History of Flight
On September 8, 2009, at approximately 1154 Pacific Daylight Time, Southwest Airlines Flight 341, a Boeing 737-3G7 (registration N670SW), was landing on runway 24R at Los Angeles International Airport when the inboard axle of the right main landing gear (RMLG) failed. The flight had departed McCarran International Airport, Las Vegas, Nevada, around 1055 PDT. Visual meteorological conditions prevailed, and an instrument flight rules flight plan was filed. The crew comprised a captain, first officer, three flight attendants, one jumpseater, and 136 passengers; none were injured. During the landing rollout, the captain felt the aircraft swerve and stopped on a taxiway. A deadheading pilot reported seeing a tire depart the aircraft. The RMLG inboard wheel and brake assembly separated and was found near the 4,000 feet remaining marker on the north side of the runway. Passengers deplaned via airstairs onto buses.
Personnel Information
The 46-year-old captain held private, flight instructor, commercial, and airline transport pilot certificates with type ratings for the Boeing 737 and SA227. His most recent first-class medical certificate was issued on June 29, 2009, with no limitations. He had accumulated 230, 77, and 4 flight hours in the last 90 days, 30 days, and 24 hours, respectively. The 39-year-old first officer held private, flight instructor, commercial, and airline transport pilot certificates with type ratings for the Boeing 737, LR-JET, DA-20, and A320 (SIC only). His most recent first-class medical certificate was issued on March 6, 2009, and he had 197, 53, and 4 flight hours in the same periods.
Aircraft Information
The Boeing 737-3G7, N670SW, had its last scheduled A-check on September 7, 2009. The RMLG was installed on November 8, 2003, and last overhauled in October 2003. The inner cylinder had accrued 48,856 hours and 36,016 cycles since new, and 18,200 hours and 14,182 cycles since overhaul. Per Boeing, the inner cylinder required overhaul every 10 years or 21,000 cycles, with a life limit of 75,000 cycles. The RMLG inboard brake was last replaced on June 27, 2009, and the inboard wheel on August 9, 2009.
Meteorological Information
At 1153, the automated weather observation at Los Angeles International Airport reported wind from 270 degrees at 10 knots, visibility 10 miles, few clouds at 9,000 feet, temperature 23°C, dew point 16°C, and an altimeter setting of 29.87 inches of mercury.
Examination
The RMLG inner cylinder assembly (P/N 65-46116-46, S/N 4P2790) was removed and examined by Boeing under NTSB supervision. The inboard axle exhibited a fracture emanating from the bolt hole at the 12 o'clock position in the brake mounting flange. The fracture displayed surface features consistent with fatigue propagation for about 3.5 inches, followed by ductile separation. The intersection of the bolt hole inner diameter and the chamfer on the outboard face showed scoring and fretting damage to the Sermetal coating, identified as the likely fatigue crack initiation area. A metallographic cross section revealed base metal heat damage in the scored area; elsewhere, the axle material met specification and drawing requirements. Southwest Airlines also measured brake mounting bolt breakaway torque on 13 brake assemblies requiring replacement and on the incident airplane's RMLG outboard brake. Per the Aircraft Maintenance Manual, installation torque for brake mounting bolts is 125–135 foot-pounds. The measured torques averaged 104 foot-pounds, with a range of 35 to 175 foot-pounds. Only three of 14 brakes had average torque above 125 foot-pounds; six had average torque below 100 foot-pounds, each with five or more bolts below 100 foot-pounds. No requirement existed to check bolt torque after installation.
Additional Information
Federal Aviation Administration Airworthiness Directive (AD) 2000-05-13 required a one-time magnetic particle or high frequency eddy current inspection of the MLG axle flange for cracking, and a detailed visual inspection of bolt holes for corrosion or fretting. This AD was performed on the incident RMLG on February 17, 2001. Flight Data Recorder data showed a normal landing with no abnormal control inputs or accelerations.