With flying colours: Pilot performance with colour-coded head-up flight symbology

Highlights

• Colour head-up symbology improves manual flying performance.

• Equivalent colour feedback performance benefits for non-pilot and expert pilots.

• No perceived subjective workload benefit of colour coded feedback.

• Usability preference of colour symbology vs. monochrome symbology.

Abstract

The manipulation of colour in display symbology design has long been recognised as a method to improve operator experience and performance. Recent developments in colour head-up display (HUD) and helmet-mounted display (HMD) technology underline the necessity to understand the human factors considerations of symbology colour coding against conventional monochrome symbology formats. In this low-fidelity desktop human-in-the-loop experiment, the colour of flight symbology on an overlaid symbology set was coded as a redundant cue to indicate the accuracy of professional and non-professional pilots’ flight profile across a range of simulated flight manoeuvres. The main finding of this study was that colour coding flight symbology supported the manual flying performance of both professional and non-professional pilots. Notably, colour-coding of the bank indicator and airspeed tape minimised performance error during turning and altitude change manoeuvres, respectively. The usability of colour coded symbology was also rated higher than the monochrome symbology. We conclude that colour coded HUD/HMD symbology is preferred by the user and may improve performance during low workload manual flying tasks. A fuller understanding of performance and workload effects will require future studies to employ higher workload flying tasks and examine the utility of colour coding within higher fidelity environments.

Introduction

The conscious attentional effort required to locate and scrutinise target information in a busy visual scene can be minimised by pre-attentive processing when the target stimuli differs from noncritical information on a single dimension [1], [2], [3], [4], [5]. When pre-attentive processing is successful in this manner the target should “pop-out” of the display [6]. An example would be the search of a single red item among a set of green distractor items. In this case, the red target pops out and summons attention with minimal interference from the green distractors. When symbology varies along multiple dimensions (e.g. colour and shape), pre-attentive processing may be able to isolate a group of likely target candidates based on one dimension (e.g. colour), but then explicit attentional resources are required to guide attention over the reduced symbology set (e.g. shapes of a specific colour). Evidently, performance will be much slower in this instance since explicit attention is required to scrutinise individual items within the reduced symbology set in serial fashion until the target is located [7], [8], [9]. Nevertheless, performance is still superior to the case where there is no colour coding. Because it is so effective, selection by colour is a common dimension of symbology that has been manipulated in the design of visual displays to improve operator experience and performance [10], [11], [12].

The above principles have been successful applied to the design of cockpit displays to improve the communication of safety critical information for more than 75 years [13]. The Federal Aviation Administration (FAA) advises that in addition to utilising visually distinct colour sets, colours in electronic flight displays must be employed only as a redundant cue and be semantically standardised (FAA Advisory circular: 25-11B [14]). For example, the progression from green to red is commonly used to semantically convey increasing degrees of threat, a potential hazard, safety criticality, or the need for flight crew awareness and/or response. There is strong body evidence that has highlighted that colour can be used to support cognitive functions, improve pilot spatial orientation, enhance accuracy, decision time and workload [13], [15], [16], [17]. In the military domain, colour has been used to support the identification of targets, smoke, flags, signal and navigation lights, and terrain differences [18]. In the commercial world, colour coding has been employed within TCAS modes of head-down navigational displays (ND) to support discrimination between TCAS proximity cautions and alerts. Future avionic applications such as the Airborne Separation Assurance System (ASAS) will utilise colour within head-down Primary Flight Displays (PFD) and ND to support pilots maintain self-assured separation during free-routing operations [19], [20]. Specifically, yellow or orange coloured “no-go” bands placed on the vertical speed tape and heading rose would represent potential conflicts between 3 and 6 min away, respectively.

Head-up displays (HUDs) and Helmet-Mounted Displays (HMDs) allow pilots to see key flight instrumentation on a transparent display whilst maintaining their view of the outside world. To achieve this both technologies optimally superimposed the symbology of the transparent display onto the user’s field of view. This collation of near and far flight information removes the need to look down at the flight instruments, resulting in increased situational awareness and greater precision in aircraft control [21]. HUD and HMD imagery is often restricted to monochrome (green) as a consequence of the single P-53 phosphor that is used to generate the imagery [22]. This results in the omission of information normally provided, or organised, by colour coding. However, recent advancements in waveguide optical technology means that the development of colour HUDs could be viable in the near future [23]. The display technology in HMDs is different and the development and design of colour displays has matured further in comparison to HUDs. For example, the United Force Airforce (USAF) has addressed several relevant HMD visual processing issues such as the appropriate luminance contrasts ratios for a colour HMD [24], [25]. Nonetheless, due to their complexity and high cost, colour HUDs and HMDs have been late in development [26]. Consequently, the related human factors considerations of colour have been largely ignored. This is reflected in the absence of specific colour guidance from the FAA regarding the presentation of information on HUDs (FAA Advisory circular: 25-11B (Federal Aviation Administration, 2014)). However, these factors have not decreased their desirability to the user [16]. With the possibility to develop more visually distinct and complex head-up and head-mounted display systems in the near future an understanding of relevant human factors has become more urgent.

Several studies have confirmed the positive impact of colour on flight performance and operations. DeMars (1975) concluded that, for certain applications, colour enhanced accuracy, decision time, and workload capability. In a study by Derefeldt et al. [27], an upgraded military colour coded head-down display was discovered to provide more target search and tracking advantages than the earlier monochrome display. Furthermore, the colour displays reduced reaction times and helped pilots to see the grouping of information on the display. Similarly, colour-coding weapon symbology of military pilot HMDs can reduce missile release time without sacrificing probability of kill [28]. Conversely, Dudfield [29], [36] found that the performance benefits of colour-coded flight symbology on a HUD far outweighed its perceived importance. However, the pilots in Dudfield’s study noted that the difficultly of the employed task, maintenance of a straight-level profile, was not sufficiently challenging.

The intention of the current study is to evaluate the performance and workload benefits of a colour coded head-up flight symbology set. Rather than creating a physical HUD or HMD platform we decided to present an artificial overlay on a computer screen that would in essence create an “artificial HUD”. This of course creates an offset in terms of visual acuity and human performance, but the primary focus of this study was to examine the cognitive effects of colour (and not assess the focal demands of the display). Manual flying performance and subjective workload of professional commercial pilots and non-professional pilots was examined in response to flying with a redundantly colour coded flight symbology set across a range of low-fidelity simulated flight trials. Similar to Dudfield (1991), symbology colour coding cues were based on economy so that colour was used only when participants flew outside pre-determined boundaries, e.g. flying off course, providing the subject with immediate feedback on the accuracy of their performance. However, we expanded on Dudfield’s study by evaluating the use of colour feedback across several flight manoeuvre types (ranging in complexity), not just straight-and-level flight. The use of colour in display design has been used frequently to facilitate learning [30], the inclusion of inexperienced participants in the current study was to determine whether the availability of the colour-coded redundant information served a cognitive purpose beyond facilitating the learning process in a novice group. In addition, subjective measures of workload and usability were measured via the NASA Task Load Index (TLX) and the Post-Study System Usability Questionnaire (PSSUQ), respectively.