B737 IRS Mode Selector Unit

B737 IRS Mode Selector Unit
P5 AFT OVERHEAD

ALIGN Light

  • Illuminated (white) –
    • Steady – The related IRS is operating in the ALIGN mode, the initial ATT mode, or the shutdown cycle.
    • Flashing – Alignment cannot be completed due to IRS detection of:
      • Significant difference between previous and entered positions or an unreasonable present position entry.
      • No present position entry.
  • Extinguished –
    • IRS not in ALIGN mode.
    • With mode selector in NAV, alignment is complete, and all IRS information is available.
    • With mode selector in ATT, attitude information is available. Heading information is available following entry of initial magnetic heading.

FAULT Light

  • Illuminated (amber) – A system fault affecting the related IRS ATT and/or NAV modes has been detected.

IRS Mode Selector

  • OFF –
    • Alignment is lost.
    • All electrical power is removed from the system after a 30-second shutdown cycle.
  • ALIGN –
    • Rotating the selector from OFF to ALIGN initiates the alignment cycle.
    • Rotating the selector from NAV to ALIGN automatically updates alignment and zeroes ground speed error.
  • NAV (detented position) –
    • System enters the NAV mode after completion of the alignment cycle and entry of present position.
    • In NAV mode, all IRS information is available to airplane systems for normal operations.
  • ATT – Provides only attitude and heading information:
    • Attitude information is invalid (attitude flag in view) until ALIGN light is extinguished.
    • Heading information is invalid (heading flags in view) until the actual magnetic heading is manually entered after the ALIGN light is extinguished.
    • Position and ground speed information is not available until the IRS is aligned on the ground.
    • The selector must be cycled to OFF before reselecting ALIGN or NAV.

ON DC Light

  • Illuminated (amber) –
    • The related IRS is operating on DC power from the switched hot battery bus (AC power not normal).
    • If on the ground, the ground-call horn in the nose wheel well sounds, providing an alert that a battery drain condition exists.
    • Momentary illumination is normal during alignment self-test.

DC FAIL Light

  • Illuminated (amber) –
    • DC power for the related IRS is not normal.
    • If the other lights are extinguished, the IRS is operating normally on AC power.

Landing System Lights

The flight management system controls these landing system lights

Global Positioning System (GPS) Light

  • Illuminated (amber) –
    • indicates failure of both GPS sensor units.
    • indicates failure of a single GPS sensor unit when either system annunciator panel is pushed to initiate a recall.
    • with single GPS sensor failure, light extinguishes when the system recall is reset.

Instrument Landing System (ILS) Light

  • Illuminated (amber) –
    • indicates failure of both ILS sensor units.
    • indicates failure of a single ILS sensor unit when either system annunciator panel is pushed to initiate a recall.
    • with single ILS sensor failure, light extinguishes when the system recall is reset.

GNSS Landing System (GLS) Light

  • Illuminated (amber) –
    • indicates failure of both GLS sensor units.
    • indicates failure of a single GLS sensor unit when either system annunciator panel is pushed to initiate a recall.
    • with single GLS sensor failure, light extinguishes when the system recall is reset.