What seemed initially to be a relatively minor runway overrun incident at Chicago's O'Hare International Airport last month has become something much more interesting... with the revelation the American Airlines Boeing 757 landed with limited electrical power.
And, according to the National Transportation Safety Board's preliminary report on the September 22 incident, it did not have to be that way. The Board notes the plane's flight crew opted to continue on their journey even after they lost electrical power, and had to switch to backup reserves.
American flight 268 departed Seattle-Tacoma International at 0802 PDT September 22, bound for New York's JFK International. While en route, the crew received cockpit warnings of a failure of the standby power bus, and trouble with the engine indicating and crew alert system (EICAS). After consulting a quick-reference handbook, the crew switched the standby power selector to the BAT position -- meaning the airliner's electrical system was now operating largely on battery power, which according to the QRH would provide bus power "for approximately 30 minutes."
According to the NTSB prelim, the crew of Flight 268 then consulted with the airline's maintenance department... and elected to continue the flight on battery power, with several items shut down to conserve electricity.
Battery power began to dwindle as the plane overflew western Michigan, around one hour and 40 minutes later. "...Several cockpit electrical systems began to fail. The airplane was over western Michigan and the captain elected to turn around and divert to ORD," the prelim states. "Also, the flight attendants discovered that public address (PA) and the cabin/cockpit interphone systems were inoperative. A flight attendant wrote a note and slipped it under the cockpit door to inform the flight crew of their communication problems.
"A short time later, the cabin crew was informed that they were diverting to ORD. One of the flight attendants then walked through the aisle informing the passengers of the unscheduled landing at Chicago."
Given the circumstances, the approach to O'Hare appears to have been fairly uneventful... but the flight crew declared an emergency as a precaution, which would soon prove to be a prescient move. >From the prelim:
"As the airplane neared the runway on final approach, the flightcrew discovered that the elevator and standby elevator trim systems were inoperative. The captain then assisted the first officer on the flight controls and the approach to land was continued. The systems required to slow the airplane on the runway appeared to indicate normal, and with the elevator control issues the flightcrew did not want to perform a go-around to land on a longer runway. Pitch control of the airplane was difficult so the flightcrew elected to stop the flap extension at 20 degrees. The touchdown was smooth despite the control issues, however, the thrust reversers and spoilers did not deploy. The captain attempted to manually deploy the thrust reversers, but still was not sure if they deployed. The captain was concerned about the brake functionality and accumulator pressure so he made one smooth application of the brakes, which did not "perform well." Due to obstructions off the end of the runway, the captain elected to veer the airplane off the left side of the runway into the grass."
None of the 192 passengers and crew onboard the airliner were injured. As a side note, neither of the aircraft's twin turbofans responded to regular shut down procedures by the flight crew... forcing the crew to depress the fire handles to command engine shutdown, while also flooding both with Halon.
The NTSB's subsequent investigation showed the 757 touched down approximately 2,500 feet down the runway, with witnesses reporting "loud pops" as the left maingear hit the runway, with touchdown of the right mains 165 feet further down. "These skid marks were visible for the entire length of the runway up until the airplane departed the pavement," the NTSB adds.
"Post incident investigation revealed a failure of the B1/B2 contacts in the K106 electrical relay. With the standby power selector in the AUTO position, this failure would have resulted in a loss of power to the battery bus and the DC standby bus, which would have resulted in the AIR/GND SYS message and illumination of the standby power bus OFF light which the flight crew received.
"With the standby power selector in the BAT position, as selected by the flight crew, the main battery provided power to the hot battery bus, the battery bus, the AC standby bus, and the DC standby bus. In addition, the main battery charger was not receiving power, and thus the battery was not being recharged. When main battery power was depleted, all 4 of the aforementioned buses became unpowered."
The NTSB's final report on the incident is still many months off... but the prelim contains enough evidence to raise eyebrows in several respects, in particular the crew's decision to continue their flight even with a serious electrical issue.
"They should have landed as soon as practical," Michael Barr, an instructor at the University of Southern California's Aviation Safety and Security Program, told USA Today. "That would have been the conservative approach. I don't see why they thought they could fly all the way across country on their backup electrical system."
The newspaper adds that so far, no one at American Airlines or its pilots union, the Allied Pilots Association, has opted to comment on the incident, or the NTSB's report.