Accident Overview
An instructional flight ended when the airplane, after requesting priority due to a difficulty, executed a steep right turn from base leg to final approach and then descended vertically to the ground. The airplane exploded on impact and burned. Witnesses observed the steep turn and vertical descent.
Fuel Status
The airplane's main fuel tanks were full (100 gallons usable) at departure for the 1.6-hour instructional flight that preceded the accident flight. The auxiliary fuel tanks were placarded inoperative. Visual inspection after that flight confirmed the auxiliary tanks were half-full. The accident flight was aloft for approximately 1.7 hours.
Radio Communications
According to a tape recording and transcript, the instructor reported entering a right downwind for runway one eight while flying a single-engine pattern. Shortly after, he called the pilot of an airplane ahead and asked, "Can we get priority to the runway? We've got a difficulty."
Aircraft Configuration
Measurements taken after the accident indicated the landing gear was retracted and the flaps were extended 5 degrees. The rudder trim tab was fully deflected to the right when the rudder was aligned with the vertical stabilizer (left rudder). The right aileron trim tab was deflected full up (aileron down, wing up). The elevator trim tab actuator indicated the tab was 15 degrees down (elevator up), which is about half of the total deflection range per the Cessna Aircraft Company.
Engine and Propeller Examination
Disassembly of the engines revealed no anomalies. Components and accessories bench-tested within service limits. Neither propeller was feathered. The left propeller was at the low pitch stop, and the right propeller was just past this position (low power).
Performance Data
According to the airplane's Climb Data and Cruise Performance Charts, each engine consumes between 14.9 and 26.6 gallons per hour at 7,500 feet msl under standard conditions, depending on power setting. During climb between 5,000 and 7,500 feet msl, each engine consumes between 6.2 and 7.5 gph.
Airworthiness Directive
Airworthiness Directive 69-15-09, titled "To Prevent Fuel Starvation During High Angle Descent," mandated compliance with Cessna Service Letter ME69-19, "Minimum Fuel and Power Setting Placards." The service letter stated: "Operation with less than five gallons of fuel in each main tank is prohibited. Useable fuel in each tank is 45 gallons. Maintain power within the green arc during descent."