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by Herbert Pirker

Problem:

According to the German BFU (www.bfu-web.de) about 15% of all FATALITIES in glider accidents, averaged over the last 10 years, were caused by MID AIR COLLISONS. But in 1998 an alarming peak of 45% (!) with 8 collisions was registered . Bill Scull (<bscull@globalnet.co.uk> , OSTIV-meeting 1999) reported also about increasing mid airs in the UK.

One mid air happened at the pre-worlds in Bayreuth. Therefore, the proposal was made at the last meeting that ANTI COLLISION COLOUR MARKINGS and/or STROBE LIGHTS be mandatory for all gliders flying at competitions.

The delegates were not sure about the effect and applicability of such measures and asked for more background information.

Information

Many pilots sent their opinion, experience, knowledge, papers etc. to the igc-info-list (igc-info-1@fai.org), to the internet discussion group (www.dejanews.com) or directly to me (bugwiper@ping.at). The German LBA (Helmut Fendt, www.lba.de)was so kind to send papers which explained why the LBA (German Regulatory Authority, equivalent to the US FAA) does not require colour markings or strobe lights on gliders. Andreas Deutsch and others sent a lot of papers, reports and publications on the subject. I would like to thank all for their great help and I feel sorry that I am not able to mention them all by name.

Cause of mid airs:

Mid airs happen mainly because pilots are not able to see each other:

1. if one glider is positioned exactly under another glider. Collision occurs by changing speed and height.
2. flying against the sun,
3. flying near the cloud base,
4. only one pilot sees the other glider. Actions may come too late. Critical situation: a thermalling pilot sees another glider heading towards him, but has to decide whether to continue his turning or not.
5. gaggle flying in thermals: one pilot flying exactly behind another trying to overtake him/her in a steep turn. Turning around different centres of the thermal, etc.
6. Looking too long into the cockpit. Some modern computers and moving maps are difficult and time consuming to operate etc.
7. Bad visual scanning method of the air space. Examples include both pilots in a double seater look at the same time down to the turn point; scan is too rapid to notice other aircraft; pilot focuses on wing tip or other close object and not at (effectively) infinity to see distant aircraft in time.
8. The so-called "blossoming" effect. Other aircraft on a constant-bearing (collision) course from the opposite direction (180°) or from any other direction down to 60° or up to 300° are seen always at the same angle (constant bearing). They therefore appear to be a stationary object in the pilots visual field. At first the object has the size of a small point or outline of some few millimetres on the canopy, which suddenly "blossoms" to its full size as it gets closer. A pilot may not notice another aircraft if 1) the angle of the other glider is constant, making it less noticeable; 2) the other glider is hidden behind an object in the pilot's visual field such as the tape of the yaw string; 3) is mistaken as some dust, debris etc. on the canopy; 4) because of the nature of the eyes: The eyes see sharp in the centre, but not in the periphery (the acuity is many times higher in the centre than in the peripheral vision). Any object which might be just seen in 1.5 km, the eyes being focussed on it, must be close to 150m before the eyes can see them in the periphery. On the other hand, the periphery is sensitive towards movement, but an object on a constant bearing does not show any movement. Also, the human eye has a blind spot where the optic nerve meets the retina.

Overall, objects are not seen under a certain minimum size (threshold of acuity). Therefore, the outline of an object is seen first, then the main colour and finally any colour markings. So, anti collision markings have little or no effect, if gliders are on a collision course. This does not necessarily mean that anti collision colour markings are not well seen on other occasions where gliders are NOT on a collision course, where they have significant changes of angle with respect to the viewer.

(For further details, please see papers collected by Andreas Deutsch, for instance:

1. "Limitations of the See-and-Avoid Principle" by the Bureau of Air Safety Investigation Research Report, 1994;
2. papers by Gerhart Berwanger: aerokurier, 2/1984, page 175, also aerokurier 4/94, page 55 and aerokurier 5/94, page 60.

3) See also lecture held at the OSTIV meeting in Bayreuth (7.8.99) by W.G. Scull [bscull@globalnet.co.uk].)

Pilots discussed also the importance of contrast, reflectivity and the camouflage-effect.

The best overall colour for the conspicuity would be: 1) black, at least for military jets 2) yellow 3) lime green or pink day glos 4) white, (TiO2 would enhance the reflectivity). Bad conspicuity: gliders painted totally in red.

Problems of colour markings

1. Temperature effect on structure. Most composite glider structure is certified only up to a temperature of about 54°C, after which its structural strength decreases. Therefore, white is the preferred colour and dark colours such as red are forbidden, unless a special permission is given by the manufacturers.
2. Fluorescent stick-on strips damage the paint of the glider and can have a negative effect on the resin in the same way as any dark colour. The thickness of any stick-on strip may cause flow separation and performance degradation.
3. Fluorescent paint can be used but can be 0.2 mm thick. Schempp-Hirth is now using thinner fluorescent paint, but the long term effect is not known.

Common opinion about anti collision colour markings:

We note a diversity on opinions about the effect of anti collision colour markings. Some pilots are enthusiastic about them, although there is no evidence of their effectiveness and on a constant bearing (collision) course the shape is always seen before any colour. However, in France, especially in the alpine regions, pilots are required to use red stripes on the wings. In Austria, the Gliding Commission decided colour markings to be mandatory for competitions. On the other hand, many pilots, the German LBA and other organisations have come to the conclusion, mainly because of investigations and detailed research work that colour markings have little or no effect at all in preventing mid air collisions. Many pilots stick on colour strips where this is mandatory and then take them off again when they return to places where such a rule does not apply.

Strobe Lights, Anti-Collision Lighting

The effectiveness of strobe lights is mainly a question of power consumption.

Tests by Richard Johnson (see Sailplane & Gliding, April/May 1990, page 79) showed that at 12 Volts and 1 Ampere and a flash frequency of 2 sec, was noticeable at a range of 480ft in bright sunlight and of 700ft in bright overcast. However, to recognise colliding gliders in time, a range of 2-3km would be necessary which might need a current of 10 Amperes. One solution could be brighter flashes at less frequency, and this is worth further research. In darker conditions such as dark overcast or twilight, strobe lights with a power consumption of 12V/300mA (strobe light from Streifeneder for instance) might work quite well, especially when the glider is viewed against dark cloud or ground.

Regulatory Authorities

Because of the foregoing reasons, the German LBA and other authorities do not insist on anti collision colour markings or strobe lights for gliders.

Proposals by CASP:

At Bayreuth members of IGC, OSTIV (SDP and SP) formed the "Collision Avoidance Sub-Panel" (CASP). According to this Panel it is important to consider improved ground training of the pilots before they use highly sophisticated electronic devices in the cockpit. Manufacturers should deliver instruments which are easy to operate (at best in a self-explaining manner, low numbers of operating steps), have a special mode for training purposes on ground and supply the pilots with good manuals and training software for home PCs. See 1^st meeting of CASP [Michael.Rehmet@t-online.de] .

Radio aids for collision warning

Transponders. Nigel L. Rotherham [nigel.r@netline.co.za] is developing a low cost (appr. 200 Dollars) 'anti collision' system based on low power transponders using a free radio frequency band.

3-D Position Reporting. According to Walter Dittel from Filser (FE_Seischab@gmx.net) the developement of the so called:

This is a new aviation surveillance concept whereby aircraft transmit their positions (usually derived from a GNSS receiver on-board the aircraft) over a radio data link. Position information is transmitted

and received by every aircraft in the vicinity so that all users have knowledge of their own location and the locations of all other surrounding aircraft. The position information may be displayed in the cockpits of suitably equipped aircraft to give new situation awareness capabilities. Also, ground vehicles and fixed ground stations can also be equipped to transmit and receive position data, allowing surveillance of all types of traffic and

a two-way data link capability. ADS-B is an enabling technologies that can help to deliver the free flight concept to airspace users.

Disadvantages. Radio methods would involve extra expense, would have to be fitted to both aircraft involved, and would be subject to "false alarms" if they had the necessary sensitivity to warn of collisions in time for pilots to take action. Also, when thermalling it would be difficult to avoid constant alarms. The problem could be solved by using simple displays in combination with the warning by a computer voice if gliders are on collision course.

Conclusions

1. The use of colour markings as an anti-collision aid is questionable. In particular when two aircraft are on a constant-bearing (collision) course, it is the size and shape of the other aircraft which is seen first, not any colour pattern. The use of colour seems obvious at short range and on the ground but this is often not the critical case in the air in a collision situation.
2. Radio methods based on transponders or reporting of 3-D GPS position should be investigated further, but it should be recognised that cost, universal fit, and false alarms are problems to be surmounted.
3. Flashing strobe lights can call attention to other aircraft in a pilot's centre and peripheral vision, as long as the strobe brightness is great enough against the sky, cloud or ground background. Further research is required to ascertain the best compromise between electrical power required, the brightness of flash, and frequency of flash. A lower frequency at a higher brightness could be a useful compromise.
4. Pilot scan techniques. Pilots should be taught proper techniques for scanning for other aircraft. A habit of systematic scanning at an angular rate which is likely to pick up other aircaft (not too fast, not too slow) should be taught, together with practice of not focussing on close objects such as wing tips, canopy, etc. Instructor committees of IGC member nations should be asked to ensure that systematic scanning is a technique which is taught in the same way as any other pilot skill.

5) Electronic devices like moving maps, variometer systems etc. should be easy to operate, in a more or less self explaining manner, without having to study voluminous operating manuals.

Annex: Diagram of the visual acuity of the human eye (Courtesy of Jane's Simulation Systems)