All posts by admin

Thermalling Rules

The following information was compiled by Jim West in response to a question about thermalling rules/etiquette at Dunstable Downs. This is of particular relevance when sharing a thermal with a different type/performance aircraft. e.g. Sailplanes.

It is not intended to be a training aid, it is compiled from information on the internet and is provided here as a convenient collation of relevant information for Dunstable Downs.


Thermalling Rules (September 2012)

The increased performance of paragliders is tempting more PG pilots to try and join other gliders already circling in lift. At Dunstable Downs these can include hang gliders and LGC gliders. Pilots need to be aware that most gliding associations have adopted common rules for joining, flying in and leaving a thermal. The important theme is that when sharing a thermal you have to accept that safety is more important than optimising the lift.

I cannot find advice for pilots encountering aircraft with vastly different thermalling speeds. An LGC glider will be flying at more than twice a paraglider’s airspeed. To maintain position in a circle, the paraglider will have to fly at about half the radius set by an LGC glider. A paraglider trying to join an LGC glider already circling at the same height, will have an interesting if not impossible challenge!

Also bear in mind that each aircraft type has blind spots. The paraglider cannot see what is above the wing. The hang glider cannot see above or behind. A modern LGC glider cannot see below or behind and an older high wing LGC glider cannot see below or behind and can have restricted visibility above!

A hang glider or paraglider will usually circle tighter than an LGC glider and should climb faster if they are in the thermal core, requiring them to also keep aircraft above them in view.

The BHPA website has links to guidance from the FAI, interpretation by competition organisers and some other country’s aviation rules. These are summarised below. Extracts below were taken in September 2012. Pilots should also ask about additional local rules (sometimes a thermalling direction when low) and rules in foreign countries.

The UK A N O (air law) (www.caa.co.uk/docs/33/CAP393.pdf ) contains nothing specific to thermalling, but has the  general statement that “it shall remain the duty of the commander of an aircraft to take all possible measures to ensure that their aircraft does not collide with any other aircraft”

The British Gliding Association (for LGC gliders) has adopted the following rules (June 2010) (www.gliding.co.uk/forms/lawsandrules.pdf)

Joining a thermal

J1) Gliders established in a thermal have right of way. All pilots shall circle in the same direction as any glider already established in the same area of lift.

J2) If there are gliders thermalling in opposite directions, the joining glider shall turn the same direction as the nearest (least vertical separation).

J3) The entry to the turn should be planned so as to keep continual visual contact with all other aircraft at or near the planned entry height.

J4) The entry should be flown at a tangent to the circle so that no aircraft, already turning, will be required to manoeuvre in order to avoid the joining aircraft.

Sharing a thermal

S1) Pilots should adhere to the principle of “see and be seen”

S2) When at a similar level, never turn inside, point at or ahead of another aircraft unless you intend to overtake and can guarantee safe separation.

S3) Leave the thermal, if in your judgement you cannot guarantee adequate separation.

S4) Look out for other aircraft joining or converging in height.

Leaving a thermal

L1) Look outside the turn and behind, before straightening.

L2) Do not manoeuvre sharply unless clear of all other aircraft.


Extracts from the FAI sporting code (May2012). (www.fai.org/fai-documents ,select Sporting Code – General Section)

Competition organisers are advised that an over aggressive pilot (e.g. trying to circle tighter in the core) can disrupt an established group, risking collisions, and should be banned after a warning is ignored.

Expands (J4) to suggest that you should plan to join the other glider’s circle where they are flying away from you. It is ideal to join when the other pilot is on the opposite side of the circle.

Warns about a visual illusion that makes you think that the other pilot is going around your circle. The thing to judge is that if you are catching up, you are turning too steeply.

Sometimes thermals seem to have multiple cores. Pilots, circling in the same direction in different parts of a large thermal will still have the potential for a head on collision if they reach the same height. The advice is for all converging pilots to widen their circle to join in one circle.  There is additional advice on optimising the lift which may be of interest to budding competition pilots.

Extracts form the BPCup website (2012). (www.bpcup.co.uk/thermalling.php)

They also warn about over aggressiveness (as per the FAI statement).

Expands on (J4) pointing out that if you fly directly at the centre of a circling group you will scare the circling pilots and maybe cause a collision. Echoes the FAI statement and adds that the joining pilot should initially fly a larger radius circle allowing them to judge the ideal time to ease in.

Has a different view of (J2) suggesting that the joining glider turns in the direction of the upper group as they are less visible. Should the lower group get near, the direction can be switched back.

Echoes the FAI advice for competition pilots.

The DHV (German association) advice to visitors. (www.bhpa.co.uk/pdf/GERMANFL.pdf)

The first pilot to enter a thermal determines the direction of rotation. All other pilots joining later must circle in the same direction.

An Italian update for sport flight (Sep 2010). (www.ehpu.org/content/news.htm)

The first pilot that enters a thermal decides the turn direction (J1).

Other pilots must turn the same way (J1).

Pilots inside thermals have right of way on pilots outside thermals (J1).

Pilots in thermals must give right of way to other pilots that rise more in the same thermal (S4).

The Australian authorities also agree (BHPA report on Clatter June 2010 incident)

When thermalling give way to the HGs & PGs that are climbing up from below (S4).