A safety alert for operators (SAFO) recently published by the US FAA warns airlines to ensure that their flight manuals and training "reflect accurate abnormal indications and inoperative systems" when an aircraft's battery is depleted.
The SAFO comes in response to a 22 September incident in which an American Airlines 757 en route from Seattle to New York's John F. Kennedy Airport diverted to Chicago O'Hare after several cockpit electrical systems began to fail, events that ultimately led to aircraft flight control problems, a runway excursion and subsequent deplaning of passengers on the runway.
Pilots of Flight 268, with two crew, five flight attendants and 185 passengers, had earlier in the flight addressed a series of error messages in the cockpit by selecting, per the quick reference handbook (QRH), the aircraft's batteries to provide standby bus power.
Though the QRH noted that "the battery will provide bus power for approximately 30 minutes," according to a separate US National Transportation Safety Board (NTSB) preliminary report of the same incident, the crew elected to continue the flight for several more hours to New York.
FAA says by "correctly following the operator's procedure" in the QRH, the pilots isolated four power buses from the remaining electrical systems, powered those systems with the main battery and deactivated the main battery charger, making the battery the "sole power source" for the four isolated buses.
According to the report, about 100min after isolating the power busses and disconnecting the battery charger, several electrical systems began to fail while flying over western Michigan, including the public address and cabin/cockpit interphone system.
"A flight attendant wrote a note and slipped it under the cockpit door to inform the flight crew of their communications problems," says the NTSB. "A short time later, the cabin crew was informed that they were diverting to Chicago. One of the flight attendants then walked through the aisle informing the passengers of the unscheduled landing in Chicago."
Once aligned with Runway 22R at O'Hare, the flight crew declared an emergency "as a precaution", says the NTSB. Closer to touchdown, the pilots discovered that the main and backup elevator trim systems were inoperative, requiring both pilots to use their control yokes to control the aircraft's pitch attitude. Given the reduced control, the crew also elected to use less than normal flap extension for the landing, trading off a lesser pitch change with a faster than normal landing speed.
"The touchdown was smooth despite the control issues, however, the thrust reversers and spoilers did not deploy," says the NTSB. The aircraft touched down approximately 762m (2,500ft) down the 2,286m (7,500ft) runway and left skid marks for its entire length. Unable to stop, the captain had "elected to veer the airplane off the left side of the runway into the grass," according to the report. The aircraft sustained "minor damage" to the landing gear.
However, the incident did not end after the aircraft stopped as the flight crew could not shut the engines down with either the fuel cutoff values or by extending the fire handles, says the NTSB. The crew ultimately shut down the engines by "depressing the fire handles". Passengers were deplaned through the two doors using portable stairs, according to the report.
The FAA in the SAFO is asking airlines safety and training departments to review QRHs and other materials "to ensure that procedures lead to problem resolution rather than complication." The regulator says that while some operators provide a list of inoperative equipment, few give "a complete list of critical systems or components rendered inoperative by complete loss of battery power."
"In most transport category airplanes, systems such as those for fire protection and detection, flight control, navigation and flight instruments, engine fuel control, braking, auto-flight functions, standby horizon and others are either fully or partially inoperative with no main battery power," the FAA writes. "If flight crews do not have appropriate understanding of the effects of lost battery power on critical airplane systems powered by the battery, they may be faced with a rapidly compounding emergency situation."