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Refer to figure.
In flight, the IRS detects a critical fault with excessive position error. Regarding the mode selector switch, what should the crew select?

  • A

    OFF than NAV, to realign the system.

  • B

    NAV and monitor the position error.

  • C

    ATT, to retain attitude information.

  • D
    OFF, since the system is unreliable.

After the alignment phase is completed, the INS will be switched from ALIGN to NAV mode, which is the normal operating mode allowing the use of all functions of the system. In NAV mode, the gyro outputs are transmitted to the INS computer that will send corrective indications to the motors to keep the platform level. The accelerometer outputs will be transmitted to the integrators to obtain the speed and distance. If for some reason the NAV mode becomes inoperative (if it loses its alignment reference during flight), the system can still be used in the ATT mode by switching the selector on the mode selector unit of the INS accordingly. In ATT mode (attitude only), the platform will supply only attitude and heading data.

The ATT selection will ensure that the IRS continues to supply pitch, roll, yaw and heading information to the servo driven instruments or EFIS.

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