On May 23, 1999, about 1845 Eastern Daylight Time, a Cessna 172RG, N9475B, was substantially damaged while landing at the Morristown Municipal Airport (MMU), Morristown, New Jersey. The certificated commercial pilot and passenger were not injured. Instrument meteorological conditions prevailed for the flight that originated from Ocean City Municipal Airport, Ocean City, Maryland. An instrument flight rules flight plan was filed for the personal flight conducted under 14 CFR Part 91. The pilot stated that she received her approach clearance about 12 miles from MMU. At that time, she lowered the landing gear and performed a "gumps" check. After lowering the landing gear, she observed a green landing gear light. The landing seemed normal, and the pilot did not hear a gear warning horn. Then the right wing lowered to the runway, and the airplane spun 180 degrees on the runway. The pilot further stated that she only performed 1 "gumps" check. According to Air Traffic Control Specialists, the airplane touched down on Runway 23, spun 180 degrees, and came to rest on the runway. The pilot radioed the control tower and advised that the right main landing gear had collapsed. Examination of the wreckage by a Federal Aviation Administration Inspector, and a local mechanic, did not reveal any pre-impact mechanical malfunctions. The Inspector observed damage to the elevator, horizontal stabilizer, right aileron, right flap, and right main landing gear. Additionally, he reported that the empennage and right wing sustained structural damage. At 1845, the reported weather at MMU was: wind calm; visibility 1 mile, mist; ceiling 400 feet overcast, temperature 64 degrees Fahrenheit, dewpoint 63 degrees, altimeter 29.80 inches of Hg.