Straight-In Autorotation - Autorotation (Part 2) - Helicopter Emergencies and Hazards

A straight-in autorotation implies an autorotation from altitude with no turns. Winds have a great effect on an autorotation. Strong headwinds cause the glide angle to be steeper due to the slower groundspeed. For example, if the helicopter is maintaining 60 knots indicated airspeed and the wind speed is 15 knots, then the groundspeed is 45 knots. The angle of descent will be much steeper, although the rate of descent remains the same. The speed at touchdown and the resulting ground run depend on the groundspeed and amount of deceleration. The greater the degree of deceleration, or flare, and the longer it is held, the slower the touchdown speed and the shorter the ground run. Caution must be exercised at this point as the tail rotor will be the closest component of the helicopter to the ground. If timing is not correct and a landing attitude not set at the appropriate time, the tail rotor may contact the ground causing a forward pitching moment of the nose and possible damage to the helicopter.

A headwind is a contributing factor in accomplishing a slow touchdown from an autorotative descent and reduces the amount of deceleration required. The lower the speed desired at touchdown is, the more accurate the timing and speed of the flare must be, especially in helicopters with low-inertia rotor systems. If too much collective pitch is applied too early during the final stages of the autorotation, the kinetic energy may be depleted, resulting in little or no cushioning effect available. This could result in a hard landing with corresponding damage to the helicopter. It is generally better practice to accept more ground run than a hard landing with minimal groundspeed. As proficiency increases, the amount of ground run may be reduced.

Technique


Refer to Figure 1 (position 1). From level flight at the appropriate airspeed (cruise or the manufacturer’s recommended airspeed), 500–700 feet above ground level (AGL), and heading into the wind, smoothly but firmly lower the collective pitch control to the full down position, maintaining rotor rpm in the green arc with collective. If the collective is in the full down position, the rotor rpm is then being controlled by the mechanical pitch stops. During maintenance, the rotor stops must be set to allow minimum autorotational rpm with a light loading. This means that some collective pitch adjustment can be made if the air density or helicopter loading changes. After entering an autorotation, collective pitch must be adjusted to maintain the desired rotor rpm.

Helicopter Emergencies and Hazards
Figure 1. Straight-in autorotation

Coordinate the collective movement with proper antitorque pedal for trim, and apply cyclic control to maintain proper airspeed. Once the collective is fully lowered, decrease throttle to ensure a clean split/separation of the needles. This means that the rotor rpm is higher than the engine rpm and a clear indication that the freewheeling unit has allowed the engine to disconnect. After splitting the needles, readjust the throttle to keep engine rpm above normal idling speed, but not high enough to cause rejoining of the needles. The manufacturer often recommends the proper rpm for that particular helicopter.

At position 2, adjust attitude with cyclic control to obtain the manufacturer’s recommended autorotation or best gliding speed. Adjust collective pitch control, as necessary, to maintain rotor rpm in the green arc. Aft cyclic movements cause an increase in rotor rpm, which is then controlled by a small increase in collective pitch control. Avoid a large collective pitch increase, which results in a rapid decay of rotor rpm, and leads to “chasing the rpm.” Avoid looking straight down in front of the aircraft. Continually crosscheck attitude, trim, rotor rpm, and airspeed.

At the altitude recommended by the manufacturer (position 3), begin the flare with aft cyclic control to reduce forward airspeed and decrease the rate of descent. Maintain heading with the antitorque pedals. During the flare maintain rotor rpm in the green range. Care must be taken in the execution of the flare so that the cyclic control is neither moved rearward so abruptly that it causes the helicopter to climb nor moved so slowly that it does not arrest the descent, which may allow the helicopter to settle so rapidly that the tail rotor strikes the ground. In most helicopters, the proper flare attitude is noticeable by an apparent groundspeed of a slow run. When forward motion decreases to the desired groundspeed, which is usually the lowest possible speed (position 4), move the cyclic control forward to place the helicopter in the proper attitude for landing.

In many light helicopters, the student pilot can sit in the pilot seat while the instructor pulls down on the helicopter’s tail until the tail rotor guard or “stinger” touches the surface. This action gives the student an idea of airframe attitude to avoid, because a pilot should never allow ground contact unless the helicopter is more nose low than that attitude. Limiting the flare to that pitch attitude may result in slightly faster touchdown speeds, but will eliminate the possibility of tail rotor impact on level surfaces.

The landing gear height at this time should be approximately 3–15 feet AGL, depending on the altitude recommended by the manufacturer. As the apparent groundspeed and altitude decrease, the helicopter must be returned to a more level attitude for touchdown by applying forward cyclic. Some helicopters can be landed on the heels in a slightly nose high attitude to help decrease the forward groundspeed whereas others must land skids or landing gear level to equally spread the landing loads to all of the landing gear. Extreme caution should be used to avoid an excessive nose high and tail low attitude below 10 feet. The helicopter must be close to the landing attitude to keep the tail rotor from contacting the surface.

At this point, if a full touchdown landing is to be made, allow the helicopter to descend vertically (position 5). Increase collective pitch, as necessary, to arrest the descent and cushion the landing. This collective application uses some of the potential energy in the rotor system to help slow the descent rate of the helicopter. Additional antitorque pedal is required to maintain heading as collective pitch is raised due to the reduction in rotor rpm and the resulting reduced effect of the tail rotor. Touch down in a level flight attitude.

Control response with increased pitch angles will be slightly different than normal. With a decrease in main rotor rpm, the antitorque authority is reduced, requiring larger control inputs to maintain heading at touchdown.

Some helicopters have a canted tail stabilizer like the Schweitzer 300. It is crucial that the student apply the appropriate pedal input at all times during the autorotation. If not the tailboom tends to swing to the right, which allows the canted stabilizer to raise the tail. This can result in a severe nose tuck which is quickly corrected with right pedal application.

A power recovery can be made during training in lieu of a full touchdown landing. Refer to the section on power recovery for the correct technique. After the helicopter has come to a complete stop after touchdown, lower the collective pitch to the full-down position. Do not try to stop the forward ground run with aft cyclic, as the main rotor blades can strike the tail boom. Rather, by lowering the collective slightly during the ground run, more weight is placed on the undercarriage, slowing the helicopter.

One common error is holding the helicopter off the surface versus cushioning the helicopter on to the surface during an autorotation. Holding the helicopter in the air by using all of the rotor rpm potential energy usually causes the helicopter to have a hard landing, which results in the blades flexing down and contacting the tail boom. The rotor rpm should be used to cushion the helicopter on to the surface for a controlled, smooth landing instead of allowing the helicopter to drop the last few inches.

Common Errors


1. Not understanding the importance of an immediate entry into autorotation upon powerplant or driveline failure.
2. Failing to use sufficient antitorque pedal when power is reduced.
3. Lowering the nose too abruptly when power is reduced, thus placing the helicopter in a dive.
4. Failing to maintain proper rotor rpm during the descent.
5. Applying up-collective pitch at an excessive altitude, resulting in a hard landing, loss of heading control, and possible damage to the tail rotor and main rotor blade stops.
6. Failing to level the helicopter or achieve the manufacturers preferred landing attitude.
7. Failure to maintain ground track in the air and keeping the landing gear aligned with the direction of travel during touchdown and ground contact.
8. Failure to minimize or eliminate lateral movement during ground contact.
9. Failure to go around if not within limits and specified criteria for safe autorotation.