Operation of 757 and 767 airplanes should be limited to the region
between 87 deg north and 87 deg south latitude, as stated in the
airplane flight manuals.
The primary roll mode
for polar operations should be LNAV, which may be used with the
heading reference switch in the NORM position. The HDG SEL or HOLD
mode is functional but requires the manual selection of TRUE heading
reference. Deviations from the planned route may be accomplished
in HDG SEL mode.
Rapid heading and track
changes occur as an airplane nears either pole. If operating in
HDG SEL or HOLD mode while near a pole, the flight crew will need
to update the heading selector frequently to reflect the rapidly
changing heading. Otherwise, the AFDS will command an unwanted turn.
When GPS updating is
available, the FMC position is updated to 87 deg north latitude
and 87 deg south latitude, which is the airplane certification limit.
Loss of both GPS units results in an increased ANP and possible
display of the UNABLE RNP message, but this normally does not prevent
Because of the convergence
of longitude and the latitude singularity at the poles, each product
improvement program FMC reverts to selection of a single IRS for
navigation with no updating before crossing the pole. This allows
the FMC position and velocity computations to work properly. Crossing
84 deg latitude flying toward the pole, the FMC reverts from triple
IRS navigation to a single IRS. All position and velocity corrections
gradually are phased out until the FMC navigation parameters equal
the selected IRU position and velocity. When crossing 83.5 deg latitude
flying away from the pole, the FMC reverts from single IRS navigation
to triple IRS navigation.
Although the Pegasus
FMC has not been certified for polar operations, it is technically
operational in the polar region with the IRS or ADIRU, or both.
For Pegasus FMC installations, the navigation function is computed
in XYZ coordinates instead of latitude and longitude. Should both
Pegasus FMCs fail, alternate navigation is available through the
alternate navigation pages on the CDU to continue the flight to
For GPS-equipped airplanes,
the loss of one or two IRUs does not significantly affect navigation
accuracy. Operation on one IRU should be limited to diversion to
the nearest suitable airport.
The PFD heading arc normally is referenced to magnetic heading.
The ND heading scale may be referenced to either TRU or MAG, as
selected on the flight control panel using the TRU/MAG switch. As
the airplane flies to latitudes greater than 73 deg north or 60
deg south, the magnetic variation of the IRU is set to 0 deg. At
this time, the PFD and ND heading scales automatically switch to
TRU displays; magnetic selection is no longer available.
The flight management
system (FMS) is considered to be in the polar region when the airplane
is above 85 deg north or south latitude. The ND replaces the HDG
(white) and TRU (cyan) legends on the heading scales with FMS TRK
TRU (magenta). The heading scale is then driven by FMS track information
to eliminate the effect of heading splits that occur between displays
when near the pole.
in the FMS evaluate the position and velocity information from each
IRU. Should miscompare occur, each FMS will select a
different IRU using a hierarchy keyed to the FMS installed position
to ensure that both FMSs do not use the same IRU source. The triple-mix
mode is available until a miscompare occurs.
The primary autopilot
coupled mode near the pole is NAV. The heading mode from the autoflight
system is available as long as the differences in heading among
IRUs are fewer than 4 deg. Heading splits appear because of position
differences as the meridians converge at the pole. Using the FMS
NAV mode eliminates the effects of such splits and allows continuous
coupled autoflight operation.
When the airplane is
within 2 nmi of the pole, the FMS is programmed to enter a wings-level
mode, which is necessary because the longitude slews rapidly during
the pole transition. As the airplane crosses the pole, the map display
will show the airplane symbol track backward as the longitude slews
to the new value. At 10 nmi from the pole, the FMS resumes flying
the flight plan track.
Note: Airlines always
should consult the operations manual for the latest navigation and
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