IRS Alignment

An IRS must be aligned and initialized with airplane present position before it can enter the NAV mode. The present position is normally entered through the FMC CDU. If the present position cannot be entered through the FMC CDU, it may be entered through the ISDU keyboard. The airplane must remain stationary during alignment.

Normal alignment, between 70° 12' north and 70° 12' south latitudes is initiated by rotating the MSU switch from OFF to NAV. The IRS performs a short power lest, during which the ON DC light illuminates. When the ON DC light extinguishes and the ALIGN light illuminates, the alignment process begins. Airplane present position should be entered at this time. The IRS will automatically enter the NAV mode after approximately 10 minutes, and the ALIGN light will extinguish.

High latitude alignment, at latitudes between 70° 12' and 78° 15', requires an extended alignment time. Alignment is initiated by rotating the MSU switch from OFF to ALIGN. After 17 minutes the MSU switch must be rotated to the NAV position. The IRS will then immediately enter the NAV mode.

Magnetic variation between 73° north and 60° south is stored in each IRS memory. The data corresponding to the present position arc combined with the true heading to determine magnetic heading. If magnetic variation information is not available, special navigation equipment is required to provide true heading to the EHSIs.

light Management,Navigation –Navigation

Systems Description

If the latitude/longitude position is not within 4 NM of the origin airport, the CDU scratchpad message VERIFY POSITION is displayed. If the entered latitude/longitude position does not pass the IRS internal comparison tests, the scratchpad message ENTER IRS POSITION is displayed.

Flashing ALIGN lights alert the crew of position entry problems. If a new airplane present position entry fails the internal check twice, the ISDU displays a fault, and the IRSs cannot be used for navigation.

When the alignment is complete, the IRSs enter the navigation mode and the airplane can be moved.

During transit or through-flight stops with brief ground times, a thirty second fast realignment and zeroing of ground speed error may be performed by selecting ALIGN while the airplane is parked. Present position should be simultaneously updated by manually entering latitude and longitude prior to selecting NAV.

Note: If the airplane is moved during alignment or fast realignment, the IRSs automatically begin the full alignment process, and shutdown is not necessary.

Дата добавления: 2015-11-16