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Sanjay S. Vakil, Alan H. Midkiff, R. John Hansman
Aeronautical Systems Laboratory Department of Aeronautics & Astronautics Massachusetts Institute of Technology Cambridge, Massachusetts USA June 1996 ASL-96-2



Chapter 1 Introduction
Current advanced commercial transport aircraft, such as the Boeing B777/B747-400, the Airbus A320/A340 and the McDonnell Douglas MD-11, rely on AutoFlight Systems (AFS) for flight management, navigation and inner loop control. These systems have evolved from straightforward autopilots into multiple computers capable ofsophisticated and interrelated tasks. These tasks span the range from high level flight management to low level inner loop control. In addition, these systems provide envelope protection to prevent pilots from committing mistakes such as stalling the aircraft or lowering flaps at high speeds. Unfortunately, as these systems have become more complex and interconnected, a new class of problems hasdeveloped associated with pilots’ interaction with the automation. Many incidents have been reported where there exists some confusion between the pilots’ expectations of the AFS and what the system is actually doing (Corwin, 1995). This confusion has been termed a Mode Awareness Problem (MAP). After a description of the AFS, a formal definition of mode awareness problems is presented in Section 1.3followed by representative incidents in which mode awareness problems are suspected as being a contributory factor.


AutoFlight System Description
The AFS in modern aircraft has three distinct mechanisms which correspond loosely to the

time constants associated with the differing types of control. At the lowest level are a set of inner loop controllers, especially in Fly By Wire (FBW)systems, which improve the handling


characteristics of the aircraft by modifying the dynamics to which the pilot is exposed and provide aircraft attitude control. Above these are a set of autopilots (A/P) which command the aircraft to a specific state by providing inputs to the lower level controllers, much as a pilot would do manually. Finally, the Flight Management Computer (FMC) is a highlevel planning tool which can be programmed by the pilot to fly a complex predefined trajectory. These levels are discussed in more detail below. 1.1.1 Fly By Wire and Inner Loop Controllers Inner loop controllers fly the aircraft to a target flight attitude. For example, if an external disturbance caused an aircraft to roll, the controller would return the aircraft to a level state, but would notcorrect for the integrated effects of the roll, such as a change in heading, or a loss of altitude. In a similar manner, the controller which maintains the aircraft pitch would compensate for a disturbance causing the pitch to change, but would not return the aircraft to the commanded altitude. In addition, these controllers also perform a secondary role in Fly By Wire (FBW) aircraft, such as theAirbus A320/A340. Even the most basic flight control devices, such as the yoke (or side stick), pedals and throttle do not provide raw inputs directly to the control surfaces in FBW aircraft. Instead, the inputs are filtered and modified in a manner designed to provide a consistent set of dynamics to the pilot. This is intended to improve the safety of the aircraft by providing consistent responses topilot inputs (Fishbein, 1995). Even in aircraft which are not FBW, a set of low level inner loop systems may exist to control certain aircraft modes. The most well known of these is the yaw damper that is used on commercial jet transports (Weiner, 1988). Other aircraft have systems which modify input from


the standard control mechanisms to allow more flexibility in flight. For example,...