Misrouted and reversed antiskid wiring by US-based Timco Aviation Services technicians led to the runway excursion of a United Air Lines A320 after landing at the Chicago O'Hare International Airport the night of 9 October 2007, according to a final report issued by the US National Transportation Safety Board (NTSB).

Timco performs heavy maintenance on United's A320s at its Lake City, Florida facility.

The incident, which damaged engine nacelles on the aircraft but did not injure any of the 125 passengers and crew onboard, appears closely related to a 25 February event in which another United A320 departed the right side of the runway after landing at the Jackson Hole Airport in Wyoming.

In both cases, wheel speed tachometers for the inboard and outboard wheels on the left main landing gear brake system had been cross-connected, causing both aircraft to exit the right side of the runway on landing.

"Such a configuration would be likely to cause the antiskid system to use the inboard wheel speed to control the outboard braking, and vice versa," the NTSB writes in its preliminary report of the Jackson Hole incident, which remains under investigation. "In such a situation, it would be likely that when the inboard tire began to skid, the antiskid system would release the pressure on the outboard brake instead of the inboard brake."

The NTSB's final report on the Chicago incident lists as a contributing factor that the Timco technicians did "not understand the entire maintenance procedures in the dual tachometer replacement". Maintenance on the landing gear system was prompted by a May 2007 FAA airworthiness directive (AD) on A320 shock absorbers.

Timco, in its own investigation, found that the tachometers had been mislabeled when removed and that technicians thought a certain post-maintenance test would have revealed the cross-wiring when in fact the test was not designed to do so.

The report also blames United's "unclear" maintenance practices as a contributing factor. Upon clarifying the maintenance documents following the incident, the carrier sent a letter to the FAA in December 2007 saying that it had performed functional checks on the tachometers of 45 Airbus A320s and had found only two instances of cross-wiring - the incident aircraft and one other.

Source: Air Transport Intelligence news