On September 27, 1998, at 2104 Eastern Daylight Time, an Allegheny Airlines Boeing DHC-8-102, N810EX, operating as USAirways Express flight 3977, received minor damage when it encountered turbulence during an approach to Long Island MacArthur Airport (ISP), Islip, New York. The 2 pilots and 22 passengers were uninjured. One additional passenger received minor injuries, and the flight attendant was seriously injured. Visual meteorological conditions prevailed during the nighttime event. The scheduled passenger flight was operating on an instrument flight rules flight plan and originated from Philadelphia International Airport (PHL), Philadelphia, Pennsylvania. The flight was being conducted under 14 CFR Part 121.

According to the pilots, the flight was being vectored for the ILS Runway 24 approach. The airplane was on a downwind leg, at 3,000 feet, about 8 nautical miles northeast of the airport, when it encountered severe turbulence. Afterwards, several unsuccessful attempts were made to contact the flight attendant. The airplane landed, and just prior to parking, a non-company flight attendant, riding as a passenger, informed the pilots that the on-duty flight attendant was injured.

The injured flight attendant later told a Federal Aviation Administration (FAA) Inspector that she had just completed her pre-landing walk-through, when she "felt a jolt send her up into the exit sign. A second jolt sent her into the galley area, followed by a third jolt that sent her to the floor of the aircraft." The flight attendant also stated that during the crew brief before the flight, she was told that thunderstorm activity was possible in the New York area. However, the flight had been smooth prior to the turbulence event.

The injured passenger stated to the Inspector that the flight was uneventful until 5 minutes before landing, when "suddenly, the aircraft made a violent up and down motion, followed by three other similar motions."

The FAA Inspector stated that, according to airport tower controllers, there were no other aircraft in the area when the turbulence occurred. Weather at MacArthur Airport, about 15 minutes prior to the event, included winds from 280 degrees magnetic, at 4 knots, with a visibility of 4 statute miles in haze, and a broken cloud layer at 10,000 feet above ground level. The captain stated that the airplane's weather radar had been set to a 20-mile scale, and showed some thunderstorms north of the airport.

The airplane's flight data recorder revealed that during the approach to MacArthur Airport, there was a gradual increase in vertical accelerations during a 15-second period. During the next 15 seconds, there were a series of jolts, with four vertical accelerations measuring approximately 2.0 positive g's, while three registered 0.5 negative g's. Vertical accelerations continued during the next 75 seconds, with eight exceeding 1.5 positive g's, and two registering about 0.0 g. Lateral and longitudinal accelerations were also noted, and corresponded in time with the vertical accelerations.

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