FLY-BY-WIRE AUGMENTED MANUAL CONTROL - BASIC DESIGN CONSIDERATIONS Dominik Niedermeier∗ , Anthony A. Lambregts∗∗ ∗Deutsches Zentrum für Luft- und Raumfahrt e.V., ∗∗Federal Aviation Administration [email protected]; [email protected] Keywords: Fly-by-Wire, Manual augmented control, C∗ control algorithm, C∗U control algorithm Abstract manufacturer allowing reduced training re- quirements After the introduction of Fly-by-Wire (FBW) flight control in the late eighties, several thousands of com- • Weight and maintenance effort reduction by re- mercial FBW aircraft are in service today. Look- placing mechanical components (e.g. pulleys, ing back, it can be concluded that FBW has brought cables) an impressive improvement to handling qualities and • Introduction of flight envelope protection func- flight safety. However, after the long time of experi- tions for enhanced safety ence with the first generations of FBW aircraft, open questions and issues with respect to the design and • Introduction of maneuver/gust load alleviation the operation of FBW have been revealed. The main functions objective of this paper is to discuss those issues and to • Optimization of aerodynamic performance (e.g. point out further research needs in this field. The char- drag reduction by aft-shifted center of gravity) acteristics of manual augmented pitch control algo- rithms implemented in commercial FBW aircraft are Manufacturer Current Models No. of deliveries discussed in detail. Subsequently the paper describes Boeing B777 1003 particular features of Airbus and Boeing FBW control B787 8 systems that are not directly related to the chosen con- Airbus A318/319/320/321 5022 trol algorithm. Open questions and resulting research A330/340 1225 needs with special focus on recent incidents and acci- A380 71 Embraer E-170/190 802 dents are discussed. Antonov An-148 16 Iljushin IL-96 26 1 Introduction Tupolev TU-204/214 74 Suchoi SSJ 100 7 Over 20 years after the introduction of the first digital Total 8106 FBW control system in commercial aircraft by Air- Table 1 Commercial FBW aircraft: No. of Deliveries bus in the late eighties, the trend to replace mechan- ical controls by electrical connections has become ir- Looking back, it can be concluded that FBW de- reversible. Today commercial FBW aircraft from six signs have been very successful and have accumulated different manufacturers, being Airbus, Boeing, Em- a flight safety record as good as, or better than the me- braer, Iljuschin, Tupolev, Suchoi and Antonov, are in chanical flight control systems of the previous aircraft service. The vast majority of them has been delivered generation. Especially with respect to aerodynamic by Boeing, Airbus and Embraer (see Table 1). stall accidents the introduction of FBW control and The main benefits of FBW control leading to its wide the related envelope protection functions have saved application include: a lot of lives. According to [14] there have been 27 stall accidents in transport operations with 848 fatali- • Standardization of control handling qualities ties between 1993-2008. None of those accidents in- between different airplane models of the same volved a FBW aircraft. However, shortly after the 1 DOMINIK NIEDERMEIER, ANTHONY A. LAMBREGTS publication of this statistics two stall accidents with safety" features, e.g. envelope protection. FBW aircraft occurred, but in both cases the envelope This type of control is implemented in Airbus protection function was not working properly due to and Boeing FBW aircraft. sensor malfunctions. Although FBW design has reduced the mechanical To reduce perceived program risks, development system complexity and maintenance effort, the appli- costs and maintain continuity of the product character- cation of FBW control has increased the functional istics, an evolutionary design approach has generally complexity of the Flight Guidance and Control Sys- been used - punctuated by radical exceptions often at tem due to additional control features, necessary mon- the time of first product offering. Airbus chose a FBW itoring functions and reversionary modes to provide design approach that creates an unconventional air- continuity of essential control functions under various craft behavior by providing a pseudo trajectory hold failure conditions. Each reversionary mode tends to feature when the pilot leaves the control loop. The have its own characteristics putting a higher level of resulting loss of classical speed stability led to the in- demand on flight crew proficiency, compared to pre- troduction of envelope protection functions to achieve vious airplane generations, to correctly handle the op- equivalent safety required for certification [7]. In con- erating scenarios for all possible failure conditions. trast, when Boeing introduced FBW control, it was Every aircraft manufacturer has developed its own decided to maintain operational consistency with the distinct design solution. Two general types of FBW previous mechanically controlled airplanes. flight control system design approaches are being The different design concepts introduced by Airbus used today: and Boeing for their first time on the Airbus A320 and the Boeing B777, respectively, were maintained on 1. Simple FBW designs with little or no stability their subsequent FBW aircraft types until today. Gen- augmentation: The traditional mechanical sig- erally aircraft manufacturers tend to keep the design naling from the pilot’s control inceptor to the choices made for their first FBW airplane model in control surfaces is replaced with electrical sig- order to maintain commonality of the handling char- naling, possibly including gain scheduling and acteristics of the subsequent FBW aircraft types. As feedback of sensor output variables that have a result, many FBW design issues continue to be the associated stability derivatives. Classical air- subject of intense debates, including plane dynamic modes and stability character- istics are maintained so that the airplane can • Control inceptor types: column wheel versus be certified under existing airworthiness regu- sidesticks; active versus passive inceptor feel lations. This type of FBW control is implemented in the • Manual augmented control algorithm types and Embraer E-170/190. the resulting aircraft handling characteristics 2. Complex FBW controller designs using non- • Application of envelope protection functions to classical stability/command augmentation: regain equivalent safety to compensate for the Typically, such designs include elements of the loss of classical speed stability and to reduce the type 1 design, as well as other design features, probability of exceeding the envelope bound- possibly including feedbacks of sensor output aries and of Loss of Control (LOC) variables that have no associated stability derivatives (e.g. attitudes or accelerations). No consensus between the different aircraft man- This type of design may also include dynamic ufacturers has emerged on these design issues. Ad- elements (filters) in the feedforward and ditionally, although FBW aircraft are extremely safe, feedback signal paths that result in an "higher incidents and accidents involving FBW aircraft from order controller", which typically alters the different manufacturers have revealed further issues fundamental airplane dynamic modes and related to the design and the operation of FBW con- airplane response to pilot control input. Such trol. Many of the issues result from the specific char- designs may not meet the basic certification acteristics of FBW aircraft and, except of Pilot In- requirement for stability and long term (trim) duced Oscillations (PIO), they do not have a counter- behavior and therefore may need "equivalent part in non FBW airplanes [20]. 2 FBW Augmented Manual Control - Basic Design Considerations This paper describes different manual augmented con- behavior of each algorithm, because its effect on the trol design approaches of commercial aircraft in detail long-term response of key airplane state variables, e.g. and discusses open questions related to the particular such as pitch attitude, airspeed or flight path angle, at design. The main focus will be put on the FBW con- constant thrust with the control inceptor at neutral is trol system design of type 2 which are implemented in at least equally important. This behavior is mainly Boeing and Airbus aircraft. Questions such as influenced by the design of the integral signal path. Depending on the control algorithm the aircraft may • How is the piloting task affected by a particular return to its trimmed airspeed, maintain pitch attitude, manual augmented control algorithm? vertical speed or flight path angle. This behavior significantly affects the pilot’s control • What is the impact of particular features of technique and monitoring task. Consequently, in this existing FBW systems on the pilot-aircraft- paper the distinction between existing control algo- interaction? rithms is made with respect to their long-term re- are discussed with respect to particular design charac- sponses at constant thrust without any pilot control teristics. Furthermore, relevant incidents or accidents input. The objective of this section is to describe the are presented. general characteristics of the control algorithm rather than dealing with the specific design chosen by the 2 Characteristics of Current Control Algorithms corresponding aircraft manufacturer. 2.1 The C∗ Control Algorithm Often the characteristics of a specific control algo- rithm are classified by their