Boeing 757-223 N197AN incident near Chicago, Illinois

Probable cause

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.

NTSB analysis

While performing the cockpit preflight, the captain noticed that the standby horizon was unpowered. After cycling the Standby Power Selector and the Battery Switch, the captain noted that five status messages were displayed on the engine indicating and crew alert system (EICAS) screen, although he could not remember the specific messages. Maintenance personnel subsequently cleared the messages and told the crew they were "good to go," without making a logbook entry. Approximately 30 minutes after takeoff several cockpit lights flickered; multiple EICAS messages appeared, including the AIR/GRD SYS caution message; the Standby Power OFF light illuminated; the autopilot disconnect warning sounded; and the autothrottles disconnected. The first officer referenced the STANDBY BUS OFF procedure in the QRH, turning the Standby Power selector to the BAT position. The QRH states that when BAT is selected the battery will provide bus power for approximately 30 minutes. Although the QRH did not instruct the crew to divert to the nearest suitable airport, it indicates that the battery will supply bus power for approximately 30 minutes and the captain reported to maintenance that the battery would be depleted. After consulting with company maintenance specialists, the crew elected to continue the flight. Battery power was depleted and numerous essential aircraft systems began to fail about 2 hours and 24 minutes after the flight crew switched the Standby Power selector to the BAT position. These systems included the stabilizer trim, the captain’s instrumentation, thrust reversers, anti-skid and others. The flight diverted to the closest airport, an emergency was declared, and the airplane was cleared to land on the 7,500-foot-long runway; the captain stated that due to problems controlling the airplane, he did not want to circle to land on a longer runway. The airplane touched down hard about 2,500 feet down the runway. The crew determined they were going to overrun the end of the runway, so the captain veered the airplane off the left side of the runway into the grass, where the airplane came to rest with seven of the eight main gear tires either blown out or deflated. Postaccident investigation revealed erosion on the B contacts within the K106 relay; a failure of the K106 electrical relay would result in the events described by the crew.

Text from the NTSB. Open the NTSB docket for CHI08IA292 for the full record.