Rotorcraft Rookie: Advanced autorotations

February 20, 2014

One second in a Robinson R22 could mean the difference between life and death. It may sound melodramatic, but it’s a fact. If the engine fails the pilot has 1.1 seconds to react. If he doesn’t lower collective to go down or pull aft cyclic to slow down and get air to come up through the main rotor, the blades will stall like an airplane’s wing and the helicopter will tumble.

It’s for this reason that autorotations are practiced constantly in helicopter training. As the student’s skill improves more advanced autorotations are introduced, usually as demonstrations. The versatility of an autorotation also increases its flexibility. Unlike an airplane, where we basically have the option of picking an airspeed and a flap setting, the helicopter can make an emergency approach at almost any angle, turning any direction on the way down. YouTube is littered with videos of autorotations without a flare at the end, autorotations where the pilot flares tail-first, and every other combination you can imagine.

In private pilot training things are tamer. We regularly practice only straight-in and 180-degree autorotations. The 180-degree maneuver is considerably more complicated, and more fun. It begins abeam the intended touchdown point at somewhere around 1,000 feet above the ground. We do the standard “3-2-1 autorotation” count and the fun begins.

My initial approach to this maneuver was airplane-like, which is to say it was completely wrong. In the same scenario in an airplane I would bleed off any additional altitude on the downwind to avoid a high base leg and the associated risks of overshooting final approach. And since an airplane obviously has to land parallel to the landing site, final approach has to be somewhat stable and straight.

In the helicopter we do the standard lowering of the collective, aft cyclic, and right pedal. Then as soon as the rpm stabilizes (takes only a second), we turn. The first few times we did this it seemed impossibly high. But there are two critical things to remember. First, the helicopter is coming down like a baby grand piano. And second, the landing spot is a small square, which means it can be approached from any angle. That means the helicopter can be flown in a long, swooping arc that can go as far beyond 180 degrees as is necessary to hit the spot.

The tricky part of this maneuver, and any autorotation that includes turns, is that rotor rpm—the critical measure of safety—varies based on the position of the cyclic. While lowering collective in the beginning helps to maintain rpm, going forward to make a safe airspeed (needed for the flare) lowers the rpm. Slowing down, which often inadvertently happens in the turn, increases rotor rpm. Turning also increases rotor rpm. It’s easy to see how the pilot can get in to a situation where he is constantly chasing rotor rpm with both cyclic and collective throughout the maneuver. Naturally holding a steady airspeed makes this easier because it takes away some of the resulting variability.

Given that each of these takes only a few seconds to perform it’s hard to see it in action. Thankfully we spent the time to fly high one day both to see how altitude impacts rotor rpm, and to do a longer autorotation down. With lots of altitude to play with, there’s plenty of time to see how small changes in the position of the cyclic (and therefore speed) impact the rotor rpm.

In this great book Learning to Fly Helicopters, Randy Padfield says that some helicopter experts say that the cyclic has such a strong impact on rotor rpm that it’s almost more important in a real emergency to bring it aft than it is to lower collective. After having played with this I can see why. Changing the collective does impact rotor rpm, but there’s a lag. With the cyclic it’s instant. To further make the point my instructor, Otto, will often bump the cyclic during an autorotation to help keep us in a safe range. Because if things weren’t complicated enough, getting the rotor rpm too high can easily damage the helicopter.

I hope to never have an engine failure in anything that flies, but if I do it’s comforting to know that the helicopter will go in any direction I choose while it’s coming down in a controlled manner on an autorotation.

Next time: Solo (I hope)!

Read all the stories in the Rotorcraft Rookie series.

Ian J. Twombly

Ian J. Twombly | "Flight Training" Editor

Flight Training Editor Ian J. Twombly joined AOPA in 2003 and is an instrument flight instructor.