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When navigating in the polar regions, magnetic heading should be considered unreliable or totally useless for navigation. Magnetic variations typically are extreme, often are not constant at the same point and change rapidly as airplane position changes. Ensure the computer flight plan shows true tracks and true headings. Grid headings may also be used as a reference for those airplanes equipped with grid heading indicators although no airplane systems use grid heading. For some high latitude airports, grid headings are shown on the instrument approach procedures. Note that unmapped areas in the GPWS terrain data base display as magenta dots on the map, regardless of the airplane altitude.
The primary roll mode for polar operations should be LNAV, which may be used with the heading reference switch in the NORM position. HDG SEL/HOLD are functional but require the manual selection of TRUE heading reference. Deviations from planned route may be accomplished in HDG SEL.
Note: Do not use HDG SEL or ROLL CWS north of 89 degrees 30 minutes North
latitude or south of 89 degrees 30 minutes South latitude, due to rapid
heading and track changes occurring near either pole. At latitudes north of
N85 or south of S85, the true heading on the RMI may differ from the True
Heading displayed on the MAP, due to position differences between IRU-L
and FMC.
Loss of both GPS units results in an increased ANP and possible display of the UNABLE REQD NAV PERF-RNP message but normally does not prevent continuing the polar flight.
If neither IRU is operating in the NAV mode, the airplane heading/track on the PFD and map is invalid. The GPS-L True Track on CDU PROGRESS 3/3 page provides a source of airplane track, which can be used as a secondary reference to update the heading for any IRS in ATT mode.

Descent Descent Speed Determination
The default FMC descent speed schedule is an economy (ECON) descent from cruise altitude to the airport speed transition altitude followed by a descent at ten knots less than this speed. The speed schedule is adjusted to accommodate waypoint speed/altitude constraints displayed on the LEGS pages, and speed/altitude constraints displayed on the DES page. If desired, the ECON speed schedule can be modified by alternate Mach, Mach/IAS, or IAS values on the DES page target speed line. If the FMC information is not available, use target speeds from the Descent Rates table in this chapter.

Descent Path
An FMC path descent is the most economical descent method. At least one waypoint-related altitude constraint below cruise altitude on a LEGS page generates a descent guidance path. The path is built from the lowest constraint upward, assuming idle thrust, or approach idle below the anti-ice altitude entered on the DESCENT FORECAST page.
The path is based on the descent speed schedule, any entered speed/altitude constraints or forecast use of anti-ice. The path reflects descent wind values entered on the DESCENT FORECAST page.

Descent Constraints
Descent constraints may be automatically entered in the route when selecting an

arrival procedure, or manually entered through the CDU.
Set all mandatory altitude restrictions and “at or above” constraints in the Mode
Control Panel (MCP) altitude window. The next altitude may be set when the
restriction has been assured or further clearance has been received.

Shallow vertical path segments may result in the autothrottle supplying partial
power to maintain the target speed. Vertical path segments steeper than an idle
descent may require the use of speedbrakes for speed control. Deceleration
requirements below cruise altitude (such as at 10,000 MSL) are accomplished
based on a rate of descent of approximately 500 fpm. When a deceleration is
required at top of descent, it is performed in level flight.

Speed Intervention (As installed)
VNAV speed intervention can be used to respond to ATC speed change requirements. VNAV SPD pitch mode responds to speed intervention by changing airplane pitch while the thrust remains at idle. VNAV PTH pitch mode may require the use of speedbrakes or increased thrust to maintain the desired airspeed.
Copyright . The Boeing Company. See title page for details.
4.18 FCT 737 (TM) October 31, 2006

Descent Preparation Using HUD System 737-600 - 737-900
If the combiner was previously stowed, the combiner should be positioned and the pilot should verify that it is properly aligned with the overhead unit. For night landings, set combiner brightness high enough to ensure that the symbology is visible over bright touchdown zone lights.

Descent Planning
Flight deck workload typically increases as the airplane descends into the terminal area. Distractions must be minimized and administrative and nonessential duties completed before descent or postponed until after landing. Perform essential duties early in the descent so more time is available during the critical approach and landing phases.
　
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