The U.S. National Transportation Safety Board (NTSB) has issued a preliminary report regarding the American Airlines Boeing 757 runway excursion incident at Chicago-O'Hare International Airport (ORD) last month. Readers will recall that on September 22, 2008, American Airlines Flight AAL 268 was en route from Seattle to New York when previously unspecified 'electrical problems' developed. The aircraft diverted to ORD where the crew performed a safe VFR landing. Unfortunately the aircraft departed the runway pavement before coming to a stop, causing 'minor damage' to the landing gear. There were no injuries reported among the seven crew members and 185 passengers on board.The NTSB's Preliminary Report [NTSB ID: CHI08IA292] about the incident, posted earlier today on the Board's website, makes for some very interesting reading. Rather than try to paraphrase or summarize the report, here is an excerpt from the report describing the sequence of events:
While en route the flightcrew received an AIR/GRD SYS message, an illumination of the standby power bus OFF light, and several advisory and status messages on the engine indicating and crew alert system (EICAS). The flight crew then referenced the quick reference handbook (QRH) regarding the AIR/GND SYS message. The flight crew then followed the procedure referenced in the QRH for STANDBY BUS OFF by turning the standby power selector to the BAT position. The QRH procedure also referenced that, "The battery will provide bus power for approximately 30 minutes." The airplane systems stabilized with several items inoperative and the captain contacted maintenance technical support and subsequently elected to continue the flight on battery power. The flightcrew then reviewed the MAIN BATTERY CHARGER procedure referenced in the QRH.Hmmm, let's see: cruising on battery, then battery power depleted; multiple systems failure; communications between flight deck and cabin in-op; no ILS; no elevator control; no thrust reversers; inadequate braking; unavoidable runway excursion (requiring a split second decision to go straight, left, or right); inability to shut down engines... Good grief! Sounds like the entire crew really had their hands full on that flight!
Approximately 1 hour and 40 minutes later, while in cruise flight, the battery power was depleted at which time several cockpit electrical systems began to fail. The airplane was over western Michigan and the captain elected to turn around and divert to ORD. 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.
While aligned with the runway to land, the flightcrew declared an emergency with the control tower as a precaution. 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.
As the airplane touched down approximately 2,500 feet down the runway witnesses heard loud pops. Skid marks from the left main gear were evident near the point of touchdown and 165 feet further down the runway skid marks from the right main gear were present. These skid marks were visible for the entire length of the runway up until the airplane departed the pavement. The airplane came to rest with all three main landing gear off the left side of the pavement and the nose of the airplane came to rest approximately 100 feet prior to the end of the blast pad pavement which extended 397 feet past the departure end of the runway.
After coming to a stop, the flightcrew was not able to shut the engines down with either the fuel cutoff valves or by extending the fire handles. The engines were subsequently shutdown by depressing the fire handles. The passengers were then deplaned through the L1 and R4 doors using portable stairs.
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.
We can anticipate that the final report from the NTSB will surely include a thorough review of the in-flight procedures entailed in this event; nevertheless, the crew avoided what could have been a catastrophic ending to the flight. I hope that the passengers who may have been 'inconvenienced' by the diversion appreciate this fact.
