SL Helicopter Flying Handbook/Helicopter Emergencies and Hazards

SECTION 10. Helicopter Emergencies and Hazards

Figure 1: Normal vs Autorotative Flight

1 Autorotations

An autorotation is a descent in a helicopter in which the rotor blades are disengaged from the engine. In an autorotation, the blades are kept turning purely by the flow of air up through the rotor system as shown in Figure 1 (see SECTION 2. Aerodynamics - Autorotation for details). The most common reason for needing to perform an autorotation is an engine failure, but an autorotation may also be used in the event of a tail rotor failure since while in autorotation, there is minimal main rotor torque. In both cases, neglected maintenance is often the root cause, but contaminated fuel, or fuel exhaustion may also lead to an engine failure.

When the engine fails, the freewheeling unit, or sprag clutch, automatically disengages the rotor system from the engine. The sprag clutch will disengage any time the engine rpm is less than the rotor rpm, similar to coasting on a bicycle.

As soon as a loss of engine power is detected by the pilot, the pilot must fully lower the collective as soon as possible (within a 1-2 seconds). This reduces the pitch of the blades allowing autorotation to occur. If the pilot delays lowering the collective, rotor RPM may decay to the point where recovery is impossible. In general, once rotor RPM falls below 80%, it may be impossible to recover RPM and a catastrophic landing is inevitable.

Once in an autorotation, the pilot may control the aircraft with the cyclic and pedals as normal to maneuver the aircraft to a suitable landing location. Use forward and backward cyclic for airspeed, and left/right cyclic to turn. Descent rate at 0 knots forward speed will be the highest, the descent rate minimized at 50-60 knots. While the pilot is free to manage airspeed as necessary to reach a touchdown during autorotation, the pilot should target a forward speed of 60 knots just prior to touchdown.

The pilot should monitor rotor RPM while executing the maneuver. If the rotor RPM begins to increase beyond normal operating RPM, the pilot should apply a small amount of up collective to slow the rotor RPM to within acceptable limits. If the RPM becomes too low, lower the collective again.

1.1 Straight-in Autorotation

A straight-in autorotation is an autorotation made without turns to a point directly in the flight path. Things to consider with straight-in autorotations include wind speed. A stronger headwind will result in a steeper angle of descent due to the reduced groundspeed.

The recommended altitude for practice autorotations is 500 feet AGL. Recommended procedures are as follows:

1. Set up a stabilized constant altitude approach to the runway at 500 feet AGL, then just before crossing the runway threshold, lower collective to minimum, roll throttle to idle, and apply right pedal as necessary to maintain coordination (use yaw string or turn coordinator for reference). Throttle can be rolled to idle using the recommended procedures for your specific helicopter. For a runway that starts at a sim edge, you can set up the autorotation before crossing the sim boundary.
2. Maintain a forward speed of 50-60 mph during the descent. This speed can be adjusted up or down slightly in order to land at a specific spot, but should be within this target range before entering the flare.
3. At approximately 30 to 50 feet AGL, begin a cyclic flare by pulling back on the cyclic.
4. At approximately 10 feet AGL, level the helicopter with cyclic, and begin pulling up on the collective to cushion the landing.

Timing of the flare and collective input is critical. Since throttle is locked in idle, RPM will being to decay as soon as collective is pulled. Pulling too soon will result in RPM decaying while still airborne with the helicopter dropping to the ground. Pulling too late will result in a hard landing.

Common Errors

1. Not lowering collective quickly enough after an engine failure
2. Failing to apply right rudder when lowering collective
3. Failing to maintain proper rotor RPM
4. Failing to apply up collective when necessary to prevent overspeed
5. Flairing too early or too late
6. Failing to level helicopter after the flair

1.2 Autorotation with Turns

Frequently, the optimal touch-down point will not be directly in front of the helicopter at the time of an engine failure. The pilot will need to maneuver the helicopter to a position from which a safe autorotation can be completed. Most of the same procedures that apply to straight-in autorotations apply to autorotations with turns. One additional factor that may apply when turning is that the increased load factor will cause the rotor RPM to increase. For this reason, the pilot must pay extra attention to rotor RPM and be prepared to lift the collective slightly during the turn to prevent overspeeding.

The recommended procedure for a practice 180-degree autototation is as follows:

1. Begin the procedure at 500 AGL flying parallel to and in the opposite direction of the runway.
2. On passing the planned touch down point, lower the collective, cut engine to idle if possible, and enter a 180 degree turn toward the runway.
3. Once aligned with the runway, roll out of the turn, and complete the maneuver as a straight-in autorotation.

1.4 Practicing Hovering Autorotations

Practice hovering autorotations from a medium height hover. The apply the following procedures:

1. Click the "idle lock" button on HUD to lock throttle in idle position.
2. Apply immediate right pedal to hold heading and maintain collective until helicopter begins to sink
3. Apply full up collective as helicopter begins to sink
4. Lower collective once helicopter is fully on the ground

The goal should be to set down softly with little or no change in direction.

2 Vortex Ring State

Vortex ring state, sometimes called "settling with power", is a dangerous condition that can occur when a helicopter is descending into its own downwash. Essentially a vortex ring system engulfs the rotors and they fail to produce lift. Once in vortex ring state, increases in power in an attempt to slow the descent will only make the condition worse, thus actually increasing the descent rate. If the condition is allowed to develop too far, or you are too close to the ground, it may be impossible to recover. You are at danger for vortex ring state when all three of the the following conditions hold:

1. Descent rate greater than 300 feet/minute
2. Airspeed less than 30 mph
3. More than 50% power

If you have alerts turned on, your helicopter will alert you when you are in vortex ring state. To recover from vortex ring state, you should lower the collective and apply forward cyclic to regain airspeed. However, the best practice is to avoid it in the first place.

3 Retreating Blade Stall

TBD

4 Low Rotor RPM and Rotor Stall

TBD