Collision avoidance

The CAA’s Class G for the 21st Century Airspace study recognises that the level of risk in UK airspace is acceptable. Close encounters – also known as ‘Airprox’ – do occur. Statistics demonstrate that the greatest collision risk to gliders are other gliders, and most collisions with gliders occur in thermals or close to airfields.

LOOKOUT is the primary method of collision avoidance in class G airspace. You can help to keep yourself and others safe by making the most of the following six attributes available to pilots;

Eyes – lookout – for at least 80% of the time – and develop a robust scanning technique

Ears – communicate by listening, and where appropriate, talking on the radio to enhance situational awareness

Foresight – fly defensively, with vigilance, courtesy and condideration for others (good airmanship!). Try to be predictable, particularly in the circuit and in thermals, on a ridge, etc.

Insight – review your understanding of operations in class G airspace, rules of the air and procedures. Be aware of and follow the Soaring Protocol.

Advertise – make your presence known with electronic conspicuity, eg FLARM

Prioritise – time share cockpit tasks to avoid distractions compromising your lookout

You can view a number of helpful short video clips here that help to illustrate all six points.

You can view the Skyway Code here.


Most pilots choose to use technology to enhance their awareness and to help to direct their lookout more effectively. Some of that technology is described below.

Please note that it is necessary to avoid any in-cockpit equipment from distracting from the visual lookout scan.


Most glider pilots fly in radio equipped gliders. Correctly used, radio is helpful in building and maintaining situational awareness, including with air traffic controllers. Pre-programming frequencies before flight can prevent head down time and other in-flight hazards caused by distraction. Information about pilot radio licences can be found here.

Use of Traffic and Collision Warning Systems

The BGA encourages the widespread use of traffic and collision warning systems in gliders, motor gliders and tugs.

Pilots should make their own decision on equipage based on compatibility with other systems and as to whether such a system is appropriate for their particular operation. Pilots are reminded that whilst electronic collision warning equipment can enhance pilots’ awareness by providing most useful warnings, such equipment cannot and must not replace a good systematic visual lookout scan.


FLARM is an increasingly popular traffic and collision warning system and particularly so within gliding where there is widespread adoption.

Each FLARM device determines its position and altitude with a highly sensitive state of the art GPS receiver. Based on speed, acceleration, heading, track, turn radius, wind, altitude, vertical speed, configured aircraft type, and other parameters, a very precise projected flight path can be calculated. The flight path is encoded and sent over an encrypted radio channel to all nearby aircraft at least once per second.

At the same time, the FLARM device receives the same encoded flight path from all surrounding aircraft. Using a combination of own and received flight paths, an intelligent motion prediction algorithm calculates a collision risk for each received aircraft based on an integrated risk model. The FLARM device communicates this, together with the direction and altitude difference to the intruding aircraft, to the connected FLARM display. The pilots are then given visual and aural warnings and can take resolutive action.

There is evidence that some FLARM aerial installations are nowhere near as effective as they could be. We advise owners check their FLARM coverage using the FLARM online tool. The tool is available on the FLARM website here. For LXNAV 9000, 8000, 8080 and Flarm mouse, the tool is here.

You can read more about FLARM technology here.

FLARM has published guidance on aerial installation. You can view it here.

Learn about checking and improving FLARM coverage here.

Other FLARM maintenance guidance is available here.

  • Transponders

A transponder provides a signal that is externally interrogated. Depending on the type of interrogation, a transponder sends back a ‘squawk code (Mode A) and/or altitude information (Mode C) to help air traffic controllers to identify the aircraft and to maintain separation. Another mode called Mode S (Mode Select) is designed to help avoiding over-interrogation of the transponder (eg many radars in busy areas) and to allow collision avoidance with similarly equipped aircraft. Mode S transponders are ‘backwards compatible’ with Modes A & C. Transponders represent very limited benefit to the users in uncontrolled airspace. Power use is an issue for any aircraft reliant on battery power.

  • ADS-B

Automatic dependent surveillance – broadcast (ADS–B) is a cooperative surveillance technology in which an aircraft determines its position via satellite navigation and periodically broadcasts it, enabling it to be tracked. The information can be received by ATC ground stations as a replacement for secondary radar. It can also be received by other aircraft to provide situational awareness and allow self separation.

ADS–B is “automatic” in that it requires no pilot or external input. It is “dependent” in that it depends on data from the aircraft’s navigation system. ADS-B data can be recorded and downloaded for post-flight analysis. ADS-B also provides the data infrastructure for inexpensive flight tracking.

The system relies on two components—a high-integrity GPS navigation source and a datalink (ADS-B unit). There are several types of certified ADS-B data links, but the most common ones operate at 1090 MHz. To obtain ADS-B Out capability at 1090 MHz, one can install a new transponder or modify an existing transponder plus install a certified GPS position source if one is not already present.

The BGA recognises that ADS-B, if correctly developed and deployed with changes to airspace rules, can provide benefits to all users. It is possible, for example, that ADS-B, if correctly deployed, could make integrated (rather than segregated, as it is now) UK airspace a reality  and thereby reduce restrictions. However, there are many hurdles to overcome to get there!

The CAA’s Conspicuity Working Group and the Future Airspace VFR Implementation Group (FASVIG), both of which are supported by the BGA, are working on a variety of future airspace issues including those relating to ADS-B and associated component and certification issues.