A Cessna 170B and a Grumman American AA-5B were participating in a five-airplane recreational formation flight when the two airplanes collided midair. After takeoff from the departure airport, the flight formed into a "Vic," or "V," formation with the Grumman in the No. 2 position to the left of the flight leader and the Cessna in the No. 3 position to the right of the flight leader. The formation flight turned south toward its destination. Due to the position of the sun, the flight leader decided to change to a left echelon formation, which would position the airplanes diagonally to his left, with each airplane stacked slightly lower than (stepped down), behind, and to the left of the airplane ahead. The Grumman remained in the No. 2 position. The Cessna was to transition across from right to left so that it would be in the No. 3 position to the left and aft of the Grumman.According to two other pilots in the formation, when the flight leader commanded the Cessna to the left, the Cessna started to move slowly to the left into position behind the Grumman. The airplanes then contacted each other, and the Grumman abruptly pitched up and looked as if it was entering a loop. The Cessna moved upward before its tail began moving to the left and disappeared from view. Witnesses driving on an interstate highway near the accident site saw the formation flight traveling southbound. The formation flight was about 1/4-mile west of the highway when the collision occurred, and they observed part of the wing from the Cessna come off before the fuselage tumbled and rapidly descend behind a tree line. The witnesses also saw the Grumman descend rapidly, almost straight down, until losing sight of it. The witnesses saw parts from both airplanes descend to the ground with one-piece landing in the median between the northbound and southbound interstate lanes.Examination of the Cessna wreckage did not reveal any evidence of preimpact failure or malfunction that would have precluded normal operation; however, the examination did reveal the presence of paint transfers that matched the trim color of the Grumman. The paint transfers were on the leading edge of the vertical stabilizer, which had been crushed back; the right horizontal stabilizer; and the right side of the aft fuselage. The Cessna's right wing flap (the wing part that witnesses saw falling) came to rest northeast of the Cessna main wreckage. The right wing flap had separated from its mounting location, and the inboard section of the right aileron was missing. The right aileron control cables and right wing Page 2 of 28 ERA17FA143flap tracks had been severed, and the area just forward of the right wing flap mounting location displayed evidence of propeller strikes and was missing large sections of its structure.Examination of the Grumman wreckage also did not reveal any evidence of preimpact failure or malfunction that would have precluded normal operation. No major portions of the airplane were missing, but the leading edges of the propeller blades were damaged and displayed semicircular gouges, which matched the diameter of the aileron cables in the Cessna's right wing. These damage signatures, combined with the descriptions of the witnesses, indicate that the Cessna contacted the Grumman from below and that the Grumman's propeller severed the Cessna's right wing flap tracks and right aileron control cables when the upper surface of the Cessna's right wing contacted the Grumman's propeller.The formation flight was composed of dissimilar airplanes; the lead airplane was a biplane, the Cessna and another airplane were configured with high wings, and the Grumman and another airplane were configured with low wings. Dissimilar airplanes in the same formation present unique safety risks including differences in visibility from one type to another and thus potentially significant blind spots. When both high- and low- wing airplanes are flying together in formation, the areas that are not visible outside of the cockpit due to airframe structural obstructions may not be complementary, which increases the risk of losing sight of other airplanes in the formation. In this case, the stepped-down nature of the left echelon formation resulted in a greater area of visual obstruction for the Cessna pilot due to the airplane's high wing configuration, and an increased risk of collision when compared to a stepped-up vertical position, which would have provided the Cessna pilot with a better view of the other airplanes in the formation ahead and to either side. The converse would be true for the low-wing airplanes operating in the formation. Therefore the decision to operate with dissimilar airplanes in the formation may have hindered the Cessna pilot's ability to see and maintain adequate clearance from the Grumman while transitioning to the echelon formation.