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Safety Advisory Notice issued to: Aerial Agricultural Association of Australia

Recommendation details
Output No: SAN19980178
Date issued: 30 October 1998
Safety action status: Closed




The lightbar unit of a global positioning system (GPS) "flagger" provides pilots with accurate tracking for successive swathe runs by means of a series of lights, portraying the alignment of each swathe run. This system replaced almost all ground marker personnel, whose use under the occupational health and safety legislation of the majority of States became considerably more restricted.

In 1995, the Bureau of Air Safety Investigation was alerted to possible safety implications resulting from difficulties experienced by pilots in the use of GPSs then being introduced into aerial agricultural operations.

A safety deficiency raised at that time identified a lack of standards in the training and use of satellite systems employed in aerial agricultural operations.

In May 1996, BASI issued Interim Recommendation IR960016, to the Civil Aviation Safety Authority (CASA), recommending that CASA, in conjunction with the aerial agricultural industry, formulate a policy covering the use and training of GPS in aerial agricultural operations.

In response to this recommendation, CASA in October 1996 advised that the Aerial Agricultural Association of Australia (AAAA) had offered to provide two or three experienced pilots to assist in developing a standard for the use of the GPS in aerial agricultural operations. BASI was also advised that CASA would consider taking up this offer at a later date as the GPS Implementation Team (GIT) progressed with its work. This matter was subsequently addressed by the GIT. Their decision was that because it related to a specialised area, it needed to be dealt with by CASA and the relevant industry body. To date, BASI has not received notification that this has been done.

CASA also stated in its response that the AAAA had advised that "the introduction of GPS into agricultural work has been relatively free of incidents and we would be interested to learn of BASI reports, if to the contrary".

In May 1998 another deficiency was raised, as a result of several occurrences, indicating that the lightbar unit used in conjunction with the GPS "flagger" may have distracted pilots during spraying operations. Subsequently, BASI reviewed the human factors issues associated with the positioning and use of the lightbar unit.


The mounting position and colour of a lightbar used in conjunction with a GPS for aerial agricultural operations may be distracting and reduce a pilot's ability to see and avoid ground-based obstacles and aircraft warning lights.


GPS-related agricultural occurrences

The use of GPS with lightbar units has been implicated in a number of recent accidents in agricultural operations.

In September 1997, an aircraft engaged in pasture spraying operations struck a tree and crashed, fatally injuring the pilot (Occurrence 9703150). A factor in the accident was considered to be pilot distraction caused by the lightbar associated with the GPS navigation system. The lightbar was positioned about one metre in front of the pilot's eyes.

In May 1998, an aircraft struck powerlines during spraying operations (Occurrence 9801676). The pilot reported he had been watching the GPS marker system lightbar, positioned on the nose of the aircraft, and thus had not seen the power poles in his peripheral vision.

In October 1997, an aircraft engaged in spraying operations suffered total engine failure during a spray run (Occurrence 9703511). The aircraft's fuel had been exhausted. The pilot reported the effectiveness of the red fuel warning light on the upper instrument panel had been reduced by the proximity of the red lights in the lightbar of the GPS marker system mounted on the aircraft coaming, ahead of the windshield.

Lightbar usage

Information from the lightbar was used primarily to accurately align each swathe run and avoid repeated or missed spraying of an area. Reliance on lightbar information has increased where lack of ground reference features, especially in broad acreages, required additional guidance. The lightbar has been useful when spraying cultivated land where the tillage was not parallel to the swathe runs. In these circumstances, the ground patterns presented very powerful visual cues, making straight flight difficult without reference to some form of guidance system.

Most spraying operations are carried out across the wind to avoid flying through the mist of the previous swathe run. Although only small drift angles are encountered in these crosswind conditions, a nose-mounted lightbar is sufficiently displaced from a pilot's forward view as to require diverting visual attention away from the flightpath.

Positioning of the lightbar

Discussions with representatives of the aerial agricultural industry produced varied opinion regarding the best position for mounting a lightbar. Although by far the most common position was near the windshield, this was not universally the case.

Lightbar units were first installed at the front of the engine cowling, as far forward as possible, to place them as near to a pilot's distant vision as possible. Through experience, most operators favour a much closer location of the lightbar. Some units have even been mounted inside the cockpit on the instrument coaming to avoid becoming obscured through dust and bug-smeared windshields.

Lack of standard lightbar displays

Presentation of information and lightbar displays, varies considerably between the GPS manufacturers and the different models. Of particular concern is the use of certain colours to light the displays. One of the accidents quoted above demonstrated how easily the criteria for the design and certification of warning systems can be negated by the installation of ancillary equipment, such as a lightbar, which may use inappropriate colours in its display.

Further investigation into the human performance aspects of the use of such systems, especially the positioning of the lightbar, was considered necessary. Expert opinion was sought and it was considered most likely that both attentional and visual aspects were influences in determining where best to locate a lightbar.


Human factors aspects

Two main human factor issues arise in relation to aerial work using GPS systems with a lightbar:

- Visual perception; and
- Attention and cognitive processing.

To some extent these two factors interact, as indicated below.

Visual perception

The positioning of the lightbar will determine its focal distance when viewed by the pilot and hence the degree of accommodative shift required for the pilot to change focus between the display and an outside object. The time taken for this readjustment of focus may well be significant during low-level flight, particularly in high performance (e.g. turbine) aircraft. Placing the lightbar inside the cockpit, e.g. on the instrument coaming, will increase the accommodative shift required. This effect will be more marked with older pilots as speed of accommodation decreases with age. Placing the lightbar further from the pilot's position, e.g. as far forward as possible on the nose of the aircraft, will reduce but not eliminate the need for accommodative shift.

Attention and cognitive processing

It is possible that because of the task importance and visual salience of the lightbar, it will disproportionately gain the pilot's attention, a condition called "cognitive capture". To the extent that cognitive capture diverts the pilot from closely attending to outside visual cues, situational awareness may be significantly compromised. This effect will be accentuated during demanding or stressful operations.

Positioning the lightbar as far as possible from the pilot will approximate the use of a head-up display (HUD), and previous work in this area may be relevant. There is evidence that pilots can become fixated on the symbology of HUD displays. Under visual fixation, pilots are less likely to process other instrument information or outside visual cues. In effect, this will reduce the pilot's field of regard (i.e. the total field of view available to the pilot when looking at a visual scene over a period of time).

Research has shown that limiting a pilot's field of regard is particularly detrimental to judgement for curved flight paths. It has been reported that 30% of pilots experience disorientation when using HUD displays. Between 1980 and 1985, the US Air Force lost 73 HUD-equipped aircraft as a result of pilots becoming disorientated or misorientated. There is evidence that HUDs may improve detection of anticipated objects but hinder detection of the truly unexpected. For example, when a large runway obstacle (i.e. an aircraft) was introduced into the simulated landing scene, almost all pilots ignored it. It is likely that cognitive capture was a significant factor in this result.

Interaction of visual and cognitive factors

It is possible that the need for the pilot to change focus may have positive as well as negative effects. When a pilot switches gaze from a panel-mounted instrument display to the outside world, there are a number of cues to indicate that a switch of attention is taking place, e.g. looking up, changing accommodation, and changing convergence. These can be strong cues to remind the pilot to switch attention. There is evidence from work on HUD systems that when the visual cues associated with cognitive switching from an instrument panel to the outside world are missing, cognitive capture is more likely to occur.

Placing the light bar in the pilot's peripheral vision may have advantages. A number of displays have been designed for peripheral vision. For example, the Malcolm horizon projects a narrow line of laser light across the instrument panel, and hence does not require eye fixation and attention during instrument scan. It has been shown that information in the periphery tends to be neglected at times of stress or high workload. This may act as a self-limiting mechanism to reduce the salience of the lightbar at times of high workload.

Use of other sensory perceptors for track guidance

An alternative means of presenting track-guidance information could be explored. Using senses other than vision may be possible. For example, providing track guidance by means of an auditory signal to the pilot through a stereo headphone set would allow the pilot's entire visual concentration to be centred on external cues. External visual cues are still the most preferred means of establishing and maintaining a flightpath.

Such a system could also be enhanced to generate aural warnings of proximity to powerlines.


Practical experience and related research suggests that a pilot may become preoccupied with monitoring information from a nose-mounted lightbar unit and yet become relatively myopic to the more distant outside information while focussed on a near lightbar installation.

The optimum location of the lightbar would appear to be in the pilot's peripheral vision. Additionally, more attention should be paid to the proximity of lightbar units to other aircraft warning lights and annunciator panel lights, especially with those units that have similar colours to aircraft warning and system status lights.

A curved approach path, such as the completion of the procedure turn onto a swathe run, appears to be a critical point at which distraction from other visual cues such as a lightbar, should be avoided. The provision of this information by other than visual cues should be researched.

Output text

The Aerial Agricultural Association of Australia should note the safety deficiency identified in this document and take appropriate action.

Initial response
Date issued:
Response from: Aerial Agricultural Association of Australia Ltd (AAAA)
Action status: Not Required
Response text:
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Last update 01 April 2011