On 22 September 2008, a Boeing 757-200 being operated by American Airlines on a scheduled passenger flight from Seattle/Tacoma WA to New York JFK lost significant electrical systems functionality en route. A diversion with an emergency declared was made to Chicago O’Hare where after making a visual daylight approach, the aircraft was intentionally steered off the landing runway when the aircraft commander perceived that an overrun would occur. None of the 192 occupants were injured and there was only minor damage to the aircraft landing gear.
An investigation was carried out by the National Transportation Safety Board (USA) (NTSB). FDR and 2 hour Cockpit Voice Recorder (CVR) data were available to the Investigation. It was established that prior to departure from Seattle-Tacoma, the flight crew had noted that the SBY AH was unpowered and after cycling the SBY Power Selector and the Battery Switch, a number of related status messages were displayed on the EICAS. Maintenance personnel attended the aircraft, cleared the messages and “told the crew they were good to go" without making any technical log entry.
Approximately 30 minutes after takeoff and just prior to levelling at FL370, it was reported that several flight deck lights had flickered, multiple EICAS messages had appeared, the Standby Power OFF light had illuminated and uncommanded disconnection of the AP and A/T had occurred. The Quick Reference Handbook (QRH) ‘STANDBY BUS OFF’ procedure had then been commenced by selecting the Standby Power selector to the BAT position. Although the QRH did not instruct the crew to divert to the nearest suitable airport, it indicated that the battery will supply bus power for approximately 30 minutes. The Battery DISCHARGE annunciation remained lit and the crew reported consulting the list of items lost if the standby busbar was not powered.
The aircraft commander decided to continue the flight aware that the electrical systems were being supplied by battery power and there were initially no further developments. However, after over two hours further flight, battery power had depleted and numerous essential aircraft systems began to annunciate failure including the stabiliser trim, all the instrumentation at the left side pilot position, the thrust reversers and the anti-skid and others. The First Officer took over from the aircraft commander as PF and a diversion to Chicago O’Hare was initiated over western Michigan with advice to ATC that it would not be possible to fly an Instrument Landing System (ILS) approach. It was found that only one R/T box remained operative and at about this time, the cabin crew had discovered that the PA and crew interphone had ceased to function and had passed a note under the locked flight deck door to that effect. The flight deck door was opened and the cabin crew had been advised of the diversion.
As the aircraft slowed down in the descent, it became apparent that both main and alternate stabiliser trim were inoperative and with the landing runway 22R now in sight, an emergency was declared and the aircraft commander began to assist the First Officer on the flight controls. The crew advised that because the aircraft had become difficult to control in pitch, flap extension was stopped at 20°. Having touched down hard approximately [[Length:: 760 metres beyond the runway threshold, the crew found that in the absence of spoilers, thrust reversers and normal braking, the aircraft was not likely to stop before the end of the 2290 metre runway where there were known to be obstructions and so the aircraft commander attempted to veer the aircraft off the runway onto the adjacent grass. The aircraft finally stopped with the nose approximately 120 metres beyond the end of the runway abeam the extended centreline with 7 out of the 8 main gear tyres either burst or deflated. The crew were unable to shut down the engines and only after RFFS personnel had applied foam to the overheated right main landing gear and company maintenance personnel had boarded and succeeded in shutting the engines down had the passengers been able to leave the aircraft using portable stairs.
During the flight after the occurrence of abnormal system indications and prior to the subsequent exhaustion of battery power whilst attempting to complete the scheduled flight, the aircraft commander consulted Company maintenance personnel at ‘Tulsa Tech’ via ARINC on two occasions. The summary of both conversations provided in the NTSB Report suggests that there was both a lack of understanding of the (relatively standard) architecture of the electrical systems and an inability to effectively communicate the actual electrical status of the aircraft. A willingness to delegate his authority in respect of whether to continue as originally planned or make an en route diversion was also apparent - although the invitation to give such advice was, unsurprisingly, not accepted during either exchange and was in any case proscribed by maintenance procedures.
In respect of the root cause of the electrical malfunction, the Investigation established that the system failure indications and symptoms experienced would occur if the K106 electrical relay failed and erosion on the ‘B’ contacts of this relay, which would facilitate that, was found. Boeing advised that although the serial numbers of small parts are not recorded during production, there was circumstantial evidence that the faulty relay had been installed when the aircraft was manufactured.
The battery/standby power system consists of the following buses: Hot Battery bus, Battery bus, Standby DC bus, and the Standby AC bus. The aircraft schematic diagrams show that when the standby power selector is in the BAT position, the main battery is the sole source of power for these busbars. In addition, the main battery charger is unpowered, and the battery will not be recharged. The purpose of battery power in all transport aircraft is to provide continued electrical power to essential services following the failure of supplies from the engine/APU driven generators or their substitutes, whereas in this case, none of those main systems were malfunctioning.
The history of recorded defects relating to the subject aircrafts electrical system was examined and between May 2008 and the incident, it was found that there had been eleven Technical Log defect entries related to the BPCU/Ground Power which had been rectified by either resetting circuit breakers, replacing fuses, or replacing the BPCU, the latter being done three times. It was also found that between June 2008 and the incident, there had been 10 defect entries in respect of the Standby DC BUS which had been addressed by operational checks, replacement of bulbs, replacement of four different electrical replays and replacement of a TRU and one of the Busbar Control Units (BPCU).
The Probable Cause of the event was determined as “the failure of an electrical relay due to eroded contacts and the flight crew's decision to continue a flight that was operating on battery power.”
Two Safety Recommendations were made as follows:
- that Boeing should revise the 757/767 procedures and training which address an illumination Standby Power Bus OFF light, to include specific steps to take so that complete loss of battery power is avoided. These steps should include landing at the nearest suitable airport before the power is depleted and actions to take if landing is not possible.
- that Boeing should, once it has revised its procedures and training as per Safety Recommendation A-09-41 for addressing an illuminated Standby Bus Power OFF light without depleting the main battery, require all operators of 757/767 airplanes to adopt these procedures.
The existence of another ‘open’ recommendation in respect of deficiencies in the American Airlines system for supporting continued airworthiness identified during a previous investigation (A-09-029) was also noted and re-iterated.
Relevant Safety Action already taken was noted as including:
- American Airlines revising their emergency procedures for Left and Right AC BUS OFF Lights in their 757/767 Operating Manual.
- The FAA issuing a SAFO on ‘The Effects of Aircraft Electrical Faults Resulting in Main Battery Depletion’.
- Boeing identifying that the electrical system condition which occurred during this incident was also applicable to the Boeing 767 fleet.
- Boeing issuing an Alert on the loss of all standby power following battery discharge.
- Boeing issuing a revised and expanded ‘STANDBY BUS OFF’ procedure.
- Boeing issuing an SB (757-24-0135) to define the addition of a relay and a wiring change which would enable power to be supplied to the battery charger during flight with the Standby Power Switch in the BAT position.
The Final Report of the Investigation CHI08IA292 was adopted on 21 October 2010.