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On April 25, 2007, about 1935 eastern daylight time, a Boeing 737-300, N648SW, encountered turbulence while descending in clouds near La Plata, Maryland. One flight attendant sustained serious injuries. The two airline transport pilots, two other flight attendants, and 98 passengers were not injured. The flight was operated by Southwest Airlines, under the provisions of 14 Code of Federal Regulations (CFR) Part 121, as scheduled passenger flight number 111. Instrument meteorological conditions prevailed and an instrument flight rules flight plan had been filed for the flight that departed Birmingham International Airport (BHM), Birmingham, Alabama, and was destined for Baltimore Washington International Airport (BWI), Baltimore, Maryland.
The airplane had been slowed to 250 knots, and was descending through a layer of broken clouds during approach for landing when it encountered turbulence. The altitude at the time of the turbulence encounter was approximately 13,000 feet above mean sea level (MSL).
The captain stated that the airplane "encountered light turbulence and one moderate jolt," which created a "momentary (less than 1 second) weightless experience." The first officer, who was the pilot flying, stated that "there were no reports of turbulence on the arrival or descent, and the radar did not show any significant weather." The airplane "entered the clouds and began to pick up some light turbulence." The crew "changed the radar to a closer range but saw nothing." The airplane then "experienced a moderate vertical jolt, and dropped suddenly." The first officer estimated that the airplane was in the turbulence "for no more than about a minute."
The injured flight attendant said that she was standing alone in the aft galley, working, when "all of a sudden the [airplane] dropped" and she "went up in the air." When she came down, her "left foot turned outwards," she felt a "pop" in her ankle, and her left foot collapsed. She ended up seated on the galley floor, and "couldn't move."
According to a second flight attendant, she and a third flight attendant were in the forward galley when they "hit some turbulence." These two flight attendants then took their seats and "buckled up." Both the injured flight attendant and one of the other flight attendants said that the seatbelt sign was illuminated. The flight crew had not advised the cabin crew to be seated before the event occurred.
The captain held an airline transport pilot certificate with multiple ratings, including a type rating for the Boeing 737. Her most recent Federal Aviation Administration (FAA) first-class medical certificate was issued November 16, 2006. The captain reported 12,700 total hours of flight experience in the Boeing 737.
The first officer held an airline transport pilot certificate with multiple ratings, including airplane multi-engine land. His most recent FAA first-class medical certificate was issued on May 4, 2006. The first officer reported 4,400 hours of flight experience in the Boeing 737.
At the time of the event, the injured flight attendant had been employed in that capacity by the operator for 36 years. In an interview with a Safety Board investigator, the injured flight attendant stated that she had been on this particular trip sequence for approximately six weeks, and was familiar with the passenger loads and the cabin service workload, which she characterized as "pretty busy." On the accident flight, she was wearing leather shoes that she referred to as "flats," which have an enclosed heel, and a heel height of approximately 1/2 inch.
The airplane was equipped with a weather radar system, and the flight crew reported that they were using the radar during the approach. The airborne weather radar system depicted atmospheric moisture, but did not directly depict atmospheric turbulence.
Terminal Area Forecast
The BWI terminal area forecast (TAF) for the period from 1700 to 2000 forecast winds from 090 degrees at 8 knots, 5 miles visibility, light rain showers and mist, and an overcast ceiling at 2,000 feet.
Surface Conditions in Vicinity of Turbulence Encounter
The 1856 surface observation at Quantico Marine Corps Airfield (NYG), Quantico, Virginia, approximately 11 miles west of the turbulence encounter, reported wind from 060 degrees at 5 knots, visibility 10 miles, broken ceiling at 7,500 feet, overcast ceiling at 9,500 feet, temperature 23 degrees Celsius (C), dew point 14 degrees C, and an altimeter setting of 29.83 inches of mercury.
The 1956 NYG surface observation reported wind from 030 degrees at 9 knots, visibility 10 miles, broken ceilings at 7,500 and 10,000 feet, temperature 21 degrees C, dew point 12 degrees C, and an altimeter setting of 29.86 inches of mercury. This report also noted that rain began at 18 minutes past the hour, and ended at 49 minutes past the hour.
The weather echoes provided by ground-based Doppler radar located in Sterling Virginia (KLWX), approximately 30 miles northwest of the turbulence encounter location, indicated that the intensity of the radar returns was gradually diminishing over the period bracketing the accident time, consistent with the advent of sunset. In addition, the cells were drifting southeast under the influence of northwesterly upper level winds.
The KLWX 1932:04 (2332:04 UTC) 3.52-degree elevation sweep, which had a beam center at about 15,400 feet above mean sea level, showed a very light (5 dBZ) echo just upwind of the turbulence encounter location. The next 3.52-degree scan at 1937:51 (2337:51 UTC) indicated that the same cell was then slightly downwind of the turbulence encounter location. Lower angle beam sweeps before and after the event indicated very light radar returns in the turbulence encounter location. The KLWX weather radar showed no signatures of rapidly developing convective clouds, which are normally associated with strong up- and down-drafts.
The flight weather packet issued to the flight crew contained a turbulence AIRMET ("Tango") which was issued at 1645, and was valid until 2300. The AIRMET warned of "moderate turbulence" for altitudes below 12,000 feet for an area which included the location where the turbulence encounter occurred, as well as BWI. The AIRMET also contained the statement "NO SGFNT TURB EXP OUTSIDE CONVTV ACT" which indicated that no significant turbulence could be expected when the airplane was not in clouds that exhibited vertical development. The weather packet did not contain any NOTAMs or PIREPs relevant to this event.
Flight recorder data provided by the operator indicated that the airplane experienced a vertical acceleration minimum value of approximately 0.4 of the force of gravity ("g"), followed 1 second later by a positive peak of approximately 2.0g. The corresponding lateral acceleration peak values were approximately 0.09g and -0.06g, respectively. Longitudinal acceleration remained approximately zero until the vertical acceleration peak, when it peaked at approximately 0.06g. The acceleration sensors were installed near the airplane's center of gravity (CG), and since an airborne airplane has freedom of motion in six degrees, the accelerations measured at the CG may not be representative of those experienced elsewhere in the airplane. Accelerations in the cockpit or the aft cabin can be significantly less than or greater than those at the CG, and can differ significantly from each other as well.
At the time the airplane experienced the cited acceleration values, the computed airspeed was 230 knots, and the pressure altitude was approximately 13,000 feet.
The operator's cockpit and cabin procedures required that the flight crew brief the cabin crew prior to each flight regarding the forecast turbulence for the trip. The injured flight attendant told investigators that the briefing was conducted, but that she had no specific recollection of the contents, except that there was nothing significant regarding the potential for turbulence. She added that there were not any enroute weather updates, particularly with respect to possible turbulence.
The operator's Flight Operations Manual (for flight crews) and the Flight Attendant Manual contained a "Turbulence Action Chart" which delineated the flight and cabin crew responses to various expected levels of turbulence. According to this chart, for moderate turbulence, the flight crew must ensure that the "Fasten Seat Belt" is on, advise the flight attendants of the expected level and duration of the turbulence, make a cabin announcement "reinforcing the need for Flight Attendants and Customers to be seated and service to be discontinued," and to "continue communication with the Flight Attendants as required." This chart specified that the flight attendants must stop all service, stow all galley equipment, "secure [themselves] in a jumpseat as soon as possible using seat belt and shoulder harness (take nearest passenger seat if necessary)," and for the "A" flight attendant to communicate the other flight attendants and the flight crew as required. Page 5 of the Flight Attendant Manual contained a caution which stated "If already experiencing [moderate turbulence] be seated in jumpseat (or nearest available seat, if necessary) immediately."
Section 5 of the Flight Attendant Handbook specifies the permissible footwear for cabin attendants. Approved footwear is subdivided into two categories, tennis shoes and leather shoes. The guidance requires that leather shoe uppers must enclose the heel of the foot, and that the maximum shoe heel height is 3 inches.