Pilot Debrief

Airbus Industrie A320 near Jackson, WY — 2008-02-26

Final reportDCA08IA044
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Date
2008-02-26
Location
Jackson, WY, USA
Airport
JAC
Aircraft
Airbus Industrie A320
Registration
N442UA
Category
Airplane
Highest injury
Minor
Fatalities
0
Phase of flight
Landing

Probable cause

the loss of braking action on the left main landing gear due to the cross connection of the wheel speed tachometer wires that was caused by inadequate maintenance performed on the airplane during the installation of the main landing gear.

NTSB narrative

The flight crew reported that prior to the approach into KJAX, the captain performed an approach briefing which included that the Mu readings indicated "good" breaking; it was determined that if the airplane did not touchdown in the first 2,000 feet of runway, they would perform a go around. The touchdown was "firm" and the captain believed it was within the first 2,000 feet of runway. The deceleration was normal until the airplane reached about 80 knots when it started to slide. The captain then applied maximum manual braking and the airplane started to slide toward the right side of the runway. The airplane exited the end of the runway and came to rest in the snow. The runway friction measurements taken before and after the incident were all above 0.45, which according to the Aeronautical Information Manual, is above the level when braking action would start to deteriorate. Therefore, the airplane braking performance was not deteriorated by the runway conditions. There were no faults logged in the maintenance systems that were relevant to the antiskid or brake systems, however, the inboard (IB) and outboard (OB) tachometer wires on the left main landing gear (LMLG) were found to be cross connected. With the wires crossed connected, when the IB tire began to skid (stop rotating) the Brake Steering and Control Unit (BSCU) would have commanded a reduction in the hydraulic pressure to the OB brake, instead of the IB brake, causing a loss of braking on the OB wheel. At the same time, the BSCU would have continued to command an increase in hydraulic pressure to the IB brake, causing it to fully skid and fail. Therefore, when the captain applied full manual braking, the braking action on the LMLG was almost fully lost while the right MLG braking remained normal, resulting in the airplane veering to the right and exiting the runway. A review of the maintenance documents and practices showed that there were no specific inspections required when the landing gear and wiring harnesses were received to ensure they were routed to the correct axle. Additionally, installation documents did not specify the exact wire number connector that needed to be installed on each tachometer for a given position. Finally, a proper functional check should have identified the cross connected tachometers, even though maintenance personnel indicated no discrepancies when tested. After the incident, United Airlines took numerous actions to prevent similar incidents from reoccurring.

Analysis

Primary failure mode
Loss of control
First missed decision gate
Go-around decision not executed after firm touchdown and loss of braking control.

NTSB coding

  • Sys/Comp malf/fail (non-power) · Landing-landing roll
  • Maintenance

Evidence available

  • CVR
  • ATC audio
  • FDR / data
  • ADS-B / radar
  • 39 docket documents
View NTSB final reportView NTSB docket

Docket documents39

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