View Full Version : HDG & TRU and Autopilot Behaviour (B744)

23rd Nov 2009, 05:52
I've noticed that if you manually select TRU (on the P2 panel) on the ground, that the ND compass and HDG bug rotate (where there is magnetic variation).
If you happened to have the A/P engaged in HDG SEL and did this, I assume the aircraft would fly to the HDG bug. If you had HDG HOLD selected, would the aircraft turn, or fly straight on?
Does the A/P memorise a magnetic heading value during HDG HOLD? (and would make the same turn as HDG SEL upon selection of TRU?)


23rd Nov 2009, 06:21
If in HDG SEL the AFDS changes to HDG HOLD when the heading ref switch is changed, you can then re select HDG SEL.

23rd Nov 2009, 08:58
Excellent. I wasn't able to find anything in our books on this.



23rd Nov 2009, 11:21
the ND compass and HDG bug rotate - how antiquated! Any explanation for the 'and'? Did Boeing just do a 'cheapo' by not allowing a variation compensation of the HDG bug? What happens if you do it in LNAV?

23rd Nov 2009, 12:25
Nothing; the aircraft continues on the current active leg.

The figures describing the desired track, change to true and have 'T' annotated against them in the FMC.

All you need to do is to re-align the heading bug against the lubber-line (if that's what you like to do)

The only confusion can arise if you are displaying VOR needles/course bars, as they will point in the wrong direction, i.e. they will be 'stuck' to the magnetic bearing and therefore will move with the compass card. ADF needles will stay on on the correct relative bearing (i.e. they will not move) but will now indicate a true bearing on the RMI.


24th Nov 2009, 05:03
Any explanation for the 'and'?

I just meant to highlight the fact that the heading bug still corresponds to the magnetic value (as previously selected on the Autopilot Mode Control Panel).

I'm not particularly familiar with polar navigation (other than the effects high/low latitudes have on ND compass orientation and Map background). We have the Track Up Map display option and, as such, our pilots have to keep adjusting the hdg bug to match the track flown (at least it stops the crew alertness monitor going off :} ).