LAX01LA275

About 33 minutes after takeoff, while cruising at FL 330, the airplane captain observed a weather cell ahead. He initially believed that passage would occur over the cell's top, but as he neared, the cell built up. The fasten seat belt sign was turned on. However, none of the flight attendants were directed to be seated. The airplane went through the top of the cell during which time 5 seconds of moderate intensity turbulence was experienced. Two standing flight attendants and two passengers fell down. One of the flight attendants was seriously injured, and the two passengers sustained minor injuries. During the mishap, the airplane's altitude varied a maximum of 400 feet.

History of flight

On August 9, 2001, about 1308 Pacific daylight time, a Boeing 737-500, N512SW, operated by Southwest Airlines as flight number 1706, encountered turbulence during cruise flight at flight level 330 about 77 nautical miles north of Klamath Falls, Oregon. The scheduled, domestic, passenger flight was performed under 14 CFR Part 121, and an instrument flight rules flight plan was filed. The airplane was not damaged. There were 8 crewmembers and 122 passengers on board. During the encounter with turbulence, none of the flight crew was injured. One standing flight attendant fell down and was seriously injured. A second flight attendant also fell down but was not injured. The third flight attendant did not report having fallen or being injured. Two passengers fell and each reported having a minor injury. No other injuries to passengers were reported. The flight originated from Portland, Oregon, about 1235. It landed without further mishap in Sacramento, California, about 1355.

According to the operator, the captain recalled that a cell had appeared in their flight path. Initially, the airplane appeared to be above it. However, as the airplane neared the cell it built up and the airplane went through its top. Turbulence lasted about 2 or 3 seconds, at which time the airplane was back in the clear.

In a statement written by the captain, he classified the intensity of the turbulence as "moderate," and it lasted about 5 seconds. The captain stated that, at the time, the seat belt sign was on.

The flight attendant who was seriously injured had completed drink service and was walking to the aft galley at the time. She was passing row 15 when the airplane made, according to her, "a little dip." The flight attendant landed on her back in the aisle. The two passengers who had fallen were located in the aft galley.

One of the uninjured flight attendants reported that prior to the turbulence encounter, several announcements had been made to the passengers to remain in their seats with their seat belts fastened until the captain turned the seatbelt sign off.

Meteorological information

The Federal Aviation Administration (FAA) reported that about the time of the mishap, there were no pilot weather reports (PIREPS), significant meteorological information (SIGMET's), or center weather advisories (CWA's) concerning turbulence for the area.

At 1253, Klamath Falls reported few clouds at 5,000 feet, and scattered clouds at 10,000 and 25,000 feet. No precipitation was reported, and the wind was calm. Cumulonimbus clouds were present in the area.

Tests and research

Recorded radar data received from the FAA's Seattle Air Route Traffic Control Center (ARTCC) indicated, in pertinent part, that between 1303 and 1307 the airplane’s altitude was 33,000 feet. At 1307:56, the airplane's altitude decreased to 32,900 feet (lowest altitude recorded during the mishap). Thereafter, at 1308:08, the altitude increased to 33,200 feet (highest altitude recorded during the mishap). At 1308:32, the airplane was again recorded at 33,000 feet. There were no other excursions during this portion of the airplane’s flight.

The airplane's flight data recorder was read out. In part, the data indicated that the airplane had experienced a vertical flight path deviation between 1307:25 and 1308:33. Specifically, between 1307:45 and 1308:05, the airplane's 33,000-foot cruise altitude decreased to a low of about 32,850 feet, and then it increased to about 33,230 feet.

The National Transportation Safety Board investigator reviewed the audio tape recording of communications between the accident airplane's flight crew and the ARTCC controller between 1257 and 1308. During this interval of time, no aircraft reported experiencing turbulence in the accident area.