Dealing With Wake Turbulence

The safety committee would ike to share this information with members (written by Brad Harris).

Wake turbulence

As we all learnt during our PG course, wake turbulence is the disturbed air left behind after a wing flies through the air.

All flying aircraft create a wake, though not all are the same. A sailplane with its long narrow wings and tiny wingtips creates very little, a Cessna somewhat more, an airliner with its high speed and huge mass to support creates a massive wake, and helicopters fall somewhere between. I’m sure we’ve all felt some wake turbulence flying behind other paragliders or hang gliders when ridge soaring, especially Ramon’s tandem or Carl’s big wing.

Wake turbulence trails behind an aircraft in a roughly half-cone shape, below and out to the sides. The wake is strongest right behind the aircraft and dissipates in power as it spreads out and slows down the further behind the aircraft it is. There are industry recognised wake clearances for different aircraft of different sizes.

We need to keep in mind that once created, the wake cone will drift downwind at the speed of the wind – so at about 20-30 km/h or 5-8 m/s in the conditions we fly in at Single. If an aircraft passes at say 300m upwind, we have about 40 seconds before it reaches us. If it’s 1000m we have almost 3 minutes. If it passes by to the crosswind side the wake may never reach us, or be sufficiently dissipated as to cause no issues. The further upwind it passes, the longer we have and the less the effects will be. With this visualisation in mind, we can anticipate where we might encounter or avoid wake from a passing aircraft.

For us flying on Single Hill helicopters probably pose the biggest risk, since most other aircraft tend to be far away on the runway line, or higher as they pass, or smaller aircraft with little wake.

A helicopter at hover creates a donut-shaped vortex under and outwards from the blades. In slow forward flight the ring elongates and trails behind and in full forward flight it trails behind like most other aircraft – in a cone below and behind the “wing” – but is still relatively large and more chaotic, for the size of the aircraft. The bigger the helicopter, the bigger the wake.

It’s worth watching this video which circulated recently that was an analysis of a helicopter wake incident in France. https://youtu.be/iHqN7PQraMs

In that particular case the helicopter was coming in to land (therefore with maximum wake effect) and passed by 100m upwind of a paraglider who was dune soaring at about the same altitude. The PG took a collapse and impacted the ground. Another video presented in that clip shows a heli passing about 200m in front of a dune-soaring PG who suffered an asymmetric and crashed into bushes. In both of these cases the PG was very low and the heli quite close. A recent incident in Australia had a large military helicopter pass in front of a soaring PG at a range of about 300m. The PG pilot was aware of it, ready when he took a collapse, handled the situation and continued flying.

I want to make it clear that I’m not an instructor, but from my study of the topic this is what I think are things to consider if you see an aircraft pass upwind of you:

  - Firstly assess what the severity might be any effects from wake. Do you think you can handle them? Evasive measures might be more dangerous than managing an incident in the air. In most cases you won’t be able to get on the ground before any dangerous wake arrives – if it’s close enough to cause strong wake you won’t have time. If it’s far enough to have time to land the wake will probably have dissipated enough when it gets to you.
  - Head to the side of the ridge that will give the greatest distance between their path and your position.
  - Get as high as you can. This will hopefully get you above the wake cone but will also give you height to manage any collapse you might take, or at worst throw your reserve and give it time to open.
  - Don’t get low. There’s less time do anything and the turbulence will get even more powerful and chaotic when it hits the ground and has to go somewhere.
  - If none of these options seem like they’ll allow you to avoid dangerous wake, then turn and run around the side of the hill and downwind. Remember that the wake will only travel as fast as the airmass it’s in, so flying downwind will add distance, giving you time to find a safe landing, turn into wind and get on the ground.

It’s worth mentioning that no one has ever reported feeling any aircraft wake turbulence at Single Hill. But it’s up to each Pilot In Command to do their research, decide whether to fly, assess the risks, take evasive action if required, or take it head on.

We fly in turbulence all the time. Sometimes we even seek it out in the form of thermals. It’s not a death sentence. We just need to be prepared.