I am having difficulties with my VNAV.  I try to engage it, and it disengages after a few seconds.  I ran the maintenance feature on the FMS and that had no effect.  Any ideas?

 

 

Any ideas?

 

Any time you need help in determining the cause of something, it's always good to provide steps to reproduce the issue.

 

Were you in trim?  Were you trying to engage the AP too soon after takeoff?  Do you have a trim wheel on your hardware that you forgot to set to neutral?  Do you use the REALISTIC AP ENGAGEMENT option?

 

...was it sunny out?  Did a black cat stumble across your path under a ladder?  All info would be helpful.

Elaborate on VNAV disconnects.

Normally, when start my flight, I engage the VNAV and the LNAV, if it will connect, before I even push back from the gate.  Of late, the VNAV disconnects shortly after I push the button.  And, once I am airborne and the autopilot has been engaged, I will try to press the VNAV again, and it still disconnects.  Not cats were involved..(grin)

Hi

When the route , sid and star are not properly inserted LNAV or VNAV not work.

 

Paolo

I have been putting the route, sid and star in correctly now for almost a hundred flights.  It has only now begun to give me troubles.

If VNAV can't be armed on the ground it's usually an indication that the PERF INIT is not complete.

 

It has nothing to do with LNAV, trim or autopilot since neither of those have an impact on VNAV on a departure.

 

Remember, look at the top of the Primary Flight Display for the Flight Mode Annunciator.  That'll tell you what's going on. Unless you actually see the phrase "VNAV DISCONNECT" on the FMC, then something else is going on.

May be the first waypont alt isn't achievable.

If VNAV can't be armed on the ground it's usually an indication that the PERF INIT is not complete.

 

It has nothing to do with LNAV, trim or autopilot since neither of those have an impact on VNAV on a departure.

 

Remember, look at the top of the Primary Flight Display for the Flight Mode Annunciator.  That'll tell you what's going on. Unless you actually see the phrase "VNAV DISCONNECT" on the FMC, then something else is going on.

 

I think that must have been it.  I made sure to complete everything on the PERF INIT page for the flight I am starting now, and it is not giving me any troubles.