Flight Situations and Performance

The information below addresses how well does the FLARM technology do in the different kinds of flight situations which might cause a collision threat for gliders.  Note: this information only relates to the FLARM glider-to-glider cases and does not include the PowerFLARM's ADS-B and transponder received messages which will be different.

The graphic below was sent by Regis Kuntz and Gerard Herbaud, which summarizes their experience and thoughts on this issue. There are more than ten gliding clubs and major soaring centers in the Southern Alps in an area about 100 sm x 100 sm, with an average of about 25 gliders per gliderport, some going upward of 50 for the entire summer months, plus towplanes and other planes. No wonder that these guys are interested in how well their collision detection and avoidance systems work! The graphic presents comparisons of how well pilots do with threat detection with how well they do with help of a “classic” FLARM, in various situations of close proximity.  

It is still unknown how the US PowerFLARM will perform.  The US PowerFLARM uses the same FLARM technology and software and adds the transponder detection and ADS-B capability; therefore, it will take some time to understand fully how the US PowerFLARM will work in these situations with the new additional inputs. 

In frontal approach our human eyes are not so good, particularly in the shade of clouds or with low sun shining on the canopy.  The worst situation for collision is when the two gliders are at nearly the same altitude and, unfortunately that is when they present their smallest profile and are the less visible. In addition, the human eye has evolved to be particularly good at detecting motion.  When two objects are on a collision course, neither object appears to move relative to the other; therefore, the eye does not see a relative motion and is less likely to detect the other glider, especially at a distance.  Frontal approach is a situation where FLARM is reported to work very well. 

FLARM also does well in relative approach from the side or back.  In this case the pilots are not seeing each other mostly because the nose, the fuselage or the wing hide the other glider from view. FLARM’s help is reported as being very good in this situation.

In the situation of one glider coming to enter a thermal in which another glider is already turning, the pilot of the turning glider is very poor at detecting the incoming glider, mostly because his attention is typically driven toward the center of the spiral, in paticular if other gliders are turning with him, and also because for at least half of the turn, the nose and/or the fuselage body of the banked glider are obstructing the line of sight toward the incoming glider.  Fortunately, FLARM is reported as providing good help to the turning glider and very good help to the incoming glider in detecting the other.

The case of two spirals intersecting each other, either two spirals slightly off-centered from each other as depicted in the graphic or two spirals in opposite directions, is normally contrary to the basic rules of safety and etiquette while thermalling. However, these cases are relatively frequent when two gliders have just arrived under a cloud and start thermalling or looking for the core of the thermal. It is good that FLARM is reported as providing good threat detection help in these cases.

The last case is the one in which two or more gliders are following each other fairly closely in the same circle.  FLARM's help is reported as poor because it may, rightfully so, send frequent alarms, which may be annoying to the pilots if they are keeping the other gliders  well in sight.  The alarm sound can be temporarily turned off, but then this is the point: In this situation where possibly several gliders try to maintain close nose-to-tail separation at the same altitude, the pilot’s lookout and avoidance skills are still the best defense.

In all situations, FLARM is a help to the “see and be seen” and not a replacement to it.  No one in his right mind should forget or even diminish the basic safety rules of look outside and proper thermal-entry and circling, just because a detection help is on-board.