Before the change
Generally, pilot actions are divided into:
1. manual control;
3. decision making.
When developing flight control system algorithms, synthesizing director signals for the display, choosing the characteristics and type of the control sticks, and developing flying qualities evaluation criteria, manual control by the pilot is being considered. In this mode, the pilot does not interfere periodically but rather constantly reacts, deflecting the control sticks in response to the perceived visual, motion, and proprioceptive cues. That is, the pilot’s interaction with the aircraft in this mode is not divided into stages prior to and after an intervention. There exists a constant interaction with the aircraft or, according to your terminology, intervention. The figure below presents a general diagram of the pilot’s interaction with other elements of the pilot-aircraft system in manual control.
These elements include the controlled element dynamics (the aircraft + flight control system), the display, and the manipulator. In case the piloting task changes, all of these so-called task variables also change, as well as the input signal i(t) and the disturbances d(t). For example, during the approach, the controlled element dynamics is described by altitude response, and by pitch response during the flare. The mathematical models defining the changes in these state variables and the displayed signals perceived by the pilot are different.
The researches carried out back in the 1950s and 1960s showed that the pilot adapts to different task variables, trying to keep the open and closed-loop pilot-aircraft system characteristics approximately the same. It requires the pilot to perform compensation actions, which ultimately impairs pilot subjective rating, accuracy of flying, and, as a result, flight safety. Consequently, the goal of an engineer developing a flight control system and other system elements is to ensure the easiest possible type of pilot’s action that does not require excessive corrective actions.
In the almost 70-year history of research into the pilot-aircraft system, methods for experimental research have been developed for the system, pilot behavior regularities have been studied, and mathematical models of his behavior, sensory and central nervous systems have been developed. These results have been used successfully in designing flight control systems of previous and current-generation aircraft.
From the interview regarding the “New active manipulator inceptor to improve accuracy and safety of flight” before the HF intervention:
This investigation started 3 years ago. It combined the flight control system with the manipulator from the pilot’s point of view. Currently Aircraft manufacturers use a side stick (with Airbus pilots using a small side-stick) where the output signal is equal to the forces exerted on the side stick and not to the deflection. And so the manipulator of the sidestick is proportional to the displacement of the aircraft.
After the change
In case of a poor choice of parameters of the of the pilot-aircraft system’s technical component, hazardous events may occur. Among such events are pilot induced oscillations (PIO), which are well-known among Russian and international experts in the field.
Since the 1990s they have also been referred to as aircraft-pilot coupling. This phenomenon has various causes. Due to the emergence of highly-automated aircraft with a lowered stability margin, the primary causes are the restricted maximum rate of the control surface deflection, performed by the actuators, and a significant phase delay in the control element dynamics of modern aircraft caused by various factors.
Seeking to perform the piloting task accurately, the pilot generates significant lead compensation, which can rapidly cause diverging oscillations. This kind of occurrence is observed in both civil and military aircraft. According to the statistics at my disposal, the number of incidents/accidents in civil aircraft has increased by 15 % due to PIO.
From the interview regarding the “New active manipulator inceptor to improve accuracy and safety of flight” after the HF intervention:
With the new active manipulator, the inceptor’s use does not depend on the force exerted on it but rather on the deflection. This is important because in the case of failure, the dynamics of the aircraft can change drastically and so avoids pilot’s need to apply excessive force. Therefore, this new active manipulator increased safety.
How and Why
In light of this, an extremely important task of decreasing the risk of such unfavorable interaction has arisen. A wide array of works is devoted to this matter. In particular, McRuer’s «Aviation safety and pilot control. Understanding and preventing unfavorable pilot-vehicle interactions», published in 1997 by the National Research Council, presents a detailed analysis of the phenomenon and an engineering approach to its suppression.
One of the means of PIO suppression is installing nonlinear prefilters in the flight control system that do not let rapid signals into the system.
We have proposed a modification of one such prefilter, which also controls the gain coefficient from the feedback signals.
In figure below (from my presentation at the 1995 AGARD conference) you can find the structure of these filters. The use of the proposed prefilter has allowed to decrease the resonance peaks of the closed-loop system, therefore lowering the probability of PIO occurrence.
Another means of suppressing the PIO effect is an active (dynamic) change in stick control force. This method, the elementary diagram of which is presented in the figure below, also allows to decrease the pilot-input lead, increase the amplitude margin of the open-loop system and, consequently, decrease the resonance peak of the closed-loop system, therefore lowering the PIO tendency.
In recent years we have worked a lot on researching new types of predictive displays which also serve as a means of improving the interaction between the pilot and aircraft. See the article on the matter, titled «Development of Pilot Mathematical Model in the Preview Manual Control Task».
We also used a mathematical model of the neuro-muscular system to understand the output in relation to the deflection of the hand on the stick (manipulator), where the output is the forces that the pilot can place on the manipulator.
Additionally, by combining the Flight Control System with the new active manipulator, there have been massive improvements. In the case of failure of the flight control system, we are able to change the stiffness of the inceptor because suddenly the pilot system becomes unstable and because the acceleration increases considerably. The inceptor has the mass and the steepness so with the help of the mathematical model of the neuro-muscular system, it is possible to change all of the parameters on the device.
By combining the Flight Control System with the new active manipulator/ inceptor the percentage of accidents has decreased by 15% in case of failure of the flight control system.