NTSB Factual Report

HISTORY OF FLIGHT

On August 31, 2006, about 1320 eastern daylight time, a Boeing 737-400 airplane, N425US, operated by US Airways as flight 431, received minor damage when the left main landing gear tires blew while landing at the Miami International Airport (MIA), Miami, Florida. The 2 pilots, 3 flight attendants, and 113 passengers were not injured. The scheduled domestic passenger flight departed Charlotte/Douglas International Airport (CLT), Charlotte, North Carolina, about 1135, and was operated under the provisions of 14 Code of Federal Regulations Part 121. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed.

According to a flightcrew that operated the airplane the day prior, the anti-skid system was determined to be inoperative during preflight. The flightcrew advised maintenance personnel of the issue and the system was deferred per the company's approved minimum equipment list (MEL), which allowed the airplane to be operated but with weight and landing distance limitations. After the anti-skid system was deferred, the flightcrew completed two flights without difficulty.

The day of the incident, a new flightcrew was assigned for the CLT to MIA flight. The crew reviewed the airplane's maintenance information and determined there were no discrepancies which restricted the landing weight of the airplane or increased runway distance requirements. During the flight, the flightcrew discussed the anti-skid situation and went over the procedure. Once near the airport, the crew requested the longest runway for landing which was granted.

The flightcrew stated to the responding Federal Aviation Administration (FAA) inspector that upon landing a "popping sound" was heard from the left side of the airplane. Shortly thereafter, the airplane settled onto the runway, and the flightcrew felt a series of bumps and vibrations. As the airplane slowed to a halt, the left main gear tires caught fire, and the captain initiated an evacuation after being advised by the local controller that smoke was coming from the right side of the airplane. Emergency personnel arrived within 2 minutes and contained the fire. One of the flight attendants stated that the passengers were calm and that there was no panic or shouting during the evacuation onto the runway.

FLIGHT RECORDERS

Immediately after the incident the airplane's cockpit voice recorder (CVR) and digital flight data recorder (DFDR) were removed and sent to the National Transportation Safety Board's Vehicle Recorder Division, in Washington, D.C., for readout. No irregularities were noted with the information retrieved from the DFDR with respect to the airplane's performance and manipulation of the airplane. The cockpit voice recorder captured the entire flight from pre-takeoff to landing. Prior to the landing, the flightcrew performed cockpit duties consistent with normal operations.

According to the CVR, the flightcrew was aware the anti-skid was inoperative and prepared for the constraints. About 2 seconds after touchdown, a loud vibration noise was recorded. One of the flightcrew members stated that "it sounds like a flat tire or something." A controller at the airport's tower communicated to the crew of the trailing smoke and sparks from one of it main gear. The crew acknowledged and requested assistance. The crew also advised the controller they would stop on the runway and to maintain an eye on it in case it materialized into anything. A few seconds later, the controller advised he saw flames. A few seconds later the decision to evacuate the airplane on the runway was communicated to the controller and a request to send Aircraft Rescue Fire Fighting assistance was made. The emergency checklist was initiated. The captain began briefing the passengers and notified the flight attendants to begin an evacuation. The recording ended several seconds later.

WRECKAGE AND IMPACT INFORMATION

Examination of the airplane showed the left main landing gear outboard tire ruptured and sections of it were separated. One-third of the wheel rim and about 2 inches of brake assembly were ground away. The inboard tire ruptured and about 3 inches of the wheel rim and the brake assembly were ground away. The wing flap section behind the left main landing gear incurred damage from the tire debris.

The operator made the necessary repairs to obtain an FAA special flight permit, and the airplane was then flown to the operator's maintenance facility for further repairs. At that time, several brake control components were removed for further examination. Once all the repairs and required inspections were completed the airplane was returned to service.

TESTS AND RESEARCH

A Systems Group, led by a Safety Board Systems Group Chairman was formed. Examination and testing of the airplane's brake control components were performed by the Systems Group. The inboard and outboard brake assemblies from the airplanes left main landing gear were examined, tested, and disassembled. The inboard brake wear indicator was within the serviceable limits. A functional test was performed and the brake assembly was disassembled. No abnormalities, except for the damage incurred during the incident, were noted that would have prevented normal operation. The outboard brake wear indicator was within the serviceable limits. The outboard brake assembly incurred damage that prevented a functional test. The outboard brake was disassembled. No abnormalities, except for the damage incurred during the incident, were noted that would have prevented normal operation. The parking brake valve assembly was visually examined and functionally tested. The component failed the electrical check portion, which would have given an anti-skid inoperative indication light. The brake metering valve assembly was functionally tested and disassembled; no abnormalities were noted that would have prevented normal operation. A photo of the damaged inboard wheel was made available to the group. The photo showed the tire bead area was intact, except for the damaged incurred during the incident, consistent with no rolling contact with the runway.

The anti-skid system is designed to provide touchdown and locked wheel protection during the air to ground transition and ground movement. The touchdown protection prevents brake application prior to wheel spin-up. Once the wheel has reached a select velocity, braking can be applied, but the system continues to protect against wheel lock until the wheel is below the preset velocity. The touchdown protection is not available when the anti-skid system is in the off position or not operative.

The manufacturer and the operator's aircraft operating manuals have detail information and instructions regarding the anti-skid system operation and limitations. The operator's Quick Reference Handbook (QRH) states after landing use minimum braking consistent with runway, and the MEL states "brake with caution."

NTSB Probable Cause

The pilot's inadvertent application of the left brakes at touchdown. Contributing to the incident was insufficient information in the quick reference handbook on the loss of touchdown protection with an inoperative anti-skid system.