# A Study of Emergency Aircraft Control During Landing
## 7 SIL Simulations  
### Tracking of Flight Trajectory Changes  
The system performance at a given configuration was evaluated on the grounds of three in-simulator control trials, which included tracking the set flight path angle changes at a constant heading, tracking the set heading changes at a constant flight path angle, as well as tracking a spiral trajectory involving aircraft's simultaneous descent and turning. During the initial simulation trials, the overall conformity between the responses of the simulator-tested system and the Simulink model with separated control channels was established. In a pure pitch manoeuvre, the tested system reached a flight path angle setpoint in a time close to that of the model. As for the heading control, it initially achieved fairly precise setpoint tracking, which improved further into the flight with extended landing gear. Meanwhile, the conducted trials revealed the extent of the cross-coupling issue between the control channels. Beginning with a levelled flight, excessive roll control leads to a rapid drop in lift and lowering of the flight path angle, which induce an aircraft's phugoid motion. The flight path angle control system attempts to compensate for the arising disturbance by increasing the collective component of thrust lever deflection \(\delta\text{T}_{\text{lon}}\). The aircraft gains speed. While coming out of a turn, an increase in lifting force combined with the equivalent airspeed above the equilibrium point results in a significant increase in the flight path angle. The dataset recorded during the trials made it possible to specify the changes to the control system necessary to minimise the effect of roll motion on the change in the aircraft's flight path angle. Thus, the attention was paid to the control loops in the heading control system, while the flight path angle control system was left unchanged. As a first step, the weights of the Q matrix were significantly reduced and the new LQRI controller settings of the roll angle control loop were assigned. This resulted in a much smoother and slower step response of rolling motion. The second adjustment involved a reduction in external loop proportional gain for heading control (from P = 1.7 to 1.1). This action resulted in earlier and less abrupt emergence from a turn, which is a key factor affecting the dynamics of the lateral movement and the associated magnitude of the disturbance in the longitudinal channel. The definitive settings of the heading control system are shown in Table 15c in Appendix C. Measurements recorded during the final flight path and heading tracking trials are provided in Figure 6. Step response characteristics of both longitudinal and lateral control systems under SIL tests, including setpoint values, rise times, settling times, and overshoot are provided in Table 15 in Appendix C.  
Setting changes in the heading control loop had no significant effect on extending the settling time of the system. It was comparable to the time values obtained previously. However, smoother changes to the roll angle resulted in an almost twofold decrease in the flight path angle disturbance magnitude in comparison to the preceding trials. In addition, a significantly smaller heading overshoot was observed during the gear-up flights. The cross-coupling issue is especially apparent while making a sharp turn in the aircraft with the retracted landing gear. As the control output \(\delta\text{T}_{\text{lon}}\) reaches its lower limit during the manoeuver and the position of one of the thrust levers is set to 0, the longitudinal control system is not capable of the rapid airspeed reduction necessary to compensate for the positive fluctuation of the flight path angle. The worst considered scenario of aircraft control using thrust levers is represented by the task of tracking a spiral trajectory (Figure 6c). In this situation, maintaining a negative flight path angle with the landing gear up is impossible.