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IRU

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Inertial Reference Unit

The inertial reference system is comprised of the IRU and the Mode Select Unit, and computes aircraft position, ground speed, and attitude data for:

It consists of a self-contained system once fully initialised. It allows the aircraft to continue navigating even if all other sources of path information are lost.

There are two IRS installed on the aircraft, labelled Left IRS and Right IRS.

The IRU itself provides all information required for avionics:

  • Pitch and roll
  • Heading
  • Acceleration
  • Wind data
  • Calculated data, including:
    • Flight path angle
    • Roll/Pitch rates
    • Vertical speed
    • Yaw data

Its primary sources for data are 3 internal laser gyros and 3 inertial accelerometers.

Thus the only external inputs required for the IRS are:

  • Initial Position
  • Barometric altitude to stabilize vertical navigation
  • True Airspeed to calculate wind speed and direction

Alignment process

Normal alignment is initiated by placing the IRS Mode Selector switch to NAV. After a short power test, the ALIGN light illuminates. It must be fully aligned prior to entering navigation mode.

Note that ADIRU alignment must not be attempted at latitudes greater than 78 degrees 15 minutes.

To initalize position, use the CDU via the POS INIT page. IRS alignment can be autofilled from GPS data, or by manually entering coordinates. Alignment times varies from 5 to 17 minutes, depending on position. Between stops, it is possible to achieve a 30-second position update by placeing the mode select switch on ALIGN and manually entering the GPS coordinates on teh IRS Display panel.

Full alignment is completed once the ALIGN lights extinguish and the PFD and Navigation Display provide all appropriate information with no flags.

ATT mode is used to level the system to provide attitude andd possibly heading information in case of a navigation failure.

Power sources

Each IRU has two power sources: a 115V AC and a 28V DC. Although each is sufficient for operation, the 28V DC power is a back-up source.

The left IRU is powered by the AC Standby Bus, while the right IRU is powered by the AC Transfer Bus 2.

Either or both systems can be switched to the Hot Battery Bus. When this occurs, however, the right IRU will only remain active for 5 minutes.

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