Helicopter Vortex Ring State
A Dangerous Helicopter State which can Lead to an Uncontrolled Crash
Helicopters do not just drop like a stone if the engine fails. But there is one aerodynamic state which can cause an uncontrolled and irreversible descent...ie a crash!
Sometimes considered as the equivalent of the fixed-wing stall, vortex ring state is a condition of powered flight in which the helicopter descends into its own downwash. Under such conditions the helicopter will descend at an extremely high rate, eventually 2500 feet per minute or more. It will randomly yaw, pitch, and roll; there will be buffeting and juddering of the airframe, with eventually total loss of control and possible structural damage…if the aircraft hasn’t hit the ground first!
What Causes Vortex Ring State?
Three factors are required to all be present: little or no airspeed, a rate of descent, and power applied. When learning to fly helicopters, the most likely time for vortex ring is the approach to landing. All three of these conditions can be present; the helicopter is descending, power is applied, the airspeed is decreasing. So it is important to always keep moving forward until the rate of descent is very low. Other manoeuvres where vortex ring must be avoided are downwind approaches, vertical descents, high hovers, autorotation recovery with low airspeed, and downwind quickstops.
How Can the Pilot Recover from it?
Recovery has to be during the incipient stage. Fully developed vortex ring can cause severe structural damage, and is very hard to recover from; some say it is irrecoverable! Recovery from incipient vortex ring is generally straightforward. All one has to do is remove one of the three factors. It is impossible to stop the descent, as this would involve adding more power. But one can get rid of the power, by entering autorotation. Or the pilot can increase the airspeed, and this is the usual way to recover. He moves the cyclic forward, the helicopter gets out of the turbulent air which is causing the problem, and starts to fly normally.
Why Does Vortex Ring Happen?
In real life, and away from the flying school, things are not simple. In any type of flying, problems occur when one is not looking out for them, and this is most likely when the pilot is overloaded or distracted. Passengers can inadvertently cause such a situation. Most pilots have encountered the situation when a passenger wants to take photos, and expects the aircraft to be put into the ideal position – the pilot slows down a little too much, descends a little, and is suddenly in incipient vortex ring.
The approach to off-airfield confined areas is another situation where vortex ring can easily occur. Again, there is often a high workload, the airspeed is likely to be low, and the pilot may have to approach downwind or not even be sure of the wind direction. There are a few less common times when vortex ring can occur; according to an experienced mountain pilot, up-flowing air in the hills can give the same situation as being in a descent. It should also be noted that conditions for vortex ring can differ depending on weight, density altitude, and helicopter type. It can also occur occasionally without warning. It is therefore something of which helicopter pilots should always be aware.
Vortex Ring is very rare, and the incipient state can easily be corrected. but it is something of which all helicopter pilots shouldbe aware at all times.
Further Reading
The Art and Science of Flying Helicopters - Shawn Coyle, Iowa State University Press 1996
Learning to Fly Helicopters - R. Randall Padfield, TAB Books 1992
