On August 4, 1995, at 2248 eastern daylight time (edt), a Boeing 757, N509US, operated as Flight 52 by Northwest Airlines, Incorporated, of Minneapolis, Minnesota, and piloted by airline transport rated crew, encountered sudden, moderate, turbulence at flight level 270. The 14 CFR Part 121 flight was operating on an IFR flight plan. One passenger received serious injuries as she was exiting the lavatory when the airplane encountered the turbulence. The seven person flight crew and 120 passengers reported no injuries. The flight departed San Francisco, California, at 1852 edt. According to the pilot's written statement, the airplane had descended from flight level 410 and had just leveled off at flight level 270, about 25 nautical miles east of Grand Rapids, Michigan, when it encountered the sudden, moderate turbulence. He said the airplane's weather radar was on the 80 mile scan and showed no weather. He said the seat belt sign had been put on and an announcement was made advising the passengers to expect "bumps" on the approach into the Detroit Metropolitan Wayne County Airport, Detroit, Michigan. The company's Director of Flight Safety said the seat belt sign was "...turned on at the top of [the] descent which would have been about 5 minutes... prior to the encounter with the turbulence." The director said the flight attendants reports concerning the incident did not "...indicate that anyone saw the lady enter the lavatory." He said it is presumed that the passenger was in the lavatory when the seat belt sign was turned on. The airplane's lavatories are equipped with a public address speaker, flight attendant call button, and a sign that reads: "Return To Seat" when the seat belt sign is illuminated in the passenger cabin. The first officer said the flight was initially cleared to flight level 230. However, according to her statement, the FAA controller changed the clearance to flight level 270 because the flight would have "...the best ride..." at that altitude. According to the airline's dispatch/meteorology department weather data, the area around Flight 52 had been experiencing 3/10's coverage of level three and four thunderstorms.