On July 25, 2001, at approximately 1210 eastern daylight time, a DC-9-51, N780NC, operated by Northwest Airlines as flight 490, encountered moderate turbulence during a descent into the Detroit Metro Airport (DTW), Detroit, Michigan. One passenger suffered a broken ankle. There were no other injuries to the two flight crewmembers, three flight attendants, or to the other 91 passengers on board. The airplane did not receive any damage. The 14 CFR Part 121 flight was operating in visual meteorological conditions and an IFR flight plan was filed. The flight departed from Atlanta, Georgia, at 1048 eastern daylight time. The captain reported that prior to departure the captain trainee made a passenger announcement which included the probability of turbulence along the route of flight, especially during the descent into Detroit. The captain reported that he turned on the weather radar prior to the descent into DTW. He reported the captain trainee turned on the seat belt sign and made a passenger announcement commenting that they should expect turbulence during the descent. The captain reported they encountered light turbulence when they started the descent and although there were no storm cells visible on the radar for their flight path, there was an indication of light rain. The captain reported that he instructed the captain trainee to call the lead flight attendant to assure they were seated and strapped in as a precaution. He reported that when the trainee captain opened the door he noticed the lead flight attendant was already seated with a shoulder harness on. The captain reported they continued the descent in light turbulence and between an altitude of 18,000 feet and 12,000 feet, they encountered several moderate "bumps." He reported that approximately 30 seconds later, they were informed that an off duty flight attendant had been out of her seat when they encountered the moderate turbulence and that she had broken her ankle. The captain reported that the remainder of the flight was uneventful with little or no turbulence. The injured off duty flight attendant reported, "Just before the descent began into Detroit, several passengers went to the lavatory and returned with no difficulty. I chose to give all other passengers the opportunity to go before I did. The flight was continuing smoothly as I made my way from my seat on the right side of the exit row, aft to the lavatory. While returning to my seat, the aircraft began to encounter some moderate turbulence as is normal while descending. I held to the seat backs to support myself as I walked forward toward my seat. As I neared my seat, the aircraft encountered a severe pocket of air with absolutely no warning whatsoever. I was lifted into the air as the aircraft dipped, then falling to the floor of the cabin. I could tell immediately that my ankle was broken because of its angle, and the terrible pain." A review of the flight data recorder data revealed the vertical acceleration of the airplane between 14,429 feet and 11,114 feet varied between .56 g's and 1.39 g's.