View Full Version : Limitations Of The See-and-Avoid Principles: The 1991 BASI Report

26th Nov 2003, 12:13
BASI Research Report

Limitations of the See-and-Avoid Principle

ISBN 0 642 16089 9

Below is only part of the BASI report from 1991. It includes only the summary, recommendations and conclusion.

If there is interest expressed to see the rest, (it is quite compelling reading) either PM me or express interest to see more on this post and i will transcribe it and put it up as time allows.

My bolding.


The see-and-avoid principle serves a number of important functions in the Australian air traffic system. However, while it undoubtedly prevents many collisions, the principle is far from reliable. The limitations of the see-and-avoid concept demand attention because increases in air traffic may impose an accelerating level of strain on see-and-avoid and other aspects of the air traffic system.

Numerous limitations, including those of the human visual system, the demands of cockpit tasks, and various physical and environmental conditions combine to make see-and-avoid an uncertain method of traffic separation. This report provides an overview of the major factors which limit the effectiveness of unalerted see-and-avoid.

Cockpit workload and other factors reduce the time that pilots spend in traffic scans. However, even when pilots are looking out, there is no guarantee that other aircraft will be sighted. Most cockpit windscreen configurations severely limit the view available to the pilot. The available view is frequently interrupted by obstructions such as window-posts which totally obscure some parts of the view and make other areas visible to only one eye. Windowposts, windscreen crazing and dirt can act as 'focal traps' and cause the pilot to involuntarily focus at a very short distance even when attempting to scan for traffic. Direct glare from the sun and veiling glare reflected from windscreens can effectively mask some areas of the view.

Visual scanning involves moving the eyes in order to bring successive areas of the visual field onto the small area of sharp vision in the centre of the eye. The process is frequently unsystematic and may leave large areas of the field of view unsearched. However, a thorough, systematic search is not a solution as in most cases it would take an impractical amount of time.

The physical limitations of the human eye are such that even the most careful search does not guarantee that traffic will be sighted. A significant proportion of the view may be masked by the blind spot in the eye, the eyes may focus at an inappropriate distance due to the effect of obstructions as outlined above or due to empty field myopia, in which, in the absence of visual cues, the eyes focus at a resting distance of around half a metre. An object which is smaller than the eye's acuity threshold is unlikely to be detected and even less likely to be identified as an approaching aircraft.

The pilot's functional visual field contracts under conditions of stress or increased workload. The resulting 'tunnel vision' reduces the chance that an approaching aircraft will be seen in peripheral vision.

The human visual system is better at detecting moving targets than stationary targets, yet in most cases, an aircraft on a collision course appears as a stationary target in the pilots visual field. The contrast between an aircraft and its background can be significantly reduced by atmospheric effects, even in conditions of good visibility.

An approaching aircraft, in many cases, presents a very small visual angle until a short time before impact. In addition, complex backgrounds such as ground features or clouds hamper the identification of aircraft via a visual effect known as 'contour interaction'. This occurs when background contours interact with the form of the aircraft, producing a less distinct image.

Even when an approaching aircraft has been sighted, there is no guarantee that evasive action will be successful. It takes a significant amount of time to recognise and respond to a collision threat and an inappropriate evasive manoeuvre may serve to increase rather than decrease the chance of a collision.

Because of its many limitations, the see-and-avoid concept should not be expected to fulfil a significant role in future air traffic systems.


The CAA should take into account the limitations of see-and-avoid when planning and managing airspace and should ensure that unalerted see-and-avoid is never the sole means of separation for aircraft providing scheduled services.

In light of the serious lim itations of the see-and-avoid concept, the CAA should continue to closely monitor the implementation of TCAS in the US and should consider the system for Australia.

The CAA should ensure that pilots are trained in effective traffic scans.

The CAA should require white strobes rather than red rotating beacons to assist visibility when the aircraft appears against dark backgrounds.

The CAA should ensure that pilots are aware of the physiological and psychological limitations of the visual system

Pilots should recognise that they cannot rely entirely on vision to avoid collisions. Consequently, they should attempt to obtain all available traffic information, whether from Air Traffic Services or a listening watch, to enable them to conduct a directed traffic search.


The see-and-avoid principle in the absence of traffic alerts is subject to serious limitations. It is likely that the historically small number of mid-air collisions has been in a large part due to low traffic density and chance as much as the successful operation of see-and-avoid.

Unalerted see-and-avoid has a limited place as a last resort means of traffic separation at low closing speeds but is not sufficiently reliable to warrant a greater role in the air traffic system. BASI considers that see-and-avoid is completely unsuitable as a primary traffic separation method for scheduled services.

Many of the limitations of see-and-avoid are associated with physical limits to human perception, however there is some scope to improve the effectiveness of see-and-avoid in other areas.

Although strobes cannot increase the visibility of an aircraft against bright sky, it is likely that high intensity white strobes would increase the conspicuity of aircraft against a dark sky or ground. There is no evidence that low intensity red rotating beacons are effective as anticollision lights in daytime.

Pilots and ATS personnel should be made aware of the limitations of the see-and-avoid procedure, particularly the psychological factors which can reduce a pilot's effective visual field. Pilots may be trained to scan more effectively and to accommodate to an appropriate distance when searching for traffic. Simply ensuring that the windscreen is clean and uncrazed will greatly increase the chance of sighting traffic.

There are important questions about the operation of see-and-avoid which can be answered by future BASI research. These include the question of how frequently Australian pilots scan for traffic and whether they scan significantly less in controlled airspace due to an over-reliance on ATS. The traffic scan training received by student pilots should be assessed. The visibility from aircraft should also be examined, with particular reference to windows and cabin obstructions.

The most effective response to the many flaws of see-and-avoid is to minimise the reliance on see-and-avoid in Australian airspace.

27th Nov 2003, 07:10

Thanks, I've had a copy of this report for some years now, but its recommendations seems to be (conveniently) overlooked or ignored in the NAS debate.

How can any airspace system improve its safety integrity, when we don't acknowledge the research or lessons from the past?

For info, there is also another study done by CASA which is worth getting hold of and having a look at re the MBZ/CTAF argument ("The Airspace Risk Model - MBZ/CTAF Analysis" by Robert Phillips Feb 2002).

Finally, Dr. Neale Fulton of the CSIRO (email [email protected]) has undertaken a number of scientific studies into "see and avoid" in Oz airspace, which have been published in "The Aeronautical Journal" (Royal Aeronautical Society mag). In one of his article overviews he states (bolded phrases are my emphasis): "In classes E, F and G airspace, there is an expectation on pilots to see and avoid other aircraft, but this is often not possible so to design airspace rules on this basis is inappropriate" and, in another article, he concludes "To achieve the desired airspace safety and reliability levels in a cost effective manner airspace design requires some form of robust communication other than visual acquisition (sic of other aircraft) whilst at the same time maximising the potential for pilots to perform visual acquisition" . Given his extensive qualifications and knowledge of risk and reliability engineering, it beats me why Dr. Fulton has not been engaged as an advisor on the NASIG. Perhaps, he knows too much and could ask too many embarrassing questions??

27th Nov 2003, 07:12
Unalerted see-and-avoid has a limited place as a last resort means of traffic separation at low closing speeds but is not sufficiently reliable to warrant a greater role in the air traffic system. BASI considers that see-and-avoid is completely unsuitable as a primary traffic separation method for scheduled services.

27th Nov 2003, 07:20
Why has the ATSB been so quiet with regard to this matter - or have I been missing something?

27th Nov 2003, 07:31
Because the ATSB's primary role is only to investigate aircraft incidents and accidents (ie "after the event" stuff), with a secondary role of pilot education and safety prevention (mainly through the crash comics).

It's CASA's job to set the safety standards and to monitor regulatory compliance and to ensure the system is implemented safely . An important point to realise here is that CASA is not required to conduct a formal safety analysis to determine if the whole NAS concept is inherently safe or not because, under the ICAO guidelines, there is no requirement to undertake a full safety analysis if the proposed airspace system is a "tried and tested" system already in operation in another part of the world.

So CASA's safety role is restricted (unfortunately) to only conducting safety analyses on the implementation of NAS not on the merits or demerits of the concept itself.

27th Nov 2003, 07:43
Don't forget of course that the ATSB, CASA, AirServices Australia are all arms of a government intent on implementing NAS.
Quite convenient really....

27th Nov 2003, 08:02
I owuld have thought that pointing out the weighting of an airspace system toward the use of a flawed system as 'accident prevention'.