This final thesis project is done at the Control Laws EXPERTISE Team at the Auto Flight Control Systems Department of THALES Avionics in Toulouse. The division currently works for both aircraft and helicopter manufacturers.
An autopilot (AP) for use in commercial aircrafts must be certified by national authorities; in the EU this body is named European Aviation Safety Agency (EASA). For landing there are different certification standards.
The first standard is called CAT I which is used when weather conditions prevents the pilot from seeing the runway during the initial approach. The CAT IIIa on the other hand is the standard needed when the weather conditions are so adverse that the pilot cannot see the runway markings until 50 ft over the ground.
This project aims to investigate the possibility to upgrade a given aircraft from a CAT IIIa to a CAT IIIb EASA certification. The CAT IIIb includes the CAT IIIa with an AP controlled runway rollout. The work includes the following:
- System analysis
- Proposition of a control law
- Implementation in non linear Simulink model
- Mock-up: Simulation visualization with synchronized Flight Mode Annunciator (FMA) and Primary Flight Display (PFD)
The primary objective is not to search for the optimal solution for a CAT IIIb upgrade but rather to identify the eventual problems that will be encountered due to system architecture and rollout dynamics.
First a system analysis is done. Here important information about the architecture is found. For example there is no connection between the nose wheel command and the auto flight system, which will be necessary, and some parts in the Fly-By-Wire (FBW) system have to be disconnected during the rollout.
Also important is an analysis of the ground reaction model made by the client i.e. calculation of friction coefficients and forces. Next step is to develop a control law and a linear model for the rollout is therefore made. The absolute speed is supposed to be the rudder and the nose wheel, we obtain a 2nd degree state space model with the yaw rate r [rd/s] and the lateral speed Vy [m/s] (AC – coordinates) states.
Since the rudder only gives effect at relatively high speeds (over ≈100 knots) and the nose wheel due to stability only can be used at low speeds (under ≈50 kn) this can be seen as two different systems and one control law can be developed for each command. The commands are then distributed proportionally between these speeds.
The maximal deviation from the runway for the nominal flight case with 15 knots of crosswind is acceptable. However, it does not converge until rather late in the rollout. Simulations with stronger crosswind and head wind have been made with good results. Wet runways or less landing mass seem to be the most degrading cases.
At touch down a yaw bar appears in the FPD and shows the control target of the AP. The FMA is complemented with the mode rollout during that phase. The connection with the simulator makes it possible for the viewer to understand the relation between the different phases and the AP logics.
The control law is stable for the nominal case and even for different masses and runway conditions. However, when these variables are changed simultaneously unstable behavior appears. The phase just after touchdown should be improved.
The FMA and PFD with proper annunciation synchronized with the flight simulator enable the uninitiated to understand the CAT IIIb operation.
Author: Wåhlstedt, Carl-Adam