On January 15, 1999, about 1120, eastern standard time, a Douglas DC-9, N300ME, operated by Midwest Express Airlines as flight 150, encountered wake turbulence while descending through flight level 235 over Plainfield, Pennsylvania. The airplane was not damaged. Two flight crewmembers, 2 flight attendants, and 55 passengers were not injured. A third flight attendant sustained a serious injury to her right ankle. Visual meteorological conditions prevailed, and an instrument flight rules flight plan had been filed for the scheduled passenger flight that departed General Mitchell International Airport (MKE), Milwaukee, Wisconsin, destined for Philadelphia International Airport, Philadelphia, Pennsylvania. The flight was conducted under 14 CFR Part 121. The captain stated that they departed MKE at 1010, and climbed to flight level 290. About 1 hour and 35 minutes after departure, he was cleared by air traffic control (ATC) to 17,000 feet msl. Upon receiving the clearance, he briefed the first officer he would to stay above a proceeding Boeing 747 descent path. The Boeing 747 was 12 nautical miles ahead and also descending to 17,000 feet MSL. The captain then initiated approximately a 3,500 foot per minute descent and slowed to 280 knots. While in the descent and passing flight level 235, the airplane encountered a "moderate jolt" which the captain assumed was from the Boeing 747. The captain pitched the airplane's nose up to exit the turbulence, and requested additional separation between the two airplanes from ATC. The flight continued to its schedule destination without further incident. During the wake turbulence encounter, a flight attendant in the galley slipped and seriously injured her right ankle. A passenger, who was also a registered nurse, applied ice, and splinted the injured ankle. The flight attendant was then seated in the aft jump-seat for landing. Radar data revealed that the accident airplane was about 2 minutes in trail, 1,000 feet below, and 3 miles to the north of the Boeing 747. Radiosonde data recorded about 23,000 feet over Pittsburgh, Pennsylvania, at 0700, showed the winds were 195 degrees at 90 knots. According to the 1998, Aeronautical Information Manual (AIM), "Flight tests have shown that the vortices from larger (transport category) aircraft sink at a rate of several hundred feet per minute, slowing their descent and diminishing in strength with time and distance behind the generating aircraft."