Boeing A 757 airplane recently experienced illumination of the Standby Power OFF light. In response, and following the Quick Reference Handbook, the flight Crew selected the Standby Power Switch to the BAT position, and elected to continue the flight. Some time later, a number of system annunciations indicated that the battery was depleted, causing loss of power to the Hot Battery Bus, Battery Bus, and 28V DC Standby Bus and 115V AC Standby Bus. The airplane diverted to an alternate airport, where the flight crew noted that all Stab Trim was unavailable, and that lateral control was also difficult. Upon landing, the inboard tires blew, due to loss of inboard antiskid. The flight crew chose to steer off of the side of the runway to avoid obstacles at the end of the runway. After the airplane stopped, the engines could not be shut down until the standby power switch was returned to the AUTO position and the generators were switched on to restore power to the battery charger and battery, thus powering the fuel shutoff valves.
Boeing analysis confirmed that selecting the Standby Power Switch to the BAT position disables the battery charger, and forces all standby power buses to receive power from the battery. Our analysis also confirmed that flight crews might be unaware of the systems that might be affected by loss of battery power when the airplane is in this configuration.
We have found this to be an airplane Safety concern, and initiated SRP 24-135 to determine the root cause and corrective action(s) for this condition. We have also confirmed that this condition is applicable to the 767, and our investigation will extend to the 767 as well.
In response to illumination of the Standby Bus OFF Light, the 757 QRH directs the flight crew to select the Standby Power Switch to the BAT position. If the Standby Bus OFF light extinguishes, the QRH then indicates that the battery will provide standby bus loads for about 30 minutes. As a part of our solution selection process under the SRP, we evaluated how well this informs the flight crew of possible system impacts when the battery discharges with the Standby Power Switch in the BAT position.
We have elected, for the 757 and 767, to revise the Quick Reference Handbook to direct the flight crew to move the Standby Power switch back to Auto until the airplane is within a short time of landing so that the battery charger will remain powered and battery power will be available throughout the flight. Additionally, we are pursuing a design change to enable battery charger operation during Standby Power operation in flight.
The QRH will be revised as discussed in the STATUS section of this article. Once operators have incorporated the wiring change to ensure battery charger remains powered with the Standby Power Switch in the BAT position, we will recommend they advise Boeing of that incorporation wo that the QRH can be returned to the exisiting wording.
Boeing production wiring change and service bulletin, including installation of a new relay, to enable power to the battery charger in the air with the Standby Power Switch in the BAT positon. We have not yet committed the release date for this change. We expect the design change to be completed in August 2009. All operators will be notified via update of this article when the retrofit release date is committed.
Boeing Fleet Team Digest 24, 14th December 2008