Arming NAV when HDG mode is presently in effect

[KirkR]

I am trying to understand the root cause of the following:  sometimes during climb after takeoff, when I have HDG mode active, then enter DIR to my first waypoint and EXEC, then hit NAV, the NAV button activates (light illuminates), but the HDG button's lights do not turn off, and the plane continues in HDG mode.  At other times, hitting NAV results in HDG automatically turning off, as expected.

My takeoff sequence is:

- N1 TO (ATS)
- Pos rate, gear up
- flaps up at appropriate speed
- A/P on
- sync down to 7.5deg
- adj. HDG (vectors), and Speed target
- after takeoff checklist
- CDU DIR to first waypoint, EXEC, then hit NAV

This is when the above issue occurs.

Any ideas on what could be the cause?

 

 

[Pils]

Are you on a course to intercept the nav track?

[KirkR]

Yes, I believe so.  Example is depart KDTW 21R, TRMML 3 SID, on runway heading shortly after takeoff following gear//flaps up, press NAV to proceed on TRMML 3 (see below).  But I will watch this more closely.  Possibly I need to initiate the initial turn to intercept crs 175 to WINRS, possibly I blew past it and it was too late for lateral NAV to engage it.  Generally how quickly can NAV be engaged reliably, agl?

[KirkR]

Confirmed that due to the intercept crs to WINRS being so quick after takeoff, I must have been too far on rw hdg for NAV to do it.  Practiced it again and basically have to arm NAV right after A/P in order to lock on it.  Thanks for the feedback.

[Pils]

1 hour ago, KirkR said:

Generally how quickly can NAV be engaged reliably, agl?

0 ft.

[KirkR]

Got me.    

If you know you have a quick turn for a SID after takeoff, do you arm NAV on the ground?  I think that's what you were saying above.

[JJM]

3 hours ago, KirkR said:

Got me.    

If you know you have a quick turn for a SID after takeoff, do you arm NAV on the ground?  I think that's what you were saying above.

Why not hand fly it until you make the turn to 094?

[Pils]

9 hours ago, KirkR said:

Got me.    

If you know you have a quick turn for a SID after takeoff, do you arm NAV on the ground?  I think that's what you were saying above.

I always arm NAV on the ground when flying an RNAV departure, no reason not to really.