On June 27, 2006, about 1120 Pacific daylight time, a Beech 76, N6720T, experienced a left main landing gear collapse during landing rollout at Gillespie Field, El Cajon, California. Scandinavian Aviation Academy was operating the airplane under the provisions of 14 CFR Part 91. The certified flight instructor (CFI), and the private pilot undergoing instruction (PUI), were not injured; the airplane sustained substantial damage. The local instructional flight departed Gillespie Field about 1020. Visual meteorological conditions prevailed, and no flight plan had been filed. The CFI submitted a written report. After departing Gillespie Field and performing maneuvers in the east practice area, the flight instructor and student proceeded to Brown Field Municipal to practice touch-and-goes. As they approached the airport traffic pattern, the student pilot moved the gear selector to the down position. Once the gear was down they noticed that the left main gear down light was not illuminated. They decided to recycle the landing gear, but still received the same indication. At this point the instructor took control of the airplane. The instructor asked the tower if they could do a low approach to verify that the gear was in the down position. The controller confirmed that the gear appeared to be down. The instructor decided to return to Gillespie Field. While en route, the instructor and student inspected the light bulbs of the indicator light and found them to be operating properly. They decided to recycle the gear one more time. While doing this, the gear motor circuit breaker popped, and they smelled smoke in the cockpit. They pulled the landing gear circuit breakers immediately and the smoke smell cleared. The pilots extended the landing gear using the manual extension checklist. Once the gear came down, they still had the same unsafe gear indication. The instructor attempted to lock the gear by yawing the airplane, but received the same unsafe gear indication. Upon arriving at Gillespie, the instructor asked for another gear confirmation from the tower. The controller said the gear appeared to be down. The instructor asked for a full stop landing on runway 27R. He executed the landing using a soft-field landing technique and the airplane touched down without incident. As the airplane decelerated, the left main landing gear collapsed and the left wing tip and the propeller struck the runway. They pulled the mixtures to idle as soon as they realized the gear was failing in an attempt to stop the propeller rotation before impact. The wing and propeller hit the ground before the engine shut down. The airplane turned left 45 degrees before stopping. They secured the airplane and exited. Emergency crews were standing by as the pilot had requested. A Federal Aviaition Administration inspector was present when maintenance personnel inspected the main landing gear. They found the left main landing gear "A" frame tube had failed at the lubrication point. Airworthiness Directive 97-06-10 requires repetitive inspection of the main landing gear "A" frame assemblies for cracks on Raytheon Aircraft Company Model 76 airplanes, and replacing any assembly found cracked. The actions specified in this AD are intended to prevent main landing gear failure because of a cracked "A" frame assembly, which could result in a loss of control of the airplane during landing operations. A review on the aircraft records indicated that AD 97-06-10 had been complied with during the last 100-hour inspection on June 21, 2006. This aircraft is operated by Scandinavian Aviation Academy, which is a flight school. The aircraft maintenance records indicated that the airplane is operated at an activity level where a 100-hour inspection is accomplished every 30 days or slightly less.