9.2 Propulsion Control of Aircraft: A Case Study in Systems Engineering

Abstract

ABSTRACTOn July 19, 1989 United Airlines Flight 232 was cruising at approximately 37,000 feet between Denver and Minneapolis when it experienced a catastrophic failure of the number two (tail‐mounted) engine. At the moment of failure the engine ejected shrapnel through the tail section, destroying all three on‐board hydraulic systems. The complete loss of hydraulics left the pilots of the McDonnell Douglas DC‐10 aircraft without the ability to move the flight controls. In contrast to what most aviation experts would have expected as an outcome, the airplane was crash‐landed at Sioux City, Iowa with 185 of the 296 passengers and crew surviving. The flight crew was able to maintain rudimentary control of the aircraft with only the use of thrust from the two remaining wing‐mounted engines by independently moving their respective power levers. It was not until the final touchdown that the crew's command of the aircraft was lost due to the imprecise nature of manual propulsion control (Propulsion Controlled Aircraft [PCA], 1999).Following the accident, the National Transportation Safety Board (NTSB) recommended that Propulsion Control of Aircraft (PCA) be evaluated as a “pilot friendly” backup system using engine thrust as the only actuating mechanism in the case of primary flight control failure. This paper examines the use of systems engineering to effectively decompose the problem, identify the solution set, and integrate the solution with the potential for improved safety and reduced life cycle costs. An overview of aircraft flight dynamics and primary control systems is provided in order to understand the basic system properties. Analysis includes physical systems as well as software systems and their interface. Finally, the use of simulation to better understand system behavior and mitigate risk while arriving at a system‐based solution is examined (Blanchard, 1998). Control the aircraft in flight after a primary flight control failure and accomplish that with an interface which is intuitive to the pilot, that is the problem. The solution could only have been reached through systems engineering.