Symbology development for a 3D conformal synthetic vision helmet-mounted display for helicopter operations in degraded visual environment

Kurzfassung

Degraded visual environment (DVE) is still one of the most challenging and dangerous situations for helicopter pilots. When operating in adverse weather with very poor or no out-the-cockpit visibility spatial disorientation is likely to occur, severely compromising flight safety. However, not only poor weather conditions but also surface properties can contribute to a loss of orientation. When operating in desert environment impaired visibility is caused by sand or dust being stirred up by the helicopter’s rotor downwash during landing or take-off. This situation is referred to as brownout. Especially in the military field brownout is responsible for multiple incidents and accidents during desert operations. According to the NATO report on Rotary-Wing Brownout Mitigation (2012) it is responsible for about 75% of helicopter mishaps within NATO member countries and the largest cause of rotary-wing airframe loss in the US services alone. So far many different proposals have been published about the design of display contents for helicopter pilot assistance during DVE landing. Currently the state-of-the-art symbology sets combine a 2D-forward and a bird’s-eye view presented on either a multicolor head-down or a monochrome helmet-mounted display. Within the project ALLFlight the German Aerospace Center (DLR) Institute of Flight Guidance is working on diminishing the problem of piloting helicopters in DVE conditions. In contrast to the already existing 2D-symbology sets, DLR is working on the development of 3D conformal symbology for helmet-mounted displays (HMDs). One crucial advantage of a HMD is the ability to view the instrument information and the outside scene in parallel by superimposing symbology in the forward field of view. This results in reduced scanning time and the prevention of eye accommodation when switching between the symbology and the outside world (Yeh et al. 1998). In contrast to head-up displays, HMDs also allow for an increased freedom in movements (Velger, 1998). Nevertheless, a detriment that is often discussed regarding superimposed symbology is the cost of clutter. It may obscure information in the outside world (Yeh, 1999) and impose difficulties in information processing and in focusing on one particular domain (Wickens, 1997). However, the cost of clutter can be mitigated by the use of conformal symbology. It allows for a better division of attention between the near and far domain, therefore supports tasks that require divided attention (Martin-Emerson & Wickens, 1997) and facilitates the mental integration of information in the outside scene and the symbology presented (Wickens, 2003). It was also found to provide benefits regarding guidance and navigation as well as reduction of attentional capture (McCann & Foyle, 1995). The authors further point out the importance of conformal symbology for nap-of-the-earth helicopter operations and low visibility approaches. The current paper introduces design solutions on conformal symbology sets for different flight demands when operating in DVE. The aim of the development is to provide an intuitive and well-arranged presentation of relevant flight information within the pilots’ forward field of view that allows for fast and accurate perception of the environment and anticipatory flying to enhance situation awareness, avoid spatial disorientation and reduce workload. The symbology will be implemented on a state-of-the-art wide field-of-view binocular HMD with a high precision magnetic head-tracker. The design solutions comprise the depiction of different types of obstacles, route information, terrain data as well as threat areas and landing zones. Obstacles for instance include power poles, masts or wind mills. Therefore information about the type of obstacle, its relative location to the helicopter, the elevation above ground, the distance from the helicopter and their width has to be delivered. The route information consists of a number of dedicated symbols indicating a sequence of waypoints following the earth elevation. Terrain data is presented as grid, the threat area symbology is designed to depict no-flight areas and the landing zone symbology is designed to provide a conformal projection of the expected landing site on earth. At first the symbology designs will be evaluated in a standardized online questionnaire. Primarily helicopter pilots from the German Armed Forces, the German Federal Police Force and the German Automobile Club (ADAC) will participate. After a briefing session, pilots will be shown in-flight pictures and video sequences of the symbology. They will assess the quality of information, e.g. whether it provides too much or is lacking information, is unnecessarily complex, supports anticipatory flying, facilitates spatial orientation and is intuitively understandable. Questions will also focus on clutter as well as attentional aspects such as attention allocation and attention fixation. Also design issues such as level of detail, terrain resolution, texture and perspective view including ratings on perceived distance, height or width will be assessed. The results will offer an important user-centered perspective on the design, usability and acceptance of conformal symbology. In a next step, part-task simulations focusing on visual, perceptual and attentional aspects will be performed on the HMD.