CHI08IA026

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.

History of flight

On October 9, 2007, about 2030 central daylight time an Airbus Industrie A320-232, N431UA, operated by United Airlines (UA) as flight 628, received minor damage when it exited runway 22R (7,500 feet by 150 feet, grooved asphalt) and impacted runway lighting at the O'Hare International Airport (ORD), Chicago, Illinois. Two pilots, 2 flight attendants, and 121 passengers were uninjured. One flight attendant and one passenger reported minor injuries. The 14 CFR Part 121 scheduled domestic passenger flight was being conducted in night visual meteorological conditions. An instrument flight rules flight plan was filed and activated. Flight 628 departed the Seattle-Tacoma International Airport, near Seattle, Washington, about 1649, and was destined for ORD.

The captain said that the preflight, taxi, takeoff, and cruise portions of the flight were uneventful. The autobrakes were set to low prior to the approach. The captain's incident report, in part, stated:

Approach was stabilized to touchdown with about one-half dot low

deviation at about 500 ft, correcting back to glide slope prior to

touchdown. Tower reported winds just prior to touchdown as 300/13.

Touch down was in the landing area, somewhat harder than

anticipated. Reverse Thrust was initiated, but there appeared to be

no autobraking.

Approximately 1500-1000 feet prior to intersection of taxiway C, I

checked airspeed, waited a couple counts for the airspeed to drop

below 100 kts and commenced braking. The aircraft immediately

swerved hard right. I corrected with full left rudder and brake, but

aircraft continued to the right. I then used Nose Wheel Steering to

attempt to straighten the aircraft, but it was ineffective. At that time

the aircraft left the right side of the runway, parallel to the runway,

contacted at least one sign, and then began to track left, back onto

the paved surface. Aircraft stopped on runway and taxied clear at

[taxiway] R.

Although I knew there was some aircraft damage, there was no

indication of fire. Since the aircraft taxied normally, I taxied to gate

B-10 and parked at the gate.

At the gate, brake temp indications showed no temp rise on the left

brakes (indicating about 60 degrees), while the right brakes were in

the overheat temperature about 375 degrees. At that point

maintenance arrived and said there was damage to the engine nacelles

and the left inboard tire was blown.

The first officer's incident report, in part, stated:

Captains leg on landing 22R wind 300/17G23 on the ATIS. Briefed

a flaps 3 landing for the crosswind. Firm touchdown in the landing

zone. Spoilers deployed and reverse activated. We were coming up

to C still above 80 knots. Applied brakes firmly and plane swerved

to the right. Checked to make sure captain had full left rudder and

full left brake. He did. As we departed the right side of the runway

saw the Captain was using steering to get us back on. He did. The

right Main and nose wheel exited the runway. Once stopped back

on the runway we informed tower of our excursion from the runway.

They told the plane behind us to go around. We determined that we

could taxi ok and decided we could clear the runway. Once clear it

felt like it was taxiing ok so we continued to the gate. We left the

flaps and spoilers in position in case of damage. The left brake

temps were around 60 degrees like they never came on and the

right were 390. I forgot to add that we had the auto brakes set to

LO. Neither of us felt them engage on the rollout.

Injuries to persons

According to the operator, one flight attendant and one passenger went to a hospital for examination. The attendant and passenger subsequently released themselves from the hospital.

Personnel information

The captain held an airline transport pilot certificate with an airplane multiengine land rating and held airplane single engine land commercial privileges. He held a flight engineer certificate. The captain held type ratings in Airbus A-320, Boeing 757, 767, 777, and Gulfstream Aerospace Corporation-1159 airplanes. The operator reported that the captain had accumulated about 14,250 hours of total flight experience, which included 2,450 hours in the Airbus A-320. The operator reported that the captain's most recent Federal Aviation Administration (FAA) first class medical certificate was issued on April 27, 2007. His last check ride was completed in June 2007.

The first officer held an airline transport pilot certificate with an airplane multiengine land rating and held airplane single engine land commercial privileges. He held a flight engineer certificate. The first officer held type ratings in Airbus A-320, Learjet, and Learjet-45 airplanes. He had held a certified flight instructors certificate with single engine, multiengine, and instrument airplane ratings. The operator reported that the first officer had accumulated about 10,206 hours of total flight experience, which included 268 hours in the Airbus A-320. The operator reported that the first officer's most recent FAA first class medical certificate was issued on March 15, 2007. His last check ride was completed in July 2007.

Aircraft information

N431UA, an Airbus Industrie A320-232, serial number 571, was a pressurized, low-wing, transport category airplane. The airplane had a full-cantilevered wing and tail surfaces, a semi-monocoque fuselage, and a fully retractable tricycle landing gear. The two wing-mounted IAE 2527 turbofan engines each produced 26,500 lbs of thrust. The airplane was configured to accommodate 138 passengers and 8 crewmembers. The airplane had a reported maximum gross takeoff weight of 169,750 lbs.

The airplane was being maintained by compliance with a FAA approved continuous airworthiness program. The airplane's last inspection was completed on 10/8/07. According to the operator, the airplane had accumulated 41,163 hours total time in service at that inspection.

The airplane was equipped with brakes and anti-skid system. The section on that system in the airplane's flight crew operating manual supplied by UA, in part, stated:

The main wheels have multidisc brakes that can be actuated by either

of two independent brake systems. The normal system uses green

hydraulic pressure: the alternate system uses the yellow hydraulic

system backed up by a hydraulic accumulator. An anti-skid system

and autobraking work through the brake system. Braking commands

come from either the brake pedals (pilot action) or the autobrake

system (deceleration rate selected by the crew). ... All braking

functions (normal and alternate braking control, anti-skid, autobraking,

brake temperature indication) are controlled by a two-channel Brake

and Steering Control Unit (BSCU). ...

Anti-skid system

The anti-skid system produces maximum braking efficiency by

maintaining the wheels just short of an impending skid. When a

wheel is on the verge of locking, the system sends brake release

orders to the normal and alternate servovalves - and to the ECAM

[electronic centralized aircraft monitoring], which displays the

released brakes. ...

The system compares the speed of each main gear wheel (given by

a tachometer) with the speed of the aircraft (reference speed). When

the speed of a wheel drops below 0.87 times the reference speed, the

system orders brake releasing in order to maintain the brake slip at

that value (best braking efficiency).

Auto brake

The purposes of this system are:

- to reduce the braking distance in case of an aborted takeoff.

- to establish and maintain a selected deceleration rate during

landing, thereby improving passenger comfort and reducing crew

workload.

Arming

The system arms when the crew presses the LO,