Boeing Bulletins

GEN-18 : Uncommanded Auxiliary Power Unit (APU) Shutdown Prior to Engine Start

22 November 2006

Effectivity

Please refer to source document.

Reason

To inform flight crews of an uncommanded APU shutdown fault prior to engine start and provide temporary operating instructions.

Information in this bulletin is recommended by The Boeing Company, but may not be FAA approved at the time of writing. In the event of conflict with the FAA approved Airplane Flight Manual (AFM), the AFM shall supersede. The Boeing Company regards the information or procedures described herein as having a direct or indirect bearing on the safe operation of this model airplane.

Background Information

Boeing has received operator reports of a fault pertaining to an uncommanded Auxiliary Power Unit (APU) shutdown on the ground prior to engine start. Boeing has confirmed the shutdown fault occurs randomly and is a result of pneumatic air reverse flow when either or both Pack Control Selectors are positioned OFF. The uncommanded APU shutdown occurs when the APU Electronic Control Unit (ECU) is unable to compensate for the pneumatic pressure pulse when either or both pack control valve(s) close. Boeing has confirmed the shutdown fault may occur under the following conditions:

Both engines shutdown;
APU running;
APU Bleed Air switch ON; and,
A Pack Control selector positioned OFF.

Therefore, flight crew recognition of this fault is primarily noticeable when accomplishing the normal "Before Engine Start" procedure after positioning the Pack Control Selectors OFF in preparation for the initial engine start. If the fault is active and an uncommanded APU shutdown occurs, the APU can be restarted without delay without any required time consideration for cool down.

Boeing has confirmed the fault can only occur upon positioning either or both Pack Control Selectors OFF with the APU as the sole source of pneumatic power. Therefore, exposure to the fault described above is isolated to ground operations and specific to APU bleed air supply for initial engine start.

Operating Instructions

For operators experiencing in-service interruption due to uncommanded APU shutdown prior to engine start, the following temporary operating instructions are recommended:

Prior to positioning the Pack Control Selectors OFF when accomplishing the "Before Start Procedure", select the APU Bleed Air switch OFF and allow the APU Bleed Air VALVE transition light to momentarily illuminate and extinguish. Then, position the left and right Pack Control Selectors OFF and allow the PACK OFF lights to illuminate. Finally, select the APU Bleed Air switch ON to restore pneumatic air to each engine’s starter, and accomplish a normal engine start.

Administrative Information (Extract)

This Flight Crew Operations Manual Bulletin will be revised to include service bulletin information when available.