Newark, NJ, USA
N462UA
Airbus A320
This incident occurred when United Airlines flight 731 experienced a fault in the airplane's AC 1 electrical bus, one of the two primary electrical distribution systems for the airplane, which in turn caused a number of other electrical busses on the aircraft to lose power. The loss of this electrical power led to the loss of a number of aircraft displays and systems. During the flights return to the airport, multiple messages on the electronic centralized aircraft monitoring (ECAM) system, which provides information to the crew regarding failures that have occurred on the aircraft along with recommended corrective actions, were being displayed then removed by the ECAM system, making it difficult for the flight crew to interpret and address the error messages. There were no faults found in the airplane wiring and the testing and teardown found don faults in the #1 integrated drive generator (IDG), bus tie contactor (BTC), and generator control unit (GCU).
HISTORY OF FLIGHT On January 25, 2008, about 0945, an Airbus A320, N462UA, operated by United Airlines as flight 731, returned to Newark Liberty International Airport (EWR), Newark, New Jersey, shortly after departure from runway 22R because three of the six electronic displays providing information to the flight crew went blank and several aircraft systems became inoperative. The flight crew landed the airplane without further incident, and no injuries were reported for anyone on board the flight. Visual meteorological conditions (VMC) prevailed at the time of the incident for the 14 Code of Federal Regulations Part 121 scheduled domestic flight, which was operating on an instrument flight rules flight plan. According to the flight crew, shortly after takeoff, the captain's primary flight display (PFD) and navigational display (ND), along with the upper electronic centralized aircraft monitoring (ECAM) display, went blank. The ND for the first officer remained functional, as did the lower ECAM display; however, the first officer reported that the attitude information on his PFD was initially not usable but that the information appeared to be reliable later in the flight. In addition, although the handle that controls the landing gear had been moved to the retract position after takeoff, the landing gear did not retract. According to the crew, all radios were inoperative and the overhead panel was blank. The pilots leveled the aircraft at their first assigned altitude of 2,500 feet, elected to return to the field, and landed at EWR with several aircraft systems inoperative, including the airplane's transponder, the traffic alert and collision avoidance system, and the standby attitude indicator. The crew later reported that the indicator (which, according to Airbus, is designed to function for about 5 minutes after it loses power) stopped functioning while the aircraft was on the downwind leg of the approach for landing. After landing, the flight crewmembers observed that the "AC ESS FEED" pushbutton on the overhead panel had an illuminated fault indication for the AC 1 electrical bus. Both crewmembers stated that this fault indication was not illuminated in flight. They then manually selected the AC essential electrical bus feed to "alternate," which reconfigured the power supply. After this selection, the captain's instruments, as well as most of the failed aircraft systems resumed functioning. PERSONNEL INFORMATION The captain was 54 years old and held an airline transport pilot certificate with an Airbus A320 type rating. He held a first-class medical certificate dated December 12, 2007, with no limitations. Records indicated he had about 14,500 hours total time with about 4,950 hours in the A320. The first officer was 40 years old and held an airline transport pilot certificate with an Airbus A320 type rating (second in command). He held a first-class medical certificate dated March 10, 2007, with no limitations. Records indicated he had about 5,000 hours total time with about 2,290 hours in the A320. AIRCRAFT INFORMATION The flight instruments in an Airbus A320 include the captain's primary flight display (PFD) and navigational display (ND), the first officer's PFD and ND, and the upper and lower electronic centralized aircraft monitoring (ECAM) displays. The ECAM system automatically displays messages and system diagrams to pilots. It provides operational assistance for both normal and abnormal airplane operational situations. The primary sources of electrical power for the A320, AC 1 and AC 2 use several electrical busses to distribute power throughout the airplane. In its normal configuration, the AC 1 electrical bus provides power to the AC essential bus, the DC essential bus, and DC 1 bus (power is provided to the DC electrical busses after first passing through a transformer-rectifier unit.) The auxiliary power unit and air driven emergency generator can also provide limited aircraft electrical power under some conditions. When a fault is detected in the AC 1 electrical bus (or it stops providing power), the other busses can be powered by an alternate configuration, which entails selecting the AC 2 electrical bus to provide power. The alternate reconfiguration of the electrical system must be performed manually by the flight crew and is accomplished by activating a pushbutton switch located on the overhead panel. TESTS AND RESEARCH Following the incident, maintenance personnel examined the #1 integrated drive generator (IDG) pylon connector for signs of arcing and #1 engine feeder cables. No defects were noted. The #1 IDG, bus tie contactor (BTC), and generator control unit (GCU) were removed for testing and examination. The IDG and GCU were examined at the Hamilton Sundstrand facilities in Rockford, Illinois. Both units were placed on the systems test bench and subjected to loading conditions representative of the conditions that existed at the time of the incident. Both units completed the test series with no faults found. In addition, a production acceptance test procedure was conducted on the IDG with no faults found. Finally, the IDG was partially disassembled for a detailed examination and no problems with the unit were noted. The BTC was tested and examined at a United Airlines maintenance facility and was found to be in good physical condition. The BTC was then tested per OEM procedures and passed all requirements. The unit was then partially disassembled and inspected with no defects noted. PREVIOUS INCIDENTS On October 22, 2005, a British Airways A319 flight from London Heathrow Airport to Budapest, Hungary, had a similar event occur. As the airplane climbed through 20,000 feet in night VMC with the autopilot and autothrust engaged, an electrical failure occurred, which resulted in five out of the six flight displays going blank. In addition, the autopilot and autothrust systems disconnected, the VHF radio and intercom became inoperative, and most of the cockpit lighting went off. After troubleshooting the problem, the flight crew was able to restore power to the displays and most of the affected systems by manually selecting the AC essential electrical bus feed to "alternate," which reconfigured the electrical system to provide power from the AC 2 electrical bus. The United Kingdom Air Accidents Investigation Branch (AAIB) investigated this incident. No definitive cause for the AC 1 bus fault could be determined. According to Airbus, as of May 2007, 49 events similar to the United Airlines flight 731 and UK events had occurred in which the failure of electrical busses resulted in the loss of flight displays and various aircraft systems. Of these 49 events, roughly 37 were due to failures of the AC 1 bus. SAFETY ACTIONS TAKEN In response to the previous incidents, Airbus issued two service bulletins: • Service Bulletin SB A320-24-1120 was issued on May 31, 2007. This service bulletin provided for the automatic reconfiguration of the AC essential bus power supply in the event that the AC 1 electrical bus fails. Along with this service bulletin, Airbus released a Retrofit Information Letter (SEOT2/916.0083/08, revision 1, dated 28 January 2008). This letter stated that the modification kits required to accomplish SB A320-23-1120 were available from Airbus free of charge. • Service Bulletin SB A320-33-1057 was issued on May 11, 2007. This service bulletin provided for the modification of the supply logic for the standby horizon and the cockpit floodlight located under the glareshield so that they are supplied by the hot electrical bus. This is designed to ensure that the standby horizon remains powered and illuminated in the event that the AC 1 electrical bus fails. In response to the circumstances of this incident, on July 22, 2008, the National Transportation Safety Board issued Safety Recommendations A-08-53 through -55 to the Federal Aviation Administration (FAA) and Safety Recommendations A-08-56 through 58 to the European Aviation Safety Agency (EASA). All recommendations have been closed-acceptable action, closed-acceptable alternate action, closed-reconsidered, or closed-no longer applicable. The FAA issued the following Airworthiness Directives and Alerts as a result of this incident and the previous incidents: • Airworthiness Directive 2009-01-04, on February 27, 2009, which requires operators of in-service A320 family aircraft that are equipped with a classic standby instrument system to incorporate the recommended modifications. Upon completion, modified airplanes were equipped with an electronically driven mechanical attitude indicator powered by an additional source that will last for a minimum of 30 minutes in the event of an AC 1 electrical bus failure. • Airworthiness Directive 2010-10-08, on June 21, 2010, which requires operators of in-service A320 family aircraft to incorporate the modifications specified in Airbus Service Bulletin A320-24-1120. • Safety Alert for Operators (SAFO) 08020, "The Loss of Flight Displays and Aircraft Systems Following Partial Electrical Power Failure on Airbus A318 Through A321 Series Airplanes," which was published on October 8, 2008. This SAFO emphasizes the necessity for operators to provide flight crews with guidance and appropriate training (1) to quickly recognize the symptoms of an AC 1 electrical bus failure and (2) to respond appropriately. The EASA issued the following Airworthiness Directives as a result of this incident and the previous incidents: • Airworthiness Directive 2009-0235, on October 29, 2009, which requires the modification of the electrical power distribution system on the A320 family of aircraft, in accordance with Airbus Service Bulletin A320-24-1120.
a fault in the airplane's AC 1 electrical bus that caused a number of other electrical busses on the aircraft to lose power.
Source: NTSB Aviation Accident Database
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