On May 15, 1999, at 1200 eastern daylight time, a Piper PA-30, N8055Y, collided with the runway when the right main gear collapsed on landing at the Hernando County Airport, in Brooksville, Florida. The dual instructional flight was operated by the certified flight instructor (CFI) under the provisions of Title 14 CFR Part 91, and visual flight rules. Visual meteorological conditions prevailed and no flight plan was filed. The CFI and the private pilot were not injured and the airplane sustained substantial damage. The flight originated from Orlando Executive Airport, Orlando, Florida, at 1130. According to the CFI, a normal preflight and run-up was conducted prior to the flight to Brooksville, Florida. The enroute portion of the flight was uneventful with no mechanical problems reported by the CFI. The instructor reported that the Automated Surface Observation System (ASOS) at the destination airport indicated that the winds were variable from the northeast at 10 to 12 knots. The CFI elected to use runway 09. A before landing checklist was completed, and the CFI reported that a visual check of the landing showed all three wheels down and locked. Upon touchdown, the airplane "suddenly and inexplicably porpoise on to the nose gear." The airplane then "ballooned" yawing left with the left wing low and, at a nose-high attitude. The CFI took control of the airplane and took the following corrective action: added full power, applied right rudder, applied right aileron and applied forward pressure on the yoke. Attempts by the CFI to correct the attitude of the airplane failed. The right main landing gear subsequently collapsed, and the airplane departed the left edge of runway 09 at a 45 degree angle. Damage to the airplane consisted of the right wing spar, right tip tank, right horizontal stabilizer, right main gear, right propeller, and the right flap lower attach points.