CHI98LA351

Flight 146, an ATR-42 was approximately 18 miles from the airport on the ILS 09R approach when it encountered wake turbulence from a Flight 554, a Boeing 737-300, which was vectored onto the approach from the north in front of Flight 146. Radar data showed Flight 554 intercepted the localizer approximately 17.5 miles from the airport while descending from 5,200 feet to 4,800 feet. Flight 146 was at an altitude of about 4,900 feet when the wake turbulence encounter occurred. Separation of the two airplanes when the encounter occurred was approximately 4.8 miles. Vertical acceleration of Flight 146 varied from 2.3679 g's to .5542 g's during the encounter.

On September 17, 1998, at 1933 central daylight time, an Aerospatiale ATR-42, operated by American Eagle as Flight 4146, encountered wake turbulence from a Boeing 737. The flight was approximately 18 miles out on approach to O'Hare International Airport when the event occurred. The flight attendant received a fractured ankle. There were no injures reported by the captain, first officer, or 13 passengers. The 14 CFR Part 121 flight was operating in visual meteorological conditions on an IFR flight plan. The flight originated at Wausau, Wisconsin, at 1836 cdt.

The flight crew reported that when they were approximately 20 miles out from the airport, they encountered wake turbulence from the airplane that was ahead of them. Shortly after the encounter they received a call from the flight attendant in the back of the airplane informing them that she was injured. The captain went to the back of the airplane where he found the flight attendant being attended to by a nurse who was on board. The captain reported he then assured that the passengers were ready for landing and he returned to the cockpit. The captain reported he informed the air traffic controller that they had an emergency. The airplane was met at the gate by emergency rescue personnel and the flight attendant was transported to the hospital.

The flight attendant reported she was in the galley when the turbulence was encountered. She reported that she attempted to get to her jumpseat and was almost seated when more turbulence was encountered. She said the airplane "rocked hard to the right" and she was thrown from the jumpseat coming to rest on the floor near the galley service door. She said she heard a "loud pop" and realized that she had broken her leg. She asked the passenger seated in seat 12B to assist her in calling the captain. The passenger handed her the interphone and she informed the captain that she was injured and asked him to come to the back of the airplane. She stated that a nurse on board applied ice to her leg and attended to her through the remainder of the flight.

A review of the communications transcript between Flight 146 and the Chicago O'Hare Approach Controller showed that approximately two minutes after being cleared for the ILS approach to runway 09R, the pilot contacted approach and reported, "...I have a minor emergency in the back of the cabin here, uh, if you could send us out some other place to hold for a bit." The controller cleared the airspace then issued vectors to turn Flight 146 off of the approach. The controller then contacted Flight 146 and asked if they needed any assistance at which time the pilot stated they would probably need an ambulance once they got on the ground. Approximately five minutes after declaring the emergency the flight crew reported they were ready to head back to the airport. The controller vectored the airplane back to the localizer and the approach continued to an uneventful landing.

Radar data was reviewed for both Flight 146 and United Flight 554, a Boeing 737-300. The review revealed that Flight 554 was turned onto the ILS approach from the north in front of Flight 146. Flight 554 intercepted the localizer approximately 17.5 miles from the airport while descending from 5,200 feet to 4,800 feet. Flight 146 was at an altitude of about 4,900 feet when the wake turbulence encounter occurred. Separation of the two airplanes when the encounter occurred was approximately 4.8 miles.

A review of the flight data recorder data from Flight 146 revealed that during the encounter the vertical acceleration varied from a maximum of 2.3679 g's to a minimum of .5542 g's.

The local winds aloft around the time of the accident were from 298 degrees at 8 knots.