According to the flight instructor, he and the student pilot were practicing short field landings. On approach to runway 28, a 5,509 feet-long by 99 feet-wide runway, the airspeed suddenly decreased when the airplane was about 20 feet above the ground. The student pilot applied full engine power, the airplane yawed to the left, and then the airplane impacted the ground prior to the runway. The flight instructor took the flight controls, decreased the engine power to idle, and applied the brakes in order to stop the airplane, but it struck a taxiway sign located on the left side of the runway, resulting in substantial damage to the left wing. The wind reported at the airport about the time of the accident was from 250 degrees at 3 knots. The flight instructor reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.