On October 31, 1997, about 2032 eastern standard time, a Dehavilland DH-8-201, N988HA, registered to Wilmington Trust Company, and operated by U.S. Air Express as flight 3040, a 14 CFR Part 121 scheduled domestic passenger flight from Jacksonville, Florida, to Tampa, Florida, encountered severe turbulence on climbout while passing through 4,500 feet. The pilot requested and received clearance back to the departure airport. Visual meteorological conditions prevailed and an IFR flight plan was filed. The airplane sustained no damage. The airline transport-rated pilot-in-command (PIC), first officer, 1 flight attendant and 14 passengers reported no injuries. One passenger sustained serious injuries. The flight originated from Jacksonville International Airport, Jacksonville, Florida, about 2 minutes before the accident, and returned to the destination airport without further incident. The PIC stated they were passing through 4,500 feet when they encountered severe turbulence which lasted about a microsecond. The weather radar was on the 50-mile range with no weather depicted, except for rain showers. The seatbelt sign was illuminated. The flight attendant informed them after the turbulence encounter that a passenger sustained a broken leg. The passenger stated to the flight attendant and the PIC, "she could not believe how stupid she was for not fastening her seat belt."