At 17:55 EDT, a Cessna 172N and a Cessna 152 collided while on approach to runway 29 at the Gilbert Airport. The flight instructor and passenger of the 172 sustained serious injuries and the student pilot of the 152 sustained minor injuries. Witnesses said the 152 had been practicing full-stop landings and had remained in the pattern. The 172 had departed Kissimmee, Fla., about 15 minutes before the accident. The witnesses heard the pilot of the 152 announce his left downwind, base and final approach positions on the Unicom. The 172 was observed making a right downwind approach and an abbreviated base to final to runway 29 above the 152. A witness who was in his aircraft at the hold short line attempted to warn the 172 over the Unicom frequency several times but there was no reply. The 172 struck the top of the 152 about 30 feet above the ground, forcing the 152 into the ground. The 172 pitched up, rolled right and nosed into the ground coming to rest on its back. The 152 landed hard on its main gear, and slid to a stop.