I dont know what going on latley but last 3 Flight ive done VNAV will not engage but LNAV will, ive done everything the same each time.. How do i trouble shoot this?

Okay so i just fixed it by adding in RESERVE fuel..... is that normal?? to be a problem?

10 hours ago, wheels1911 said:

Okay so i just fixed it by adding in RESERVE fuel..... is that normal?? to be a problem?

Sounds more like you forgot something else, and managed to correct it in going back to add that in.

Last time I had this problem I forgot the CI's value .

I also had this annoing problem whith VNAV.  I looked a litle back on this item in this forum, and there it was told to check the PERF INIT in FMC.

  Fill out necessary data here, and when Execute light comes on, activate.  That functioned, and the VNAV woorks.

B. Karlsen

 

Is the green VNAV light supposed to be on when I get to the final approach phase of the flight? I noticed that it was not on at this point during my latest flight, and the aircraft would not descend. When I pressed the VNAV button, the green light came on, and the aircraft immediately started its descent. To be honest, this seems to be happening quite often, and I am not really sure why. I seem to be "winging" it when it comes to final approaches, so I am not confident that I have everything under complete control. For the record, I limit the cruise altitude of my short flights to 5000 feet (because I like to look at the scenery when I am flying), but I quite often have to "force" the aircraft to descend. Even this is a bit "hit or miss". Sometimes clicking on the VNAV button starts the descent, and at other times the APP button. I am copying some stuff directly from the Tutorial #1 calculations (eg. the assumed temperature, and also the automatic selection of CLB-1....even though it always points to CLB-2) , so maybe there is an issue with this?

3 hours ago, Christopher Low said:

Is the green VNAV light supposed to be on when I get to the final approach phase of the flight? I noticed that it was not on at this point during my latest flight, and the aircraft would not descend. When I pressed the VNAV button, the green light came on, and the aircraft immediately started its descent. To be honest, this seems to be happening quite often, and I am not really sure why.

I assume you begin the flight with VNAV armed and it becomes active after TOGA.  The only way to drop out of VNAV is to activate another pitch control mode such as FLCH or VS or ALT HOLD. You are describing a problem during arrival and descent, but the problem actually occurs earlier and there is no way to know without more information.

You should always be aware of the pitch mode using the FMA.  In fact any time that you wonder what it is doing the first place you look is the FMA.  It tells you what the automation is doing in regard to thrust, roll and pitch and it vital information in all phases of flight.  If you want to descend, that is related to pitch control mode.  If it is not VNAV then what was it?

3 hours ago, Christopher Low said:

I am copying some stuff directly from the Tutorial #1 calculations (eg. the assumed temperature, and also the automatic selection of CLB-1....even though it always points to CLB-2) , so maybe there is an issue with this?

Not knowing what the "stuff" is there is no way of telling. The CLB-2 selection could be due to different takeoff weight?

 

EDIT:  One more thing Christopher, this should be a separate thread.  I encourage you to start a new thread for each new topic or problem.  Your problem does not fit the title of this thread and you will be assured support from more users is you had a new thread titled something like Why Do I have To Use VNAV to Descend or whatever.

10 hours ago, Christopher Low said:

Sometimes clicking on the VNAV button starts the descent, and at other times the APP button

 

I think there is some common misconception about the MCP. 

Vnav is a vertical mode that just follows a programmed vertical path in combination with the speed. The app-button arms or eventually activates the approach mode of the flight director. 

The green lights on the MCP mean nothing but the mode can be deactivated by pressing the button again.

Given the autopilot is engaged: If vnav is activated, pressing it will deactivate vnav and automatically activate vs and speed or alt hold and speed. As long as the app mode is only armed it has a green light which, again, means it can be deactivated —> the FD remains in the still active mode like vnav/lnav or FLCH, whatever is selected. As soon as app mode is activated the light extinguishes and you can not deactivate it anymore except you turn both flight directors off or go around. 

 

However you can not read the actual FD mode from the mode control panel. Therefore you have to look at the FMA which is the voice of your airplane. You talk to the airplane using the MCP, the airplane talks to you via the FMA. 

I have made significant progress after reading a few other threads on this forum about VNAV/LVL CHG/VS modes etc. I put that to the test in my latest flight this morning, and everything worked perfectly. For the first time, I felt in complete control of the aircraft during the descent and final approach

On 11/4/2017 at 5:34 PM, Ephedrin said:

Given the autopilot is engaged: If vnav is activated, pressing it will deactivate vnav and automatically activate vs and speed or alt hold and speed.

Wouldn't it go to CWS-P?

4 hours ago, Spin737 said:

Wouldn't it go to CWS-P?

You are right... I guess I mixed it up with the 777. 

Hallo,

I had the same prblem.
The reason was:

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

All necessary data must be entered