In the vortex
Factors conspire for an inescapable descent
In reality, for helicopter pilots, it’s much scarier. A vortex ring state is dangerous space where rotor lift is degraded to the extent that, even with full power, the helicopter will continue to sink at a dangerous rate, which can result in catastrophe. This was the case for a Eurocopter EC130 pilot flying over the Tennessee River with three passengers onboard, one of whom did not survive the crash.
The pilot departed on a warm August evening for a sightseeing flight. After an hour he circled over the Tennessee River for a landing on his property adjacent to the river. According to interviews with the NTSB, the pilot planned to descend to about 75 feet over the water and then hover taxi to his property. He descended at what he estimated was a 25-degree angle, dropping 300 feet per minute with little to no airspeed.
What is vortex ring state?
Vortex ring state is an especially dangerous condition for a helicopter because it typically happens close to the ground, especially in downwind approaches. The rapid and uncontrolled descent only takes place when three conditions are met: There is power, the helicopter is below effective translational lift, and the descent rate is more than 300 feet per minute. Take away any of those and vortex ring state won’t happen.
Illustration by Charles Floyd
Attempting to arrest the descent and transition to hover, the pilot noticed the helicopter kept descending. He pulled the collective to maximum power, but the powerful Eurocopter continued to drop, as if “the helicopter didn’t have any power.” The aircraft continued its drop to the water, tilted left when a blade hit, then torqued hard to the left and began to sink. The pilot and two of the passengers egressed safely; the fourth occupant did not get out and drowned.
The NTSB estimated the helicopter to be almost 1,000 pounds below maximum gross weight. Data downloaded from the digital engine control unit showed no signs of abnormal engine operation. Examination of the flight controls and components indicated no signs of pre-impact mechanical malfunction.
Even in the warm temperatures, with light to no wind, the 747-horsepower turbine engine had plenty of power for the intended flight. But not enough to overcome a sink rate with rotors not generating lift. Entering a vortex ring state seems the most logical explanation for the uncontrollable sink.
All rotor pilots learn about the vortex ring state during training. The condition can occur when horizontal velocity is below effective translational lift. Translational lift is when forward airspeed is enough to move the rotor out of its vortices and into undisturbed air, typically around 15 to 25 knots.
Helicopter pilots can begin to “normalize deviance,” pushing greater and greater angles of descent, with slower horizontal airspeeds, and it all works fine until one day, the winds, temperature, and helicopter aerodynamics all conspire for perfect vortex conditions.Pilots can fly into a vortex ring state in a light wind or tailwind, attempting to hover out of ground effect, or descending at steep angles with the engine between 20 and 100 percent of power. Exiting a vortex ring state is simple; two techniques are taught, both effective…if. The first technique—taught for years—is to lower the collective and push forward on the cyclic (stick) to move forward, out of the dirty air. The second technique is known as the Vuichard Recovery technique. Fixed-wing pilots will recognize this as similar to a slip: pull the collective to climb power, apply cyclic opposite the direction of the main rotor (right for a counterclockwise rotor) and then opposite anti-torque (rudder) pedal. The objective in either method is to move horizontally out of the dirty air.
The big if with either technique is recognition. Vortex ring states occur during critical phases of flight, and it’s easy for the pilot to notice lift degradation, but not recognize the cause and just think they need more power until it’s too late. Vortex ring states used to be aptly termed “settling with power.” Similar to density altitude traps, vortex ring states can be hard to grasp, and difficult to visualize.
New pilots fear the vortex ring state and stay well clear. Experienced pilots have likely scared themselves and learned the signs for recognition. This pilot had logged more than 10,000 hours pilot-in-command time in all aircraft, but only 300 total hours in the Eurocopter. Three hundred hours is enough to gain experience and build some confidence. It’s also about the right time to get ambushed by the effects of “normalizing deviance.” Helicopter pilots get used to the vertical capabilities of their aircraft, and as they gain experience and confidence, continue to work close to the envelope, even perhaps flying near the vortex ring state, but not recognizing it since they don’t experience the lift degradation.
It’s easy to imagine that over time, a pilot can drop it from conscious awareness while focusing on obstacle clearance, hover mechanics, and the myriad elements they assess approaching a landing site. Helicopter pilots can begin to “normalize deviance,” pushing greater and greater angles of descent, with slower horizontal airspeeds, and it all works fine until one day, the winds, temperature, and helicopter aerodynamics all conspire for perfect vortex conditions.
The fix is to stay vigilant and know the vortex ring state is lurking at slow horizontal airspeeds, high rates of descent, and high angles of vertical movement. Pilots must be sensitive to changes in lift and increasing needs for power. Vortex ring states steadily get worse, so if more and more power is needed for the desired vertical change, pilots must be ready to adjust their vector with cyclic movements to slide out of a developing ring.
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