True north - mag north -switch in a 737NG

[choffmann]

Could any of the professional pilots here give me a clue, where exactly on the 737NG overhead panel I can find the switch to change between magnetic and true north? (Remark: I do know, this is not found in most 737NGs, but it´s an option for customers who fly the keyhole area or polar routes - although polar routes are not "usually" flown in a 737).
One more question: Is a third FMC obligatory or on the MEL for polar routes or routes in MNPS airspace?
A picture would be excellent!
Thx in advance!

Chris

[Callsign Kilo]

Taken from the 737NG FCOM - Opeartional Limitations. Aircraft operational in areas of magnetic variation. No option to switch to degrees true.


Maximum flight operating latitude – 82° North and 82° South, except for the region between 80° West and 130 ° West longitude, the maximum flight operating latitude is 70° North, and the region between 120° East and 160° East longitude, the maximum flight operating latitude is 60° South.


MNPS requires Dual FMC and HF comms fitted under our regulations. I can't speak for flight within Polar Airspace, however I'd imagine requirements are the same. :ok:

[choffmann]

Thank you, callsign kilo.

In Boeing´s Aeromagazine No.16 it says: ..."
Polar navigation option.
These latitude limitations
on flight operation do not
apply to 747-400, 777, and
737-600/-700/-800/-900
airplanes equipped with
the polar navigation option
(heading reference switch
and FMC Update U10.3 or
later) and dispatched with
the following equipment
operational: CDU, left
GPS, both IRUs in navigation
(NAV) mode, and both display
electronic units.
Flight crews of such equipped airplanes
should not use HDG SEL or ROLL CWS
north of 89 deg 30 min north latitude
or south of 89 deg 30 min south latitude.
They also should not use the heads-up
display system, if installed, when the
heading reference switch is in TRUE.

So, any comment would be appreciated.

Thanks again.

Chris

[Callsign Kilo]

For the B737NG I cannot comment on the Polar Nav option as we do not operate the aircraft within these regions. Hence the published limitation pertaining to latitude.

On the B744 there is a heading reference switch within the proximity of the Nav DUs for 'Norm' and 'True' selection. I believe the FMS updates position depending on its selection. When in magnetic, position is based on place, bearing, distance from two radio aids, IRS position and GPS. Above 82N and below 82S split IRS updating occurs with GPS comparison. Radio aid updating is inhibited. I believe this function is automatic at these latitudes.

In response to the Boeing article, for polar ops, GPS and two independent IRUs are needed for position updating and comparison. Single DEU operation is non dispatchable for any operation and when airborne precludes a host of functions. I believe Boeing aircraft have now reached version 10.8 or 10.9 for the FMS update. I suppose the version which you install is dependent on your operation.

[Denti]

As far as i know the last certified version is 10.8A. 10.8 was pulled after just a couple weeks of usage since it contained some very serious bugs, like flying non precision approaches 200ft below the intendet flight path. Can't comment on the polar option, we never bought it as we do not fly in those regions. For MNPS 2 HF and two FMC are enough.