"LNAV maintains last HDG"

[Hardy Heinlin]

Good evening,

as we know, "LNAV maintains last heading" is a standard phrase used in the manuals for various functions. For example, in the event when executing a MOD RTE that puts the active leg away from the current aircraft position, or when the end of the route is overflown etc., the FMC copies the last heading and LNAV will maintain that copied value.

A question for the astronauts: What if this event occurs while the aircraft is banking in a turn? Will the FMC really copy the current heading during the bank, or will it first command wings-level and then copy that wings-level heading?

The AFDS mode HDG HOLD, for example, first commands wings level, and when the bank angle is less than 3° the AFDS copies the current heading and will from then on use that as a target heading.


Regards,

|-|ardy

[Britjet]

I've no idea, as usual, but I would guess that it reverts to the HDG HOLD logic, using the last HDG that it was trying to actually maintain...
Peter

[Captain_al]

I would guess the same as Peter, if you overfly a discontinuity, end of route, or the end of offset, the jet goes into a HDG HOLD sub-mode. So I would think if you timed it to turn just prior to reaching that point, that it would just roll out and probably hold that heading.

So, I think it would be your second case Hardy, command wings level, then copy that heading. Of course, normally this is not encountered when turning.

But, being an astronaut I am more comfortable with orbital dynamics, SuperDraco Thrusters, and docking rather than HDG HOLD Submodes     : )

[Hardy Heinlin]

The HDG HOLD wings-level logic is now implemented in LNAV as well, in PSX 10.115:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy