A danger not only for paragliders
Helicopters and paragliders often share the same airspace. This means increased stress and a potentially higher risk for both helicopter crews and paraglider pilots, particularly during transport and rescue missions in busy flight areas.
Author: Martin Graf, SHeV
Wake turbulence occurs as a result of a helicopter generating lift. Their strength depends on the weight, rotor diameter and speed of the helicopter. This turbulence is most intense at low speeds, for example during take-off and landing. The lifetime of wake turbulence varies depending on the wind and atmospheric conditions. They can be carried further by the wind, which makes them difficult to predict. As a guide, wake turbulence can persist for up to five minutes after it has formed. Careful planning of approach sequences is therefore essential.
Several accidents involving paraglider pilots
The Swiss Transportation Safety Investigation Board (STSB) had investigated several accidents involving paraglider pilots in Switzerland (final reports 2335 & 2352) and came to the conclusion that wake turbulence from helicopters led to collapses and thus to paraglider crashes. A lack of awareness of the danger of wake turbulence among both helicopter and paraglider pilots was identified as a systemic cause. There are also en one documented case in which wake turbulence caused the loss of control of a small aircraft.
"Downwash" and "Wake Turbulences"
In addition to the downwash, the aerodynamic generation of lift by means of a rotor also creates vortices, which in their entirety are referred to as wake turbulence, as the STSB further explains. In forward flight, both airplanes and helicopters generate a counter-rotating pair of vortices that remain behind the aircraft. The rotor downwash appears as an immediate, powerful, partly turbulent airflow that is directed almost vertically downwards in hovering flight. The intensity of this flow depends primarily on the mass of the helicopter and the rotor diameter. The flow is strongest when hovering or moving slowly forward.
Wake turbulence, consisting of a pair of vortices rotating in opposite directions, also arises as a direct consequence of the generation of lift. In calm air, these vortices sink almost vertically, drift apart laterally and can persist for up to two minutes with a greatly reduced effect. Wind influences both the direction of movement of the vortices and their development over time.
More powerful than fixed-wing aircraft
Studies have shown that the intensity of wake turbulence is more pronounced in helicopters than in fixed-wing aircraft of the same mass. As with fixed-wing aircraft, the flight speed of helicopters also plays a significant role. The occurrence of wake turbulence is particularly high in helicopters in the speed range from around 20 to 50 knots.
This turbulence is invisible and can pose a danger to following aircraft for several minutes after an aircraft has flown through. In contrast, the danger zone of the rotor downwash, which develops its effect directly and locally below the helicopter, is easier to predict.
Several cases are known from the USA in which wake turbulence from helicopters even caused small aircraft to crash. The French Bureau d'Enquêtes et d'Analyses pour la sécurité de l'aviation civile (BEA) launched comprehensive investigations into the topic in 2021 and subsequently published explanatory videos on YouTube. According to their studies, wake turbulence can persist for up to five minutes after it occurs.
Important sensitization
The main problem is that paraglider pilots are often not sufficiently informed about the danger, and helicopter pilots can hardly take responsibility for the airspace they left five minutes ago. I myself have seen paraglider pilots cross my flight path several times after I had left the area. In principle, paragliders, as non-motorized airspace users, have priority over helicopters.
Recommended measures in compliance with the applicable airspace regulations:
For helicopters:
- Flying slower with high paraglider traffic.
- If possible, fly around clusters of paragliders.
- Raise awareness of potential wake turbulence.
For paragliders:
- Use bright colors on the paraglider to be better recognized.
- Swing or turn when a helicopter approaches to attract attention.
- Use a Flarm Beacon.
- Always keep at least 500 meters away from helicopters.
The Swiss Helicopter Association (SHeV) offers awareness-raising presentations on the subject at paragliding clubs and schools as required.