To provide a revised "STANDBY BUS OFF" procedure (Chapter NNC - Section 6) for the Quick Reference Handbook (QRH).
An electrical system failure event was recently confirmed by Boeing, the National Transportation Safety Board (NTSB), and the Federal Aviation Administration (FAA) regarding the partial loss of the DC-powered electrical bus system on a 757 airplane. The information provided by this Flight Crew Operations Manual Bulletin (FCOMB) is related to FAA Safety Alert Flight Operations (SAFO) 09001, Subject: Effects of Electrical Faults Resulting in Main Battery Depletion, Dated: January 13, 2009.
The failure event resulted in the loss of all equipment solely powered by the main airplane battery even though power to both AC buses was available. Boeing and the FAA have determined interim corrective action is needed for all 757 airplanes to mitigate the occurrence of this specific failure event.
During normal operation with the Standby Power selector in the AUTO position, the AC buses supply power to the battery and standby DC buses through a transformer-rectifier unit and to the standby AC bus. The main battery charger, powered by the ground service bus, provides DC power to the hot battery bus. If the Standby Power selector is in the BAT position, the battery charger is disabled and the main battery becomes the sole source of electrical power to all equipment powered from the hot battery bus, the battery bus, the standby DC bus, and the standby AC bus. On some airplanes, the APU battery is paralleled to the main battery, which provides increased time these buses can be powered with the Standby Power selector in the BAT position.
The Quick Reference Handbook (QRH) replacement pages provided by this FCOMB provide either one or two revised STANDBY BUS OFF checklists, depending upon the operator's fleet configuration of the alternate stabilizer trim control system. This STANDBY BUS OFF checklist(s) contains the recommend flight crew guidance in the event the STANDBY BUS OFF advisory-level alert message is shown on EICAS.
Boeing will be issuing Service Bulletin 757-24-0132 during or before 3rd-quarter 2010. This service bulletin will be applicable to all 757 variant airplanes and will provide an FAA-approved retrofit kit of parts to bypass the battery-charger cutout with the Standby Power selector in the BAT position during flight.
The QRH replacement pages provided by this FCOMB are valid for use on airplane configurations both unmodified and modified by this service bulletin in a mixed-fleet configuration. When an operator reports to Boeing that all 757 airplanes have been modified by Service Bulletin 757-24-0132, a final revised STANDBY BUS OFF checklist will be issued to the operator's Boeing-published QRH that is compatible to a pure-fleet, SB-modified 757 configuration. This eliminates FAA and Boeing concerns of multiple STANDBY BUS OFF checklists being contained within an operators QRH, or operator equivalent, and the flight crew potential of erroneously accomplishing a STANDBY BUS OFF checklist not compatible with the airplane wiring configuration.
IMPORTANT: All operators are reminded to properly report all Service Bulletin incorporation to Boeing in a timely manner. This will ensure proper manual pages are provided commensurate with the reported airplane fleet configuration.
The effects of this Flight Crew Operations Manual Bulletin (FCOMB) are further modified by pure-fleet incorporation within an operator's fleet of Boeing Service Bulletin 757-24-0132. This Flight Crew Operations Manual Bulletin will be "CANCELLED" (CANC) upon operator notification to Boeing that all affected airplanes covered in this Flight Crew Operations Manual have been modified by Boeing Service Bulletin 757-24-0132.