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.
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.
| Time | Event | Page |
|---|---|---|
| 09:49:00-10:00:57 CDT | Before takeoff airspeed invalid (FDAU not receiving data); main battery 28 VDC | 5 |
| 10:00:57 CDT | Takeoff roll began; climb to 37,000 ft, level at 10:24 | 5 |
| 10:20:47 CDT | Faults recorded for left and right main pitot heat and left AOA probe heat | 5 |
| 10:25:35 CDT | Main battery voltage spiked to about 31 VDC | 5 |
| 10:30:55 CDT | Pitot/AOA heat faults cleared; main battery indicated discharging at -24 A and voltage began decreasing (battery carrying loads - generator/bus loss) | 5 |
| 12:53:32 CDT | Main battery discharge indicated normal; battery voltage 15 VDC | 5 |
| 12:54:37-12:56:07 CDT | Left main pitot heat fault (12:54:37), computed airspeed cycling 0-320 kts from 12:55:31, left AOA heat fault (12:55:37), right main pitot heat fault (12:56:07) | 5 |
| 12:56:47 CDT | Lowest main battery voltage 5 VDC | 5 |
| 12:57:32-12:57:38 CDT | Computed airspeed and pressure altitude invalid to the end of the recording; Air Data Computer failure recorded | 5 |
| 13:17:47 CDT | Descent began (from pitch and engine indications, air data being invalid) | 5 |
| 13:41:58 CDT | Gear weight-on-wheels indicated ground (landing ORD 22R) | 5 |
| 13:42:00 CDT | FDR recording ended (recorder lost power at the air/ground transition) | 5 |
944 transcribed utterances from 11:43:23 to 13:46:45, 27 sources. Final entries:
| 13:46:37 | HOT-1 | okay they're gonna put up some stairs. |
| 13:46:41 | HOT-2 | unbelievable. |
| 13:46:44 | CAM | [sound similar to power interruption] CHI08IA292 |
| 13:46:44 | CAM-? | okay. |
| 13:46:45 | CAM | [sound similar to engine spooldown] |
Docket: data.ntsb.gov/Docket/?NTSBNumber=CHI08IA292. This brief is an analytical aid assembled from public records; it is not an investigative finding.