Primary flight control inceptor being an important interface of a pilot with the aircraft, it is a best tool to estimate pilot workload, and to understand, ‘how hard the pilot is working on the control stick’. Workload Buildup Flight Test Technique (WB FTT) is hypothesized to systematically elevate pilot gain for HQ stress testing. Objective   is to investigate the relationship ofPilot Inceptor Workload (PIW) and pilot workload and its relationship with WBFTT for aircraft HQ Evaluation. The statistical model of the present research involved three independent-variables, each affecting the pilot workload in different characteristics. Two studies were undertaken on a fixed-base variable-stability HQ research flight simulator with military test pilots. The inflight studies were undertaken in an Advanced Jet Aircraft, from target tracking during Air-to-Ground dive attacks, to investigate pilot inceptor movement in high pilot gain tasks. The results of simulator studies validated the relationships of PIW and WBFTT with statistical significance from a wide set of data of 20 Test Pilots. The inflight studies validated the effects of high pilot gain and proximity to ground (boundary), on the pilot inceptor movements, in training combat manoeuvres. Studies manifested PIW as a simple and direct measure to estimate pilot workload, to address the need to have an analytical approach of HQ Evaluation and supplement the subjective method of pilot opinion rating.

An important aspect of flight controls system design and flight testing of piloted aircraft is satisfactory Handling Qualities (HQ). Design challenges in achieving good HQ of an aircraft include varying degrees of requirements. Cooper-Harper Rating (CHR) (Cooper [1984]) is a subjective pilot opinion rating scale that is being widely used for HQ evaluation of a piloted aircraft. Awarding of CHR is based on success in task performance (precision) and pilot workload (ease) demanded to achieve the required performance (Cooper [1984]). The pilot control inceptor is the most important interface between pilots and aircraft for piloting the aircraft. Hence, it is prudent to say that pilot workload as perceived by the pilot can be studied based on the control inceptor movements. Pilot Inceptor Workload (PIW) defines the pilot workload expended in achieving the aircraft control through primary flight control inceptor, which is also addressed as a control stick or wheel. PIW measures are simple to be correlated by pilots in terms of workload perceived qualitatively during a flight control task.

The present research involves the investigation of PIW during flight control tasks in a simulator environment and as well as inflight. In this study, the Aircraft FQ level is considered as one of the factors to study the dependent variables of PIW, investigated during the research, to vary the pilot workload. The success in achieving secondary mission tasks by a pilot while performing the primary flight control task will depend on spare mental capacity available to the pilot. Hence, if the primary flight control task is very demanding, which is likely to be so, in the case of an aircraft with degraded FQ, then there would be very little spare mental capacity available to the pilot for performing secondary mission tasks. Hence, secondary task conditions, which is categorised in this research as Task Difficulty (TD) levels, is considered as the second factor to investigate the dependent variables of PIW in two different conditions of pilot workload. Workload Buildup Flight Test Technique (WB FTT) is a flight test technique based on Boundary Avoidance Tracking (BAT) theory (Gray [2008]). BAT theory hypothesizes that the closing and opposing boundaries imposed during the flight control of the aircraft affect the pilot’s state (Gray [2004]) and hence the pilot workload. Hence, ‘Boundary Size (BS) levels’ which defines the boundaries imposed during the simulator visual task, is considered as the third factor against the dependent variables of PIW investigated in the study, which systematically increases pilot workload.

In nutshell, PIW is investigated in the present research, against three factors of pilot workload, namely, aircraft FQ levels, secondary Tasks Difficulty (TD) levels and closing – opposing Boundary Size (BS) levels.

Two simulator studies were undertaken, to investigate PIW against the three factors, in a high-end variable stability HQ research simulator. The experiments were simulated under ideal conditions, i.e, without air traffic controller communication, additional aircraft traffic and adverse weather conditions.   Finally, an Inflight study was undertaken to investigate the pilot inceptor movement during high precision ground target tracking tasks. The study design, aircraft details and methods of inflight study are explained in the Inflight Study section.

The simulator studies showed that pilots’ inceptor workload is affected by pilots’ flying strategy during high workload tasks, involving poor aircraft FQ, secondary mission tasks and closing – opposing boundaries in a simulator environment.  The subsequent question was, “Can similar results be obtained in a real world flying conditions during high pilot gain tasks of Category A flight phase”. The present section describes an inflight study undertaken in BAES Hawk aircraft undergoing flight profiles including all the three categories of flight phases, in particular, air to ground dive attacks training missions, which is a representative combat mission under the category A flight phase, comprising the effects of proximity to ground, as a boundary imposed to pilot. A test flight was undertaken in Hawk aircraft while undertaking routine training operational tasks. Test Flight comprised of seven Air to Ground Attack tasks to investigate the closing boundary effects on pilot gain.

2D – PIW Plots.        Although Dutycycle and Aggressiveness as individual metric represent different characteristics of the pilot inceptor movements characteristics, together represented as PIW, can be investigated in an X-Y plane 2D Plot as shown in Figure (Gray [2008]). The theoretical graph of PIW placed as Figure, is self-explanatory in terms of interpretations of the 2D PIW plot as different regions mentioned in the corners of the plot. The PIW results of the simulator studies of 20, are plotted in an X-Y plane 2D Plots are placed BELOW. Further, for better interpretation of results the first and third quartile values of both Dutycycle and Aggressiveness, respectively, are represented as straight lines in the plot. The quartile lines divide the 2D plot into nine salient rectangular regions which can be used to interpret and categorise the inceptor control strategy of each pilot. The pilots inceptor control strategy can be predominantly categorised into three categories based on, following the trendlines of PIW from bottom left corner to top right corner of both the plot, as low PIW, medium PIW, and High PIW. Further, it is interesting to note that, the values of Dutycycle and Aggressiveness of preliminary study are substantially lesser than that of the final study and the reason for the same is attributed to the difference in modal parameters between the SoS signals used in the two studies. The frequency and the RMS speed of the inceptor movements are directly related to the frequency and the amplitude of the target movement. Hence, it can be concluded that the target command SoS signals have significant effects on the results of the PIW.

Figure 1 .  PIW in terms of Dutycycle and Aggressiveness (Gray [2008])

WB FTT which is based on BAT theory systematically elevates the affected state of the pilot while undertaking a tracking task. Both studies confirmed that the WB FTT can systematically increase PIW further that pilot inceptor workload increases after boundary awareness. Aircraft FQ was found to have effects on the boundary effects on the pilot; Poor HQ aircraft was having lesser effects due to the presence of boundaries. It can be explained as due to the increased oscillations of poor FQ, because of which the pilot had to entirely focus on controlling the aircraft to reduce the tracking error, and hence the boundary awareness could not take place throughout the task. Hence, WB FTT will not be effective in the case of poor HQ aircraft, to increase PIW.

Figure 2 . Plot of Simulator Study Results

The experiments concerning this research were undertaken in a buildup approach comprising two sets of simulator experiments in an HQ research simulator. An incremental approach was followed in terms of increasing order of pilot sample size and number of cases considered for each set of simulator experiments. The present research with the support of a comprehensive set of pilot-in-the-loop test data, more accurately characterise the Pilot inceptor workload, BAT theory, WB FTT and their relationship with aircraft flying qualities and pilot workload due to secondary tasks. The results and outcome of this research are believed to have answered the research questions and also have opened new questions in Handling Qualities Evaluation conducted on aircraft. The research on handling qualities of an aircraft is considered to be a need of the day and is believed to provide great impetus to the design and development of modern flight control systems.

Author: Wg Cdr M Dilli Babu

Link for the presentation Video :

Link to the paper: 10.13140/RG.2.2.25311.69289


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