L Nav does not initially engage

[Kerrydancer 2]

Hi Guys,

 

I've been using the 747 400X Queen of the Skies for some time now and have a problem to which I can't find the answer. 

 

On most take-offs, everything works out well and falls into place, but every so often L Nav fails to engage, even when the aircraft is heading directly for the initial way point.  I try disconnecting and re connecting the autopilot, but this has no effect.  I try taking the plane off heading, then back to the correct heading, but again this has no effect.

 

Then, for seemingly no reason at all, and after 10 or 15 attempts, it engages and my flight continues.

 

Has anyone had similar problems and has found an answer to this.  When it happens, I can see no reason why it does not engage as normally it will do so within several degrees of the desired way point heading.  

 

Ron

[pschlute 37]

LNAV will engage above 50 feet, and within 2.5 miles of the planned track.

 

If on an intercept course to the track it will arm. If not on an intercept course the FMC will warn you in the scratchpad.

 

Heading directily to your first waypoint is not enough to engage LNAV if the above are not satisfied.

 

btw... how are you navigating from runway to the first waypoint ?   Are you using a SID ?

[Kerrydancer 2]

pschlute,

 

Thanks for your response.

I'm just coming to grips with SIDs and STARs, so I don't always use them.

The last time this happened, I was certainly above 50 ft., was on an intercept course and within 2.5 miles of the track as the waypoint was directly on the departure runway heading.  So I'm sure all requirements were met.  However I will bear these in mind when engaging L Nav again and see what happens.

Thanks anyway.

 

Ron