Flight-Propulsion Response Requirements for Directional Stability and Control

The vertical stabilizer is a primary aerodynamic surface responsible for providing directional stability of an aircraft. Directional stability is further augmented by the yaw damping capability of the rudder using feedback control. When the rudder experiences faults or the vertical stabilizer is damaged, directional stability of an aircraft will be compromised. In the worst-case scenario, the vertical stabilizer would become completely separated that would cause a loss of control. While rare, this occurrence was a causal factor for the crash of the American Flight 587 in 2001.1 In an event that directional stability of an aircraft is compromised, aircraft engines can potentially be used in an autopilot mode to regain aircraft stability. The use of engine differential thrust for directional control has been demonstrated by the United Flight 132 accident in Sioux City, Iowa, in 19892 and the DHL incident in Baghdad, Iraq, in 2003.3 As a control actuator, aircraft engines are generally slower than the conventional flight control actuators, and therefore may not be able to respond quickly to changes in aircraft dynamics in order to maintain aircraft stability. In order to integrate aircraft engines into a flight control systems, aircraft engines would need to have a faster response time as well as additional thrust capability. This study examines the engine response time requirements in severe damage situations associated with the vertical stabilizer of a generic transport aircraft. The results of the study could be used to assess requirements for engine design for fast response in an emergency situation.