Vortex Ring: the ‘Helicopter Stall’

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Vortex ring is actually very rare, image by Helen Krasner

When is Vortex Ring Likely to Occur?

Vortex ring can happen quite easily under certain specific conditions.  When learning to fly helicopters, the most likely time for incipient vortex ring is the approach to landing.  All three conditions can be present; the helicopter is descending, power is applied, the airspeed is decreasing.  So it is important to keep moving forward until the rate of descent is very low.  Newer students sometimes have a tendency to try to slow down if they are afraid of overshooting their landing point, and this can put the helicopter into vortex ring, so instructors tend to urge them to keep up their airspeed, and keep going forward and down.

Later in the course there are other manoeuvres where vortex ring must be carefully avoided – downwind approaches, vertical descents, high hovers, autorotation recovery with low airspeed, and downwind quickstops.  So when attempting all of these complex and relatively unusual manoeuvres, pilots must be very aware of the possibility of vortex ring, and keep looking out for it.

However, in real life and away from the flying school, vortex ring is most likely to occur when the pilot is overloaded or distracted.  Passengers can inadvertently cause such a situation, especially if they want to take photos and expect you to put the helicopter into an ideal position.  They may ask you to slow down, descend a bit, then stop in a high hover – and all the conditions for vortex ring can be present before you realise it!  Of course, so long as the pilot recognises what is happening, it is easy to increase speed and get out of the incipient stage of vortex ring; there is only a problem if the pilot is distracted and/or does not realise what is going on.

So helicopters can indeed just fall out of the sky!  However, pilots are trained carefully to avoid it, and they extensively practise getting out of vortex ring when learning to fly, so it is actually very unusual for an accident to be caused by this phenomenon.

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