The idea of gliding a helicopter to a safe landing may seem to go against the laws of aerodynamics. But the same trusty laws that keep airplanes flying are hard at work here, too. As with an airplane forced landing, a forced landing in a helicopter requires good timing, quick thinking, and lots of careful practice.
To fully understand how autorotation works, we need to review some basic aerodynamics. The rotor blades spin to produce airflow and consequently lift and drag. The rotational speed of the rotor system is held constant, and lift is created by increasing each rotor blade's angle of attack. This is accomplished by raising the collective pitch control in the cockpit. Basic aerodynamic theory tells us that an increase in lift will be accompanied by an increase in drag. This is one reason that, as a helicopter pilot raises the collective pitch control, he or she must increase engine power to maintain the correct rotor rpm. In most piston-powered helicopters (the kind primarily used as trainers), turning the throttle at the end of the collective control increases engine power. Turbine-powered helicopters, and some advanced piston ones, have governors that increase and decrease engine power automatically.
Remember Newton's Third Law from high school physics class? It states that for every action there is an equal and opposite reaction. Since the helicopter's rotors turn in one direction, the fuselage wants to spin in the opposite direction. This is the reason for the tail rotor; it supplies thrust to keep the fuselage stationary. The tail rotor is connected to the main rotor transmission and spins at a constant rpm. Gearing allows it to spin faster than the main rotor, and its rpm varies in direct relation to main rotor rpm. To rotate the fuselage around the main rotor axis, the pilot changes the tail rotor blades' angle of attack, and hence thrust, with anti-torque pedals in the cockpit.
The more power being used, the higher the torque (the force that tends to produce rotational motion) and the stronger the tendency for the fuselage to yaw. In helicopters with rotor systems that spin counterclockwise, the pilot must apply left pedal when increasing power. When a helicopter's engine stops, the torque drops to zero and the tail rotor thrust must be neutralized. This is done by applying almost full right pedal.
Now let's look at what happens when the helicopter is established in cruise flight and the engine fails. The helicopter's only source of power is gone, and therefore the machine no longer has a means of producing energy. However, at the instant that the engine quits, the helicopter has stored energy in the form of altitude, airspeed, and rotor rpm. A successful autorotation is the effective use of that energy to safely land the helicopter. It is worth noting that this same energy, if not properly used, can destroy the helicopter and its occupants.
So how do we use the stored energy to our advantage? First, and most important, the pilot must eliminate as much drag as possible to prevent the rotor system from slowing down and stalling. If the rotors stall, the blades will either "blow back" and cut off the tail cone or just stop flying. This situation is unrecoverable, and the helicopter will crash - likely with fatal results. I know this sounds scary, but that's why good instructors ensure that their students have the skills and confidence to recognize problems before they become critical.
One big source of drag is the dead engine; therefore it must be quickly disengaged from the rotor system. Fortunately, a freewheeling clutch disengages the engine from the rotor system automatically whenever engine rpm becomes less than rotor rpm.
The next big source of drag comes from the angle of attack of the rotor system. Remember, the higher the collective pitch setting, the higher the drag. At high pitch settings, the pilot will only have a couple of seconds to lower the collective control to reduce the angle of attack and prevent the associated drag from slowing down the rotor system. Of course, doing this also reduces lift, and the helicopter starts to descend. The de-scent forces air up through the rotors and maintains rotor rpm. Essentially, the pilot is consuming the energy stored in the aircraft's altitude to maintain rotor rpm. Generally, this will result in a 1,500-foot-per-minute descent.
While this is happening, the loss of engine torque will cause the helicopter to yaw severely to the left because the tail rotor is still producing thrust. In forward flight, the rear vertical stabilizer fin prevents the fuselage from spinning and gives the pilot time to apply right pedal. In a hover, or at very low airspeeds, the pilot must be quick to press the right pedal, because the fuselage will start to spin and, if not arrested, can build up a fast rotational velocity. Also, bringing the tail rotor to flat pitch removes drag from the system and helps to stop the rpm of the main rotor from decaying.
Once the helicopter is established in an autorotative descent, the pilot must choose the best airspeed to achieve the goal of landing on the desired spot. Because of complex aerodynamics, the rotor system is most efficient at about 60 knots. That means the helicopter will have the lowest rate of descent at that airspeed so it will stay aloft for the longest period of time. This is not to be confused with the speed that produces the best glide range, which is slightly higher. Normally, these speeds are published in the flight manual, and every student should commit them to memory.
Now that the helicopter is stabilized in autorotation at an appropriate speed, the pilot must maneuver the helicopter through turns and speed adjustments to reach the spot of intended landing. This is done by using the cyclic control for pitch (to control airspeed) and bank (used for turns) in the same manner as during powered flight. In addition, the collective control is raised or lowered to maintain proper main rotor rpm by adding or removing drag. The anti-torque pedals control trim. There is a relationship between airspeed, rotor rpm, and rate of descent. For example, increasing airspeed will reduce rpm and increase rate of descent while decreasing airspeed will have the opposite effect. Like a well-choreographed dance, the pilot's control movements interact to produce the desired response.
With the helicopter set up to glide to the landing spot, the pilot waits until the helicopter reaches a height of about 40 to 150 feet above the ground and starts to rapidly reduce airspeed with aft cyclic (commonly referred to as the flare). The exact height to begin the flare depends on the kind of helicopter and the size of the area of intended landing. Helicopters with large, heavy (referred to as high inertia) rotor systems can be flared at higher altitudes. Lighter-weight (low inertia) rotor systems have much less stored energy and should be flared lower to the ground.
When the pilot reduces the airspeed during the flare, the helicopter's rate of descent decreases and the rotor rpm increases. The objective is to bring the helicopter to zero rate of descent and zero airspeed simultaneously while at a height of five to 10 feet above the landing spot. At this point, the helicopter's airspeed and altitude energy are used up, and the helicopter starts to fall. This is where the energy stored in the rotor system is used to cushion the landing. As the helicopter approaches the ground, the pilot raises the collective pitch control, increasing the rotor's angle of attack and receiving a burst of lift before the increasing drag slows the rotor system to a stall. With the right timing, the helicopter will be on the ground with the blades slowing to a stop.
If the engine stops in a low hover (that is, at a height of five to 10 feet with little or no airspeed), immediate action is required. The pilot must apply right pedal, establish a level attitude, and use the collective pitch to cushion the landing - all within a few seconds.
Now, this is the textbook explanation. In an actual autorotation, things can get a little more complicated. For example, a Bell JetRanger helicopter departed an airport with two pilots on board for a company checkride. At about 300 feet above the ground, the engine stopped while the pilot who was being examined was at the controls. He panicked and made no attempt to enter autorotation. The check airman grabbed the controls and lowered the collective to initiate the autorotation and stop the rapid decay of rotor rpm. The rotor rpm at this point was dangerously low at about 70 to 80 percent. With a set of power lines in front of the helicopter, the check airman decided to raise the collective to get a little more lift and clear the lines; however, this pulled the rotor rpm even lower. Once clear of the lines, and with the rotor rpm below 60 percent, the helicopter stopped flying and dropped vertically from 100 feet, landing on a road. The impact crushed the lower seven vertebrae in the backs of both pilots, but they survived.
In this case, the engine failed at a critical time - that is, when the helicopter was low to the ground, had low airspeed, and had a high power setting. Each of these factors played a part in reducing the pilot's ability to perform a successful autorotation. The high power setting meant that rotor blades were operating at a high angle of attack. The significant level of drag at the instant the engine quit started a rapid decay of rotor rpm. The low altitude and slow airspeed gave the pilot very little time to react and no time to recover the rotor rpm. As is often the case, the situation was complicated by the need to maneuver around obstacles.
This raises the question of whether it is prudent to avoid flying in certain areas of the flight envelope. The answer is definitely yes. In fact, helicopter manufacturers publish a chart in the flight manual that depicts combinations of airspeed and altitude that should be avoided. It is commonly referred to as the height-velocity diagram. At low airspeeds and low altitudes, the helicopter would not have enough altitude to allow the pilot to lower the nose and accelerate to an airspeed that would allow a flare. Flight in the high-speed, low-altitude portion of the envelope does not allow the pilot sufficient reaction time to establish a level attitude and may require an aggressive flare that could result in the tail boom striking the ground.
With few exceptions, the height-velocity diagram is located in the performance section of the flight manual, not the limitations section, so the pilot is not prohibited from flying in the danger areas. It reminds the pilot that a successful autorotation may not be possible under certain conditions. It is wise for pilots to be conservative when other factors such as high-density altitude, high power settings, and operations close to gross weight are involved.
The only way for pilots to become proficient at performing autorotations is to practice. The practical test standards require that students demonstrate an autorotation to a point straight ahead, a point behind the helicopter by making a 180-degree turn (although this requirement was recently removed for private pilots), and from a hover.
Since the most likely point for damage to the helicopter to occur is at touchdown, many instructors practice what is called a power recovery autorotation. Since the engine is still running - just not connected to the rotor system - the pilot can roll up the engine rpm with the throttle and re-engage it to the rotors. This is normally done as the pilot flares. At this point, full power is applied to bring the helicopter to a hover. This has become more common because practicing "full touchdown" autorotations has damaged many helicopters. Regardless, instructors must be careful not to overestimate their ability to recover a practice autorotation gone wrong.
Just such a thing happened to actor Harrison Ford. Ford, a certificated helicopter pilot, had been practicing autorotations with an instructor in his Bell LongRanger. The instructor reported that Ford began the flare at about 150 to 200 feet above a dry riverbed and then began to restore engine power to stop the helicopter in a hover. The instructor stated that the engine failed to respond, forcing the pilots to land. The helicopter landed hard and slid forward in loose sand. The left landing skid hit a log, and the helicopter rolled onto its left side. Neither Ford nor the instructor was injured.
Learning to land a helicopter without engine power is exhilarating and satisfying. It is a skill that every helicopter pilot needs to keep practicing - carefully, I might add - because even the most experienced pilot can never be too expert at the art of autorotation.