On August 27, 2000, about 1850 hours Pacific daylight time, a Pitts S1S, N9PJ, sustained substantial damage when it collided in-flight with a Cessna 172RG, N6225R, near Westminster, California. Both airplanes were maneuvering during a formation flight when the collision occurred. Both airplanes made successful landings at the Fullerton, California, airport following the collision. Their respective owners were operating the airplanes under 14 CFR Part 91. The Pitts S1S sustained substantial damage to the empennage, while the Cessna 172RG incurred minor propeller damage. The commercial pilots, the sole occupants of their respective airplanes, were not injured. The personal flights departed Dougherty Field, Long Beach, California, en route to the Cable Airport, Upland, California, about 1840. Visual meteorological conditions prevailed and no flight plans had been filed. The pilots stated they were flying each other's airplane. The Pitts departed first, and the Cessna followed. Once over the coastline and clear of the Long Beach airport traffic area, they established radio and visual contact with each other. The Pitts circled and assumed a trail position behind the Cessna. The Pitts pilot said he varied his distance and position relative to the Cessna. Cruise altitude was about 1,000 feet. As they approached the Huntington Beach Pier, the Cessna turned left toward Disneyland and began a gradual climb to 1,500 feet. About 1,400 feet, the Pitts pilot advised the Cessna pilot to hold still and the Cessna pilot acknowledged. The Cessna pilot leveled off and held his course. He knew from their years of flying together that the Pitts pilot would come below him. As the Pitts maneuvered from behind to in front of the Cessna, the Cessna's propeller severed the top half of the Pitts' vertical stabilizer and rudder. The Cessna's damaged propeller set up a vibration. The Pitts pilot maintained control of his airplane. The pilots discussed their options and elected to divert to Fullerton. They advised the tower of their situation and asked for emergency equipment to be on hand. The Cessna landed first, and the Pitts pilot evaluated the amount of control available. After the emergency equipment arrived, the Pitts landed uneventfully.