuff then your autopilot will not engage properly as it will find the aircraft out of trim.

Takeoff trim/climb power etc. is always set correctly, based on the value provided by the FMC when setting up the aircraft for departure. It's worth noting here that the preparation for the 747-400 is almost identical to that for the 737 NGX, but the VNAV mode always remains active (with VNAV PTH displayed on the FMA).