Aircraft Autopilot Analysis and Envelope Protection for Operation Under Icing Conditions

Abstract

The behavior of a typical autopilot structure is studied for flight under icing conditions. The study is based on aT win Otter aircraft model and focuses on the pitch attitude behavior. A quadratic stability analysis using linear matrix inequalities is conducted to show that the time-varying closed-loop system maintains quadratic stability under icing conditions. In addition, the problem of envelope protection in the presence of icing is considered; in particular, how to maintain the angle-of-attack within the stall limits which are time varying. Based on steady-state behavior, a practical envelope protection scheme is developed and tested via simulation. I. Introduction A IRCRAFT accidents continue to occur due to the formation of ice on aircraft in flight. The development of safer, more reliable and affordable aircraft must include better solutions for flight in icing and bad weather conditions. The primary cause of these accidents is the effect of ice accretion on the performance, stability, and control of the aircraft. 1 Accidents occur when aircraft are not properly protected against ice accretion either on the ground or in flight. As mentioned in Ref. 1, this situation may result from an inadequate ice protection system or aircraft operation outside of the iced aircraft flight envelope. Icing accidents can be prevented in two ways: 1) icing conditions can be avoided, and 2) the aircraft system can be designed and operated in an ice-tolerant manner. For all aircraft, ice avoidance is the desired goal for increased safety. However, for commercial aircraft, where revenues and schedules must be maintained, ice tolerance will continue to be the preferred method for all but the most severe icing conditions. Currently a project on developing smart icing systems (SIS) to deal with this problem of icing is being carried out by a group of researchers at the University of Illinois. 2 The goal of the SIS research is to improve the safety of operations in icing conditions. This paper is a result of this SIS research. It studies the effects of ice accretion on the autopilot stability and performance and also develops techniques for envelope protection (EP) with the autopilot in operation under icing conditions. Current practice recommendations dictate that the autopilot system be shut down immediately when ice accretion is reported during the flight. The research reported here aims at taking a step in the direction of developing an autopilot system that is reliable and robust to icing conditions. This autopilot system will be tested by integrating it into a high-fidelity real-time simulator platform, the Icing Encounter Flight Simulator (IEFS), 3 currently being developed at the University of Illinois.