POLAR ROUTE NAVIGATION

B777

The 777 primary and preferred roll mode for polar operations is LNAV, which may be used with the heading reference switch in the NORM position. HDG SEL/HOLD and TRK SEL/HOLD are functional but require the manual selection of TRUE for the heading reference. Deviations from the planned route may be accomplished in TRK SEL or HDG SEL mode. When operating the autopilot in the polar region in a roll mode other than LNAV, the TRUE position on the heading reference switch must be selected.

The ND track and magenta lines may exhibit ratcheting when transiting routes in close vicinity of the pole. When operating in the polar region with the ND PLAN mode displayed, the airplane position symbol disappears. This occurs when flying into the polar region.

When a North Pole (NPOLE) or South Pole (99SP, S90EXXXXX, or S90WXXXXX) waypoint is used, a rapid heading and track reversal occurs as the airplane passes over the polar waypoint. If operating in HDG/TRK SEL or HOLD mode while near either pole, the flight crew will need to rapidly update the heading or track selector to reflect the changing or reversed heading or track. Otherwise, the AFDS will command an unwanted turn. LNAV is the preferred roll mode.

Loss of both GPS units results in an increased actual navigation performance (ANP) and possible display of the NAV UNABLE RNP message, but this normally does not prevent polar operation.

The 777 air data inertial reference units (ADIRU) are fault tolerant. Total failure is extremely unlikely because a number of independent failures must occur before all navigation functions are lost. In the unlikely event the ADIRU does fail, the Quick Reference Handbook non-normal checklist provides the crew with a list of items rendered inoperable and necessary crew actions. With at least one GPS operational, the ND is operational and accurately displays the FMC route and airplane track and position. LNAV is inoperative. A heading reference must be entered into the FMC to regain use of the compass rose. Because of the large and rapidly changing magnetic variations in the polar region, it may be more practical to enter the true track as a heading reference while in the polar region. This provides a more intuitive ND and allows tracking of the planned route in HDG SEL mode. True track may be obtained from the computer flight plan or from the ND. Magnetic compass information should be used, if available, to update the heading reference when departing the polar region. In the event of total ADIRU failure, flight crews should plan a raw data instrument landing system approach or a non precision approach.

REASON

Polar route can be the shortest "still air distance" to travel. It is really all about shortest distance. With the caveat that, when air is moving, the shortest distance must take the air movement into account, to build a new shortest track. To save fuel and time, airlines get pretty close to ideal "as the crow flies" shortest route between long

haul cities, subject to adequate emergency landing runways along the way. So, for example, the shortest route from Boston to Beijing is this (assuming still air): http://www.gcmap.com/mapui?P=BOS-PEK They will try to save fuel (=money) and time (=money) by making a journey as short as safely possible. If there are no legal runways on the route, within necessary limits, then they will make a new route that is the shortest legal, and safe route to take.

a nonprecision approach.