Pan American Airways Flight 428 taxied to Havana’s runway 05 with instructions to hold at the intersection while another aircraft was landing. On arrival at the intersection the engines were run up and the pre-flight check accomplished. At this time when operating the right engine on the right magneto there was fluctuation of 30 BMEP (Brake Mean Effective Pressure), and the engine backfired several times. The engine was operated at 30 inches of manifold pressure until the cylinder head temperature increased from 130 to 200 degrees Centigrade. At this temperature the magnetos were tested again and the engines were operating within normal limits. When cleared for takeoff, the first officer started the takeoff run. At an air speed of approximately 95 knots, back pressure was applied to the controls to take off. The captain, sensing an unusual vibration at this time, took over control, closed the throttles and applied the brakes. The aircraft had progressed down the runway about 2,600 feet when the brakes were applied. The left outboard and the right inboard tires blew out within a short distance. As the speed of the aircraft was still fairly high, the captain used the emergency air brakes. However, the aircraft rolled off the end of the paved runway, continued 300 feet to the boundary of the airport where it struck a ditch and skidded to a stop 315 feet beyond the airport boundary. PROBABLE CAUSE: “The Board determines that the probable cause of this accident was inability to stop the aircraft under marginal conditions of stopping distance because of inadequate braking effectiveness.”