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 on approach the flight crew noted that all Stab Trim was unavailable resulting in high column forces, and lateral control that was described as sluggish. Upon landing, the speedbrakes only partially deployed, reverser thrust was not available and all four 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 forces all standby power buses to receive power from the battery and disables the battery charger. We are currently evaluating the existing language in the QRH, and whether other changes might be needed as well.
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 that airplane as well.
This incident is being investigated by the NTSB (ID CHI08IA292) and Boeing is assisting with the investigation. Boeing is evaluating possible corrective actions for the loss of all standby power under the conditions experienced by this airplane. 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 Service Bulletin 757-24-0135 is scheduled for release in October 2010 to define a relay addition and wiring to enable power to the battery charger in the air with the Standby Power Switch in the BAT positon.
Boeing Fleet Team Digest 24, 14th December 2008