NTSB Factual Report

HISTORY OF FLIGHT

On September 8, 2009, approximately 1154 Pacific Daylight Time (PDT), Southwest Airlines (SWA) Flight 341, a Boeing 737-3G7, N670SW, experienced a failure of the inboard axle on the right main landing gear (RMLG) during landing on runway 24R at Los Angeles International Airport (LAX). The pilot, first officer, 3 flight attendants, 1 jumpseater, and 136 passengers were not injured. Visual meteorological conditions prevailed and an instrument flight rules (IFR) flight plan was filed for the 14 Code of Federal Regulations (CFR) Part 121 scheduled domestic passenger flight. The flight originated at McCarran International Airport, Las Vegas, Nevada, about 1055 PDT.

According to the captain, he felt the aircraft swerve during the landing rollout and stopped the airplane on the taxiway. A flight attendant called the cockpit and reported that a deadheading SWA pilot reported that he observed a tire depart the airplane. The RMLG inboard wheel and brake assembly separated from the aircraft and was recovered on the north side of the runway near the 4,000 feet remaining marker. The airplane was taxied off the runway to taxiway Y where the passengers were deplaned by airstairs onto waiting buses.

PERSONNEL INFORMATION

Captain

The 46-year-old captain held private, flight instructor, commercial and airline transport pilot certificates with type ratings for B737 and SA227 airplanes and a first class medical certificate. His most recent medical certificate was issued on June 29, 2009, with no limitations and his most recent flight review was performed on May 16, 2009. He had accrued 230, 77, and 4 flight hours in the last 90 days, 30 days, and 24 hours, respectively.

First Officer

The 39-year-old first officer held private, flight instructor, commercial and airline transport pilot certificates with a type ratings for B737, LR-JET, DA-20, and A320 (SIC only) airplanes and a first class medical certificate. His most recent medical certificate was issued on March 6, 2009, with no limitations and his most recent flight review was performed on March 2, 2009. He had accrued 197, 53, and 4 flight hours in the last 90 days, 30 days, and 24 hours, respectively.

AIRPLANE INFORMATION

The Boeing 737-3G7 airplane, N670SW, was last inspected during a scheduled A-check on September 7, 2009. According to SWA maintenance records, the RMLG was installed on the airplane on November 8, 2003. The RMLG was most recently overhauled in October 2003 and the inner cylinder had accrued 48,856 hours and 36,016 cycles since new and 18,200 hours and 14,182 cycles since overhaul at the time of the incident. The RMLG inner cylinder should be overhauled every 10 years or 21,000 cycles and has a life limit of 75,000 cycles according to Boeing. The RMLG inboard brake was last replaced on June 27, 2009 and the RMLG inboard wheel was last replaced on August 9, 2009.

METEOROLOGICAL INFORMATION

The KLAX METAR reported the following weather at 1153; wind from 270 degrees at 10 knots, visibility 10 miles, few clouds at 9000 feet, temperature 23 degrees Celsius, dew point 16 degrees Celsius, and an altimeter of 29.87 inches of Mercury.

TEST AND RESEARCH

The RMLG Inner Cylinder Assembly (P/N 65-46116-46, S/N 4P2790) was removed from the aircraft and sent to The Boeing Company. The teardown and examination of the inner cylinder was accomplished under the supervision of the NTSB. The inboard axle exhibited a fracture that emanated from the bolt hole at the 12:00 position in the brake mounting flange. The fracture exhibited surface features consistent with fatigue propagation for approximately 3.5 inches. The remainder of the fracture exhibited surface features consistent with ductile separation. The area where the bolt hole inner diameter intersected the chamfer on the outboard face of the flange exhibited scoring and fretting damage to the Sermetal coating and microscopic inspection indicated this was the likely area of fatigue crack initiation. A metallographic cross section of the crack initiation point was prepared and the microstructure examination revealed the presence of base metal heat damage in the area of scoring damage. Away from this area the axle material was consistent with the specification and drawing requirements.

At the request of the NTSB, SWA gathered data on the in-service brake mounting bolt breakaway torque on 13 brake assemblies that required brake replacement due to wear. The breakaway torque on the RMLG outboard brake from the incident airplane was also measured. The brake mounting bolts are to be installed with a final torque of 125-135 foot-pounds per the Aircraft Maintenance Manual, chapter 32-41-41. The in-service mounting bolt torque values on the 14 brake assemblies measured averaged 104 foot-pounds with a minimum measured torque of 35 foot-pounds and a maximum of 175 foot-pounds. The minimum and maximum torque values were recorded on different brakes. Eight of the 14 brakes measured had average torque values greater than 100 foot-pounds but only 3 brakes had average torque values greater than 125 foot-pounds. Six of the 14 brakes had 5 or more brake mounting bolts with torque values less than 100 foot-pounds and these six brakes all had average torque values less than 100 foot-pounds. There is no requirement or guidance to check the bolt torque after installation.

ADDITIONAL INFORMATION

Federal Aviation Administration Airworthiness Directive (AD) 2000-05-13 mandates that a one-time magnetic particle inspection or a one-time high frequency eddy current inspection of the MLG axle flange be performed to detect cracking and a detailed visual inspection of the bolt holes be performed to detect corrosion or fretting. The AD was performed on the incident RMLG on February 17, 2001.

Examination of the Flight Data Recorder data indicated the landing was essentially normal with no abnormal control inputs or accelerations recorded.

NTSB Probable Cause

The failure of the right main landing gear inboard axle due to fatigue cracking as a result of fretting damage in one of the brake mounting bolt holes.