NTSB Identification: CHI08IA026
Scheduled 14 CFR Part 121: Air Carrier operation of UNITED AIR LINES INC
Incident occurred Tuesday, October 09, 2007 in Chicago, IL
Probable Cause Approval Date: 12/24/2008
Aircraft: Airbus Industrie A320-232, registration: N431UA
Injuries: 2 Minor,125 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

The airplane's left main landing gear inboard and outboard anti skid tachometers wiring were reversed during scheduled maintenance at a vendor. During landing, the inboard left main landing gear's wheels went to a high braking level and the outboard wheel did not apply braking. The airplane sustained minor damage when it exited the runway and impacted runway lighting. Post incident examination of the airplane and brake components revealed no further anomalies. The vendor that performed the scheduled maintenance indicated that their personnel did not understand that the built in test equipment test they performed did not check for cross-wired tachometers. A different specified test called out in the airplane maintenance manual and in an operator's reference document indicated this specified check would verify a cross-wired condition. The operator reported that the reference documentation associated with the scheduled maintenance involving both of the left main landing gear's tachometers was unclear and that the procedure for that maintenance was revised.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

The misrouted and reversed antiskid wiring by vendor maintenance personnel leading to the runway excursion. Contributing to the incident were the runway sign, the vendor's maintenance personnel not understanding the entire maintenance procedures in the dual tachometer replacement, and the operator's maintenance procedures being unclear to the maintenance personnel.

Full narrative available

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