B737 APU Switch and Warning Lights

B737 APU Warning Lights
B737 APU SW

Panel Location: Overhead P5 FWD

APU Door (DOOR) Light

  • Illuminated (amber) – APU door is not in the commanded position:
    • APU door is open when APU is shut down (in air or on ground).
    • In flight – Door is not in flight position.
    • On ground – Door is not in ground position.

APU OVERSPEED Light

  • Illuminated (amber)
    • APU RPM limit has been exceeded, resulting in an automatic shutdown.
    • Overspeed shutdown protection feature has failed a self-test during a normal APU shutdown.
    • If the light is illuminated when the APU switch is placed to OFF, the light extinguishes after 5 minutes.
    • The light is disarmed when the APU switch is in the OFF position.

APU FAULT Light

  • Illuminated (amber)
    • A malfunction exists, causing the APU to initiate an automatic shutdown.
    • If the light is illuminated when the APU switch is placed to OFF, the light extinguishes after 5 minutes.
    • The light is disarmed when the APU switch is in the OFF position.

APU LOW OIL PRESSURE Light

  • Illuminated (amber)
    • During start until the APU oil pressure is normal.
    • Oil pressure is low, causing an automatic shutdown (after the start cycle is complete).
    • If the light is illuminated when the APU switch is placed to OFF, the light extinguishes after 5 minutes.
    • The light is disarmed when the APU switch is in the OFF position.

APU Switch

  • OFF – Normal position when APU is not running.
    • Positioning the switch to OFF with the APU running trips the APU generator off the bus(es), if connected, and closes the APU bleed air valve. The APU continues to run for a 60-second cooling period.
    • APU air inlet door automatically closes after shutdown.
  • ON – Normal position when APU is running.
  • START (momentary) – Positioning the APU switch from OFF to START and releasing it to ON initiates an automatic start sequence.