Autorotation | Helicopter Emergencies and Hazards

The most common reason for an autorotation is failure of the engine or drive line, but autorotation may also be performed in the event of a complete tail rotor failure, since there is virtually no torque produced in an autorotation. In both cases, maintenance has often been a contributing factor to the failure. Engine failures are also caused by fuel contamination or exhaustion as well resulting in a forced autorotation.

If the engine fails, the freewheeling unit automatically disengages the engine from the main rotor, allowing it to rotate freely. Essentially, the freewheeling unit disengages anytime the engine revolutions per minute (rpm) is less than the rotor rpm.

At the instant of engine failure, the main rotor blades are producing lift and thrust from their angle of attack (AOA) and velocity. By lowering the collective (which must be done immediately in case of an engine failure), lift and drag are reduced, and the helicopter begins an immediate descent, thus producing an upward flow of air through the rotor disk. This upward flow of air through the rotor disk provides sufficient thrust to maintain rotor rpm throughout the descent. Since the tail rotor is driven by the main rotor transmission during autorotation, heading control is maintained with the antitorque pedals as in normal flight.

Several factors affect the rate of descent in autorotation: bank angle, density altitude, gross weight, rotor rpm, trim condition, and airspeed. The primary ways to control the rate of descent are with airspeed and rotor rpm. Higher or lower airspeed is obtained with the cyclic pitch control just as in normal powered flight. In theory, a pilot has a choice in the angle of descent, varying, from straight vertical to maximum horizontal range (which is the minimum angle of descent). Rate of descent is high at zero airspeed and decreases to a minimum at approximately 50–60 knots, depending upon the particular helicopter and the factors just mentioned. As the airspeed increases beyond that which gives minimum rate of descent, the rate of descent increases again.

When landing from an autorotation, the only energy available to arrest the descent rate and ensure a soft landing is the kinetic energy stored in the rotor blades. Tip weights can greatly increase this stored energy. A greater amount of rotor energy is required to stop a helicopter with a high rate of descent than is required to stop a helicopter that is descending more slowly. Therefore, autorotative descents at very low or very high airspeeds are more critical than those performed at the minimum rate of descent airspeed. Refer to the height/velocity diagram discussion in Helicopter Performance section.

Each type of helicopter has a specific airspeed and rotor rpm at which a power-off glide is most efficient. The specific airspeed is somewhat different for each type of helicopter, but certain factors affect all configurations in the same manner. In general, rotor rpm maintained in the low green area (see Figure 2) gives more distance in an autorotation.

It is important that pilots experience autorotations from various airspeeds. This provides better understanding of the necessary flight control inputs to achieve the desired airspeed, rotor rpm and autorotation performance, such as the maximum glide or minimum descent airspeed. The decision to use the appropriate airspeed and rotor rpm for the given conditions should be instinctive to reach a suitable landing area. The helicopter glide ratio is much less than that of a fixed-wing aircraft and takes some getting used to. The flare to land at 80 knots indicated airspeed (KIAS) will be significantly greater than that from 55 KIAS. Rotor rpm control is critical at these points to ensure adequate rotor energy for cushioning the landing.

Use collective pitch control to manage rotor rpm. If rotor rpm builds too high during an autorotation, raise the collective sufficiently to decrease rpm back to the normal operating range, then reduce the collective to maintain proper rotor rpm. If the collective increase is held too long, the rotor rpm may decay rapidly. The pilot would have to lower the collective in order to regain rotor rpm. If the rpm begins decreasing, the pilot must again lower the collective. Always keep the rotor rpm within the established recommended range for the helicopter being flown.

RPM Control

Rotor rpm in low inertia rotor systems has been studied in simulator flight evaluations which indicate that the simultaneous application of aft cyclic, down collective, and alignment with the relative wind (trim) at a wide range of airspeeds, including cruise airspeeds, is critical for all operations during the entry of an autorotation. The applicable Rotorcraft Flight Manual (RFM) should be consulted to determine the appropriate procedure(s) for safely entering an autorotation. This is vitally important since the procedure(s) for safely entering an autorotation may vary with specific makes and/or models of helicopters. A basic discussion of the aerodynamics and control inputs for single rotor systems is in order here.

Helicopter pilots must understand the use of the collective for rotor rpm control during power off autorotations in a turn. Upward movement of the collective reduces the rpm and downward movement increases the rpm. Cyclic movement is primarily associated with attitude/airspeed control in powered flight but may not be given the credit appropriate for rotor rpm control during practice and emergency power off autorotations. As long as the line of cyclic movement is parallel with the flight path of the helicopter (trimmed), the aft movement of the cyclic also creates greater air flow up through the bottom of the rotor disk and contributes to an increase in rotor rpm. If the flight path is 10 degrees to the right of the longitudinal axis of the helicopter, theoretically, the cyclic should be moved 10 degrees aft and left of the longitudinal axis to get maximum air up through the rotor system.

As the pilot lowers the collective in reaction to a loss of power during cruise flight there may be a tendency for the nose of the helicopter to pitch down. As a result, the pilot may tend to lean forward slightly, which delays the application of simultaneous aft cyclic to prevent the pitch change and associated loss of rotor rpm. A slight gain in altitude at cruise airspeed during the power off entry into an autorotation should not be of great concern as is the case for the execution of practice or actual quick stops.

Various accident investigations have concluded that, when faced with a real power failure at cruise airspeed, pilots are not simultaneously applying down collective, aft cyclic, and antitorque pedal inputs in a timely manner. Low inertia rotor systems store less kinetic energy during autorotation and, as a result, rotor rpm decays rapidly during deceleration and touchdown. Conversely, less energy is required to regain safe rotor rpm during autorotation entry and autorotative descent. The pilot should immediately apply simultaneous down collective, aft cyclic and trim the helicopter for entry into an autorotation initiated at cruise airspeed. If rotor rpm has been allowed to decrease, or has inadvertently decreased below acceptable limits, an application of aft cyclic may help rebuild rotor rpm. This application of aft cyclic must be made at least at a moderate rate and may be combined with a turn, either left or right, to increase airflow through the rotor system. This will work to increase rotor rpm. Care should be maintained to not over-speed the rotor system as this is attempted.

Risk Management during Autorotation Training

The following sections describe enhanced guidelines for autorotations during rotorcraft/helicopter flight training, as stated in Advisory Circular (AC) 61-140. There are risks inherent in performing autorotations in the training environment, and in particular the 180-degree autorotation. This section describes an acceptable means, but not the only means, of training applicants for a rotorcraft/helicopter airman certificate to meet the qualifications for various rotorcraft/helicopter ratings. You may use alternate methods for training if you establish that those methods meet the requirements of the Helicopter Flying Handbook (HFH), FAA practical test standards (PTS), and the Rotorcraft Flight Manual (RFM).

Straight-In Autorotation

A straight-in autorotation is one made 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 KIAS 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 component of the helicopter closest 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, the more accurate the timing and speed of the flare must be, especially in helicopters with low-inertia rotor disks. If too much collective 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 harder landing with minimal groundspeed. As proficiency increases, the amount of ground run may be reduced.

Technique (How to Practice)

Refer to Figure 3 (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 to the full down position. Use aft cyclic to prevent a nose low attitude while 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 collective will still be able to be reduced even under conditions of extreme reduction of vertical loading (e.g., very low helicopter weight, at very low-density altitude). After entering an autorotation, collective pitch must be adjusted to maintain the desired rotor rpm.

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 increases to a rate higher than that of the engine—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. See the RFM for the manufacturer’s recommendations for autorotation rate of descent.

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. Failing to minimize or eliminate lateral movement during ground contact. (Similar for items 8 and 9)
8. Failing to maintain ground track in the air and keeping the landing gear aligned with the direction of travel during touchdown and ground contact.
9. Failing (in a practice run) to go around if not within limits and specified criteria for safe autorotation.

Autorotation with Turns

Turns (or a series of turns) can be made during autorotation to facilitate landing into the wind or avoiding obstacles. Turns during autorotation should be made early so that the remainder of the autorotation is flown identically to a straight-in autorotation. The most common turns in an autorotation are 90 degrees and 180 degrees. The following technique describes an autorotation with a 180-degree turn.

The pilot establishes the aircraft on a downwind heading at the recommended airspeed, and parallel to the intended touchdown point. Then, taking the wind into account, the pilot establishes the ground track approximately 200 feet laterally from the desired course line to the touchdown point. In strong crosswind conditions, the pilot should be prepared to adjust the downwind leg closer or farther out, as appropriate. The pilot uses the autorotation entry airspeed recommended by the RFM. When abeam the intended touchdown point, the pilot smoothly reduces collective, then reduces power to the engine to show a split between the rotor rpm and engine rpm and simultaneously applies appropriate anti-torque pedal and cyclic to maintain proper attitude/airspeed. Throughout the autorotation, the pilot should continually crosscheck the helicopter’s attitude, rotor rpm, airspeed, and verify that the helicopter is in trim (centered trim ball).

After the descent and autorotation airspeed is established, the pilot initiates the 180-degree turn. For training operations, initially roll into a bank of at least 30 degrees, but no more than 60 degrees. It is important to maintain the proper airspeed, rotor rpm, and trim (centered trim ball) throughout the turn. Changes in the helicopter’s attitude and the angle of bank causes a corresponding change in rotor rpm within normal limits. Do not allow the nose to pitch up or down excessively during the maneuver, as it may cause undesirable rotor rpm excursions.

Pitot-static airspeed indications may be unreliable or lag during an autorotational turn. The pilot should exercise caution to avoid using excessive aircraft pitch attitudes and to avoid chasing airspeed indications in an autorotational turn.

Note: Approaching the 90-degree point, check the position of the landing area. The second 90 degrees of the turn should end with a roll-out on a course line to the landing area. If the helicopter is too close, decrease the bank angle (to increase the radius of turn); if too far out, increase the bank angle (to decrease the radius of the turn). A bank angle of no more than 60 degrees should be encountered during this turn. Monitor the trim ball (along with one’s kinesthetic sense) and adjust as necessary with cyclic and anti-torque pedal to maintain coordinated flight. Prior to passing through 200 feet above ground level (AGL), if landing or making a surface-level power recovery, the turn should be completed, and the helicopter aligned with the intended touchdown area. Upon reaching the course line, set the appropriate crosswind correction. If the collective pitch was increased to control the rpm, it may need to be lowered on rollout to prevent decay in rotor rpm.

This maneuver should be aborted at any point the following criteria is not met: if the helicopter is not in a stabilized approach to landing profile (i.e., it is not aligned as close as possible into the wind with the touchdown point, after completing the 180-degree turn); if the rotor rpm is not within limits; if the helicopter is not at a proper attitude/airspeed; or if the helicopter is not under proper control at 200 feet AGL. It is essential that the pilot on the controls (or a certificated flight instructor (CFI), when intervening) immediately abort the maneuver and execute a smooth power recovery and go-around. It is important for the CFI who is intervening at this point to remember that the go-around is a far safer option than trying to recover lost rotor rpm and reestablish or recover to the hover or even the preferred hover taxi.

From all entry positions, but particularly true of the 180-degree entry, a primary concern is getting the aircraft into the course line with as much altitude as possible. Once the collective has been lowered and the engine set to flight idle, the helicopter will lose altitude. A delayed turn will result in a lower altitude when arriving on the course line. Additionally, an uncoordinated flight condition (trim-ball not centered) results in an increased sink rate, which may be unrecoverable if not corrected.

During the turn to the course line, the pilot should use a scan pattern to see outside as well as inside the cockpit. Of primary importance outside is maintaining the appropriate descending attitude and a proper turn rate. Essential items to scan inside are rotor rpm and centered trim ball. Rotor rpm will build anytime “G” forces are applied to the rotor system. Usually, this occurs in the turn to the course line and during the deceleration flare.

Throughout the maneuver, rotor rpm should be maintained in the range recommended in the RFM. Rotor rpm outside of the recommended range results in a higher rate of descent and less glide-ratio. When the rotor rpm exceeds the desired value as a result of increased G load in the turn, timely use of up collective will increase the pitch of the blades and slow the rotor to the desired rpm. In an autorotation, rotor rpm is the most critical element, as it provides the lift required to stabilize an acceptable rate of descent and the energy necessary to cushion the landing. Collective should be lowered to the full down position to maintain rotor rpm immediately following a loss of power. However, rapid or abrupt collective movement could lead to mast bumping in some rotorcraft with teetering rotor systems.

Energy is a very important property of all rotating components, and the kinetic energy stored in the rotor system is used to cushion the landing. More lift is produced at the bottom of an autorotation by raising the collective, which increases the angle of attack of the blades. The rotor rpm will also rapidly decay at this point and it is essential to properly time the flare and the final collective pull to fully arrest the descent and cushion the landing. Upon arriving into the course line prior to the flare, the scan should focus almost entirely outside. The scan should include:

• The horizon for attitude, ground track, and nose alignment;
• the altitude to set the flare and for closure (groundspeed); and
• the instrument cross-check of airspeed, rotor rpm, and engine rpm in the descent.

Every autorotational flare will be different depending on the existing wind conditions, airspeed, density altitude (DA), and the aircraft gross weight. A pilot operating a helicopter at a high DA needs to take into account the effects on the control of the helicopter when recovering from an aborted autorotation.

Some effects to consider are:

• Higher rate of descent.
• Reduced rotor rpm builds in autorotation.
• Low initial rotor rpm response in autorotation.
• The requirement for a higher flare height.
• Reduced engine power performance.

Common Errors

The following common errors should be prevented:

1. Entering the maneuver at an improper altitude or airspeed.
2. Entering the maneuver without a level attitude (or not in coordinated flight).
3. Entering the maneuver and not correcting from the initial deceleration to a steady state attitude (which allows excessive airspeed loss in the descent).
4. Improper transition into the descent on entry.
5. Improper use of anti-torque on entry.
6. Failure to establish the appropriate crosswind correction, allowing the aircraft to drift.
7. Failure to maintain coordinated flight through the tum.
8. Failure to maintain rotor rpm within the RFM recommended range.
9. Excessive yaw when increasing collective to slow rate of descent during power recovery autorotations.
10. During power recovery autorotations, a delay in reapplying power.
11. Initial collective pull either too high or too low.
12. Improper flare (too much or not enough).
13. Flaring too low or too high (AGL).
14. Failure to maintain heading when reapplying power.
15. Not landing with a level attitude.
16. Landing with aircraft not aligned with the direction of travel.
17. Insufficient collective cushioning during full autorotations.
18. Abrupt control inputs on touchdown during full autorotations.

Practice Autorotation with a Power Recovery

A power recovery is used to terminate practice autorotations at a point prior to actual touchdown. After the power recovery, a landing can be made or a go-around initiated.

Technique (How to Practice)

At approximately 3–15 feet landing gear height AGL, depending upon the helicopter being used, begin to level the helicopter with forward cyclic control. Avoid excessive nose-high, tail-low attitude below 10 feet. Just prior to achieving level attitude, with the nose still slightly up, coordinate upward collective pitch control with an increase in the throttle to join the needles at operating rpm. The throttle and collective pitch must be coordinated properly.

If the throttle is increased too fast or too much, an engine overspeed can occur; if throttle is increased too slowly or too little in proportion to the increase in collective pitch, a loss of rotor rpm results. Use sufficient collective pitch to stop the descent, but keep in mind that the collective pitch application must be gradual to allow for engine response. Coordinate proper antitorque pedal pressure to maintain heading. When a landing is to be made following the power recovery, bring the helicopter to a hover and then descend to a landing.

In nearly all helicopters, when practicing autorotations with power recovery, the throttle should be at the flight setting at the beginning of the flare. As the rotor disk begins to dissipate its energy, the engine is up to speed as the needles join when the rotor decreases into the normal flight rpm.

Helicopters that do not have the throttle control located on the collective are generally exceptions to basic technique and require some additional prudence. The autorotation should be initiated with the power levers left in the “flight,” or normal, position. If a full touchdown is to be practiced, it is common technique to move the power levers to the idle position once the landing area can safely be reached. In most helicopters, the pilot is fully committed at that point to make a power-off landing. However, it may be possible to make a power recovery prior to passing through 100 feet AGL if the powerplant can recover within that time period and the instructor is very proficient. The pilot should comply with the RFM instructions in all cases.

When practicing autorotations to a power recovery, the differences between reciprocating engines and turbines may be profound. The reciprocating powerplant generally responds very quickly to power changes, especially power increases. Some turbines have delay times depending on the type of fuel control or governing system installed. Any reciprocating engine needing turbocharged boost to develop rated horse power may have significant delays to demands for increased power, such as in the power recovery. Power recovery in those helicopters with slower engine response times must have the engines begin to develop enough power to rejoin the needles by approximately 100 feet AGL.

If a go-around is to be made, the cyclic control should be moved forward to resume forward flight. In transition from a practice autorotation to a go-around, exercise caution to avoid an altitude-airspeed combination that would place the helicopter in an unsafe area of its height/velocity diagram.

This is one of the most difficult maneuvers to perform due to the concentration needed when transitioning from powered flight to autorotation and then back again to powered flight. For helicopters equipped with the power control on the collective, engine power must be brought from flight power to idle power and then back to a flight power setting. A delay during any of these transitions can seriously affect rotor rpm placing the helicopter in a situation that cannot be recovered.

The cyclic must be adjusted to maintain the required airspeed without power, and then used for the deceleration flare, followed by the transition to level hovering flight. Additionally, the cyclic must be adjusted to remove the compensation for translating tendency. The tail rotor is no longer needed to produce antitorque thrust until almost maximum power is applied to the rotor disk for hovering flight, when the tail rotor must again compensate for the main rotor torque, which also demands compensation for the tail rotor thrust and translating tendency.

The pedals must be adjusted from a powered flight anti-torque trim setting to the opposite trim setting to compensate for transmission drag and any unneeded vertical fin thrust countering the now nonexistent torque and then reset to compensate for the high power required for hovering flight.

All of the above must be accomplished during the 23 seconds of the autorotation, and the quick, precise control inputs must be made in the last 5 seconds of the maneuver.

Common Errors

1. Initiating recovery too late, which requires a rapid application of controls and results in overcontrolling.
2. Failure to obtain and maintain a level attitude near the surface.
3. Failure to coordinate throttle and collective pitch properly, which results in either an engine overspeed or a loss of rotor rpm.
4. Failure to coordinate proper antitorque pedal with the increase in power.
5. Late engine power engagement causing excessive temperature or torque, or rpm drop.
6. Failure to go around if not within limits and specified criteria for safe autorotation.

Practicing Power Failure in a Hover

Power failure in a hover, also called hovering autorotation, is practiced so that a pilot can automatically make the correct response when confronted with engine stoppage or certain other emergencies while hovering. The techniques discussed in this section are for helicopters with a counterclockwise rotor disk and an antitorque rotor.

Technique (How to Practice)

To practice hovering autorotation, establish a normal hovering height (approximately 2–3 feet) for the particular helicopter being used, considering load and atmospheric conditions. Keep the helicopter headed into the wind and hold maximum allowable rpm.

To simulate a power failure, firmly roll the throttle to the engine idle position. This disengages the driving force of the engine from the rotor, thus eliminating torque effect. As the throttle is closed, apply proper antitorque pedal to maintain heading. Usually, a slight amount of right cyclic control is necessary to keep the helicopter from drifting to the left, to compensate for the loss of tail rotor thrust. However, use cyclic control, as required, to ensure a vertical descent and a level attitude. Do not adjust the collective on entry.

Helicopters with low inertia rotor disks settle immediately. Keep a level attitude and ensure a vertical descent with cyclic control while maintaining heading with the pedals. Any lateral movement must be avoided to prevent dynamic rollover. As rotor rpm decays, cyclic response decreases, so compensation for the winds will require more cyclic input. At approximately 1 foot AGL, apply upward collective control, as necessary, to slow the descent and cushion the landing without arresting the rate of descent above the surface. Usually, the full amount of collective is required just as the landing gear touches the surface. As upward collective control is applied, the throttle must be held in the idle detent position to prevent the engine from re-engaging. The idle detention position is a ridged stop position between idle and off in which the idle release button snaps into, prevent accidental throttle off.

Helicopters with high-inertia rotor disks settle more slowly after the throttle is closed. In this case, when the helicopter has settled to approximately 1 foot AGL, apply upward collective control while holding the throttle in the idle detent position to slow the descent and cushion the landing. The timing of collective control application and the rate at which it is applied depend upon the particular helicopter being used, its gross weight, and the existing atmospheric conditions. Cyclic control is used to maintain a level attitude and to ensure a vertical descent. Maintain heading with antitorque pedals.

When the weight of the helicopter is entirely resting on the landing gear, cease application of upward collective. When the helicopter has come to a complete stop, lower the collective pitch to the full-down position.

The timing of the collective movement is a very important consideration. If it is applied too soon, the remaining rpm may not be sufficient to make a soft landing. On the other hand, if it is applied too late, surface contact may be made before sufficient blade pitch is available to cushion the landing. The collective must not be used to hold the helicopter off the surface, causing a blade stall. Low rotor rpm and ensuing blade stall can result in a total loss of rotor lift, allowing the helicopter to fall to the surface and possibly resulting in blade strikes to the tail boom and other airframe damage such as landing gear damage, transmission mount deformation, and fuselage cracking.

Common Errors

1. Failure to use sufficient proper antitorque pedal when power is reduced.
2. Failure to stop all sideward or backward movement prior to touchdown.
3. Failure to apply up-collective pitch properly, resulting in a hard touchdown.
4. Failure to touch down in a level attitude.
5. Failure to roll the throttle completely to idle.
6. Failure to hover at a safe altitude for the helicopter type, atmospheric conditions, and the level of training/proficiency of the pilot.
7. Failure to go around if not within limits and specified criteria for safe autorotation.