Worthless Trivia

Polar Navigation Function

A feature on jet aircraft

flight management computer (FMC) position is north of 84 deg north latitude (or north of 83.5 deg north latitude after having been north of 84 deg north latitude).

When the computed position enters the region north of 84 deg north latitude or the region south of 84 deg south latitude, each FMC shifts down from the triple-mix inertial reference system (IRS) position to a single inertial reference unit (IRU) position. The SPLIT IRS OPERATION message is displayed on the control display unit (CDU) scratchpad message area. This reversion to the single IRS position is necessary because longitude and latitude singularity converge at the poles. The UNABLE RNP message is inhibited during the down mode to a single IRU position. Each FMC selects the corresponding valid IRU position. For the left FMC, the order of selection is left, center, and right. For the right FMC, the order is right, center, and left. Each IRU position is compared with the other two. The FMC then chooses the two IRU positions that are the closest together. The FMC navigation function gradually changes the computed FMC position from the triple-mix position to the single IRU position to prevent sudden position jumps. Ultimately, the FMC position is equal to the single IRU position. When the computed airplane position of both FMCs is between 83.5 deg north latitude and 83.5 deg south latitude, the FMC returns to triple-mix position updating. The FMC gradually shifts from the single IRS mode to the triple-mix mode. If there is a detected IRS failure while the FMC position is greater than 89 deg north or south latitude, the FMC position immediately becomes the IRS position. The primary roll mode for polar operations should be lateral navigation (LNAV), which may be used with the heading reference switch in the NORM position. Manual selection of a magnetic or true heading reference is accomplished by using the HDG REF TRUE/NORM switch. When the airplane is operating in a region where the IRS does not compute magnetic heading, the reference is automatically changed to true, independent of the position of the HDG REF TRUE/NORM switch. When the option to extend magnetic variation is selected, the region comprises the area north of 82 deg north latitude (or north of 70 deg north latitude between 80 deg and 130 deg west longitudes) or south of 82 deg south latitude (or south of 60 deg south latitude between 120 deg and 160 deg east longitudes). When the option to extend magnetic variation is not selected, the region comprises the area north of 73 deg north latitude and south of 60 deg south latitude. Upon leaving this region, the heading reference again is determined by the position of the HDG TRUE/NORM switch. When operating in the true reference mode, bearing information entered by the flight crew is assumed to be a true bearing reference. When a North Pole (N90EXXXXX or N90WXXXXX) or South Pole (S90EXXXXX or S90WXXXXX) waypoint is used near the poles, a rapid heading and track reversal occurs as the airplane passes over the waypoint. If the airplane is operating in HDG SEL or HOLD mode while near either pole, the flight crew will need to rapidly update the heading selector to reflect the changing or reversed heading. Otherwise, the autopilot flight director system (AFDS) will command an unwanted turn. For autopilot operation in the polar region using a roll mode other than LNAV, the TRUE position on the heading reference switch should be selected. However, LNAV is the preferred roll mode.

When no global positioning system (GPS) updating occurs, all position and velocity corrections gradually are phased out until the FMC navigation parameters equal the selected IRU position and velocity. When GPS updating is available, it is no longer used when crossing 88.5 deg latitude flying toward a pole, and all position and velocity corrections are phased out gradually before the pole is crossed. When crossing 88 deg latitude flying away from the pole, GPS updating is enabled. When crossing 83.5 deg latitude flying away from the pole, the FMC reverts from single IRS navigation to triple IRS navigation, and the UNABLE RNP mode is operational. The heading display on the primary flight display and navigational display (PFD/ND) and that on the radio magnetic indicator (RMI) may differ within approximately 30 nmi of the pole. This results from differences among IRU positions selected by the FMCs for the PFD/ND and the fixed IRU position on the RMI. Loss of one or two IRUs will not significantly affect navigation accuracy. Operation on one remaining IRU should be limited to diversion to the nearest suitable airport. Navigation can be accomplished after losing both FMCs by using the alternate navigation pages on the CDU.

From Wikipedia, the free encyclopedia

Points in the polar coordinate system with pole O and polar axis L. In green, the point with radial coordinate 3 and angular coordinate 60 degrees or (3,60°). In blue, the point (4,210°).

In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction.

The reference point (analogous to the origin of a Cartesian system) is called the pole, and the ray from the pole in the reference direction is the polar axis. The distance from the pole is called the radial coordinate or radius, and the angle is called the angular coordinate, polar angle, or azimuth.^[1]