NTSB Factual Report

On September 13, 2004, at 2003 Pacific daylight time, a Boeing 737-3H4 transport category airplane, N601WN, operated by Southwest Airlines as flight 1437, landed uneventfully after encountering wake turbulence while on approach to the Los Angeles International Airport (LAX), Los Angeles, California. Two flight crew, 2 cabin attendants, and 42 passengers were uninjured, while 1 flight attendant sustained serious injuries. The airplane was not damaged. Southwest Airlines was operating the airplane, registered to a trustee and leased to Southwest, under the provisions of 14 CFR Part 121 as a scheduled domestic passenger flight. The flight originated from Salt Lake City, Utah, and was destined for LAX. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed.

According to a written statement provided by Southwest Airlines safety department, the airplane was on the SADDE 6 arrival into LAX, and was following a Boeing 747 located approximately 5 miles ahead at 10,000 feet. The accident flight was about 8 miles west of the Santa Monica (SMO) very-high frequency omni-directional radio range navigation aid and at 10,000 feet and 250 knots, when the flight crew experienced a "quick and violent roll of about 35 degrees to the left due to wake turbulence from the preceding [Boeing] 747." The captain righted the airplane and the first officer informed air traffic control that they had encountered wake turbulence and were going to offset to the north of the 747's course to avoid any additional wake turbulence. The remaining portion of the flight was uneventful.

According to air traffic control records (from the Southern California Terminal Radar Control), controllers cautioned Southwest flight 1437 of wake turbulence from the preceding heavy Boeing 747. The flight crew responded and indicated that they had the Boeing 747 in sight.

According to Southwest Airlines written statement, the fasten seat belt sign was illuminated and none of the passengers were out of their seats. However, the flight attendants had not been seated yet (usual time for flight attendants to be seated is during descent out of 10,000 feet). A flight attendant, who was standing in the aft galley at the time of the turbulence encounter, was injured in the cabin area. Southwest Airlines learned the extent of the flight attendant's injuries (broken ribs) 2 days following the event and then notified the National Transportation Safety Board Southwest Regional Office.

The Safety Board authorized Southwest Airlines to download the data from the flight data recorder (FDR) for review. The information gleaned from the FDR revealed that the airplane experienced between +0.734 and +1.547 Gs in vertical acceleration, -0.028 and +0.104 Gs in longitudinal acceleration, and -0.181 and +0.271 Gs in lateral acceleration.

According to the Federal Aviation Administration's (FAA) safety product brochure titled "CAUTION Wake Turbulence" (www.asy.faa.gov/safety _products/wake.htm), the intensity or strength of the vortex is primarily a function of aircraft weight and configuration. The strongest vortices are produced by heavy aircraft, flying slowly, in a clean configuration (i.e., flaps and landing gear retracted). For example, a large or heavy aircraft that must reduce its speed to 250 knots below 10,000 feet and is flying in a clean configuration while descending, produces a very strong wake. Extra caution is needed when flying below and behind such aircraft. The FAA recommends the following to avoid wake turbulence when following another aircraft: "Stay either on or above the preceding aircraft's flight path, upwind, or at least 1,000 feet below."

NTSB Probable Cause

the flight's encounter with the wake turbulence from a preceding heavy airplane while on approach.