What happened
The DC-9 departed and climbed to flight level 290. Approximately 1 hour and 35 minutes after the departure, air traffic control cleared the aircraft to descend to 17,000 feet msl. Following this clearance, the captain began a descent at approximately 3,500 feet per minute and reduced the airspeed to 280 knots.
While descending and passing through flight level 235, the aircraft encountered the wake turbulence generated by a cargo Boeing 747. The larger aircraft was positioned 12 nautical miles ahead of the DC-9 and was also descending to 17,000 feet msl. To exit the turbulence, the captain pitched the nose of the airplane up and contacted ATC to request increased separation from the preceding aircraft.
During the period of wake turbulence encounter, a flight attendant located in the galley slipped, resulting in one serious injury to her right ankle.