Flight Management Navigation -
Inertial Reference System

The Inertial Reference System (IRS) calculates airplane position, acceleration, track, vertical speed, ground speed, true and magnetic heading, wind speed and direction. It also supplies attitude data for the displays, flight management system, flight controls, engine controls and other systems.

• 757-200. The IRS consists of three Inertial Reference Units (IRUs) and the IRS Mode Selector Panel.
• 757-300. The IRS consists of three Air Data Inertial Reference Units (ADIRUs) and the IRS Mode Selector Panel.

Air Data Inertial Reference Unit (-300)

The Air Data Inertial Reference Units (ADIRUs) combine the function of an Air Data Computer with an Inertial Reference Unit. The Air Data and IRS functions of the ADIRUs are controlled separately and are described as separate systems.

Inertial Reference System - Operation

The Inertial Reference System is controlled by the IRS Mode Control Panel on the overhead panel. When operating in the navigation mode, the IRS provides attitude, acceleration, ground speed, track, true and magnetic heading, present latitude and longitude, and wind speed and direction to other systems. In normal operation, the FMC is used to input data to the IRS and to monitor system operation.

Magnetic heading and track are not available in polar regions. Magnetic reference is provided between N73° and S60° latitude. Above these latitudes only true headings are available.

IRS Alignment

An IRS must be aligned before it can enter the NAV mode. Rotating the IRS mode selector from OFF to NAV begins the IRS alignment. The IRS performs a short power test, during which the ON DC light illuminates. When the ON DC light extinguishes the ALIGN light illuminates. Alignment requires approximately ten minutes.

Present position (latitude and longitude) must be entered on the CDU Position Initialisation page to complete the alignment. If the present position cannot be entered through the CDU, it may be entered through the IRS mode selector keyboard.

If the latitude/longitude position is not near 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.

Alignment can be accomplished only when the airplane is parked. Alignment stops if an IRU detects motion during alignment. When the motion stops, some units restart the alignment automatically. Other units flash the ALIGN lights until the alignment is manually restarted.

Manual restarts are accomplished by moving the IRS Mode Selector to OFF. Then, when the ALIGN Light extinguishes, reselect NAV.

The IRS is aligned when all IRUs enter the navigation mode. The latitude and longitude display on the SET IRS POS line of the CDU POS INIT page then blanks. Alignment is lost if the selector is moved out of the NAV position.

High Latitude Alignment

High latitude (between 70°12.0' and 78°15.0') alignments requires an extended alignment time. This extended alignment is accomplished by rotating the Mode Selector from OFF to the ALIGN position and allowing the IRS to align for a minimum of 17 minutes. Present position is entered while in the align mode. After the extended alignment, navigation mode is entered by rotating the mode selector to the NAV position.

Fast Alignment

Following operation in the navigation mode and with the airplane parked, performing a fast alignment removes accumulated track, ground speed, and attitude errors, levels the system, and updates present position. This is accomplished by positioning selectors to ALIGN, entering present position, and repositioning selectors to NAV. Fast alignment completes in approximately 30 seconds.

Fast alignment can be accomplished without entering present position. However, greater navigational accuracy is attained by entering present position.

A full alignment must be accomplished when the time from the last full alignment to the to the completion of the next flight exceeds 18 hours.

IRS Attitude

If an IRS loses both AC and DC power, alignment is lost. Alignment can also be lost if the selector is moved out of the NAV position.

If alignment is lost in flight, the navigation mode is inoperative for the remainder of the flight. Attitude information can be obtained by moving the selector to ATT. The IRU enters the Align mode for 30 seconds during which the airplane should stay in straight and level flight. This re-levels the system and provides attitude displays on the ADI. Some attitude errors may occur during acceleration. After acceleration, errors are slowly removed.

Heading information can be provided in the ATT Mode if a heading entry is made on the CDU POS INIT page or IRS mode selector panel. Magnetic heading must be updated periodically.

IRS Power

Normally the IRSs operate on AC power from the left and right electrical systems. The main airplane battery is used as an alternate power source. The ON DC light illuminates and the EICAS message IRS ON DC is displayed when AC power is lost and DC power is being used. The DC FAIL light illuminates and the EICAS message IRS DC FAIL is displayed when DC power is lost and AC power is being used. Both lights extinguish if both AC and DC power are on or off.

If all AC power sources are lost the IRSs are powered by the Standby Power System.

As Installed

• ETOPs:
  (Hydraulic Driven Generator): Standby power to the right IRS is limited to 5 minutes to save battery power.

• Non-ETOPs:
  Standby power to the left and centre IRS is limited to 5 minutes to save battery power.

Ground Crew Call Horn

Radio Navigation System