Flying qualities reduction of fly-by-wire commercial aircraft with reconfiguration flight control laws

As to guarantee flight safety when system failures occur, modern fly-by-wire flight control systems for commercial aircraft incorporate reconfiguration control modes. While relaxed static stabilities (RSS) are utilized on most new designed fly-by-wire airliners, the flying qualities design become a major concern when reconfiguration flight control laws are developed. The aim of this paper is to analyze the reduction of flying qualities when reconfiguration laws are activated. Longitudinal and lateral-directional control laws, along with their alternative and direct modes, were developed for a commercial aircraft with relaxed longitudinal and directional static stabilities. Flying qualities assessment with reconfiguration laws were carried out based on the handling qualities rating method (HQRM), which has certification levels connected to military specifications such as MIL-STD-1797A. The analysis indicated that short period mode and Dutch roll mode are most affected in the downgrade of flight control law. It is suggested that qualities of these modes should be guaranteed as priority when reconfiguration laws are designed.