The two airplanes collided in mid-air at approximately 2500 feet msl at 1653 Central time. One aircraft was destroyed by ground impact and fire; its pilot, co-pilot, and passenger were fatally injured. The other aircraft was substantially damaged but its crew made a successful emergency landing at nearby Dodge County Airport, Juneau, Wis. Visual conditions prevailed; both airplanes departed the General Mitchell International Airport. The flights were performed to verify the fuel flow rates for auxiliary fuel tanks that had been recently installed on both airplanes. Prior to departure, both flight crews decided that they would join up after departure in order to take video and still photography of each airplane. Subsequently, one aircraft came up along the left side of the other and flew in formation with approximately 100-150 feet lateral separation. The crew of the leftmost aircraft announced they would turn right and descend below the other aircraft. The leftmost aircrafts left wing then impacted the left wing and engine of the other airplane. The surviving crew experienced a complete hydraulic failure and performed an emergency landing with flaps retracted and a partially extended landing gear. The airplane overran the end of the runway, coming to rest about 100 feet from the departure threshold. The left aileron from the destroyed aircraft was found on the runway where the surviving crew had landed.