True Heading

[fujiwaranaoya]

I have a question on true heading.

I was flying northern Russia around 75-77North, East side.

Navigation heading was MAG.  It is correct.

But headings in legs pages were TRU and GRID heading was shown on upper right corner of ND.

Is that correct?

Regards,

[Hardy Heinlin]

Hello,

yes, all correct.

See page 289 for TRU and NORM references.

See page 302 for the GRID display.


Regards,

|-|ardy

[fujiwaranaoya]

Thank you. I will check.

Regards,

[fujiwaranaoya]

Dear Hardy,

I understand GRID display trigger.  But still, FMC legs page should be in MAG mode outside the TRU range, isn't it?

Regards,

[Hardy Heinlin]

Hello,

no, it's all correct. Here's the text from page 289:

==================
NORM:

Heading and track reference in FMS and active CDU leg is magnetic north for locations between 73°N and 60°S, otherwise true north.
===================


Note: "active CDU leg" refers to standby navigation using the CDU's in-built electronics which can be used when the FMS fails. See pages 333 and 334.


Regards,

|-|ardy

[Hardy Heinlin]

Hello,

no, it's all correct. Here's the text from page 289:

==================
NORM:

Heading and track reference in FMS and active CDU leg is magnetic north for locations between 73°N and 60°S, otherwise true north.

Heading and track reference for PFDs, NDs, and AFDS is magnetic north when the aircraft is outside the orange areas shown on the figures to the left; inside these areas it is true north.
===================


Note: "active CDU leg" refers to standby navigation using the CDU's in-built electronics which can be used when the FMS fails. See pages 333 and 334.


Regards,

|-|ardy

[fujiwaranaoya]

Dear Hardy,

Now I understood.  I was not sure what was "active CDU leg" exactly.  But now OK.

I had been long assuming that heading in legs page and actual heading are always the same when LNAV is active.

Thank you.

Regards,

[Hardy Heinlin]

The "active CDU leg" was not your problem. I'm sure you were navigating with the FMS, not with the CDU's standby navigation system.

In NORM mode, any leg in the FMS (not just the active leg) refers to MAG if the leg is between 73°N and 60°S, otherwise to TRU.

In NORM mode, when the FMS has failed, the active leg in the CDU's standby navigation system refers to MAG if the leg is between 73°N and 60°S. All other CDU legs always refer to TRU.


Background:

There are 3 CDUs on the flight deck.

There are 2 FMCs outside the flight deck in the belly of the aircraft.

When the FMCs are operative, 2 of the CDUs can be used as a "remote control" to enter data into the FMCs, and to see what the FMCs compute.

When the FMCs fail, the CDUs can no longer be used as a "remote control" to operate the FMCs. The FMCs are then inoperative: there is no VNAV anymore, no LNAV, no autothrottle, no thrust limit reference etc. But all 3 CDUs can then be used for standby navigation. Each CDU has an in-built standby navigation system; this is a very simplified system, much simpler than the FMC system.

To see on the captain's ND what the CDU's standby navigation system generates, move the captain's NAV source selector (left of the captain's clock) to CDU L or CDU C.


Regards,

|-|ardy

[fujiwaranaoya]

Thank you for your kind explanations.

Now whole things are much clearer on standby navigation system over 73N.  I will fly with standby navigation system on the same route again.

Regards,

[Hardy Heinlin]

I will fly with standby navigation system on the same route again.

Why do you want to do this? :-)

To test the standby navigation?

Or to solve a "problem"?

(There was no problem. You just "thought" there was a problem :-) But there was no problem.)


Regards,

|-|ardy

[fujiwaranaoya]

Dear Hardy,

Do not worry.  My problem is clear with your kind explanations.  And now I am interested in standby navigation system.  I have never tried it.  Flying long distance without radio navigation station by just looking at true heading numbers on CDU is now curious to me.  Just it.

Thank you again.

Regards,