My VNAV keeps disconnecting.  Any idea what would cause this?

Hello! What was the state of flight? What did you program in the fms? Any unplanned descent/offset? Situation? That would help, on vnav not staying connected it usually comes down to something being wrong with the route planning/state of flight not matching what the fms "thinks". In this cases also don't be afraid to revert to a lower state of automation. If it's not working it usually is because the wrong information has been given to your tools. ..

Hello! What was the state of flight? What did you program in the fms? Any unplanned descent/offset? Situation? That would help, on vnav not staying connected it usually comes down to something being wrong with the route planning/state of flight not matching what the fms "thinks". In this cases also don't be afraid to revert to a lower state of automation. If it's not working it usually is because the wrong information has been given to your tools. ..

I entered a route from JFK to DEN, normal stuff.  On ascent, the VNAV would not stay connected, and I could not connect it for the rest of the flight.

Did you have any other indications like DRAG REQUIRED?

Check if you have a "route discontinuity" in your FMC's flight plan.

 

John

I've had that happen a time or two and I noticed that the flight plan in the FMC had not filled in with altitudes. Seems to happen out of Seattle going west more than other airports. Pretty infrequent, though.

I've had that happen a time or two and I noticed that the flight plan in the FMC had not filled in with altitudes. Seems to happen out of Seattle going west more than other airports. Pretty infrequent, though.

I have the same problem on an infrequent basis with no altitudes in the legs pages and VNAV will not arm or engage on the ground or in the air. The solution for me has been to select a runway and STAR at the destination and execute. Then VNAV works fine.

 

This started with routes of KIAD to KTPA and KMIA. These routes were copied and pasted from Flightaware into PFPX. The route was built and exported to the NGX's FMC.

You always have to enter a rwy for departure and arrival, preferably with SID and STAR and no DISCO's.

 

John

You always have to enter a rwy for departure and arrival, preferably with SID and STAR and no DISCO's.

 

John

Lots of STARs have DISCOs, so you can have them.

You always have to enter a rwy for departure and arrival, preferably with SID and STAR and no DISCO's.

 

John

If you are correct how was I able to use this route from Flightaware for a TNCM-KMIA flight? Route from Flightaware copied into PFPX and exported to NGX FMC was B520 DDP A555 HARDE Y587 COZIE FLIPR FLIPR3. I added runway 10 and the BOPAT3 SID for TNCM and cleared the disco between JUICE transition and DDPNB. No runway was entered for KMIA and the FMC dropped the FLIPR3 STAR. The route and legs terminate at the fix FLIPR. I have altitudes in FMC legs pages and VNAV was armed on the ground and engaged in the air.