SL Helicopter Flying Handbook/Helicopter Emergencies and Hazards

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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
  7. Landing with aircraft not aligned with direction of travel

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.

Common Errors

  1. Not lowering collective quickly enough after an engine failure
  2. Not beginning turn immediately as the autorotation is established
  3. Failing to apply right rudder when lowering collective
  4. Failing to maintain proper rotor RPM
  5. Failing to apply up collective when necessary to prevent overspeed
  6. Flairing too early or too late
  7. Failing to level helicopter after the flair
  8. Landing with aircraft not aligned with direction of travel

1.3 Autorotations from a Hover

A power failure in a hover requires a different technique called a hovering autorotation. To practice this technique use the following procedure:

  1. Simulate power failure by rolling throttle to idle
  2. Apply immediate right pedal to stop the turn due to the left pedal applied for torque
  3. As helicopter begins to settle, raise collective to cushion landing
  4. Once helicopter is on the ground, lower collective and neutralize controls

Common Errors

  1. Failure to maintain heading with proper pedal usage
  2. Failure to stop any sideways or forward/back movement with cyclic
  3. Applying collective too soon or too late resulting in ballooning or a hard landing
  4. Failure to touch down in level attitude
  5. Failure to lower collective after touchdown to prevent rotor damage
1.3.1 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

Figure 2: Vortex Ring State

Vortex ring state (Figure 2), 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 knots
  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.

When practicing recovery from vortex ring state, choose a high enough altitude that a recovery can be made before ground contact. Try to enter the maneuver at least 1000 AGL. Then use the following procedure:

  1. Bring helicopter to an OGE hover with zero forward airspeed
  2. Lower collective and allow helicopter to descend vertically
  3. Once helicopter is descending at more than 500 feet/minute, pull up collective until vortex ring state develops.
  4. Recover by simultaneously lowering collective and applying forward cyclic
  5. Resume normal forward flight at above 30 knots

Common Errors

  1. Having too much lateral speed to enter VRS
  2. Excessive use of collective
  3. Loosing too much altitude due to slow recovery

3 Retreating Blade Stall

Retreating blade stall, which occurs when airflow over the retreating blade becomes to low causing that side of the rotor disk to stall, was first discussed in SECTION 2. Aerodynamics - Retreating Blade Stall. It usually occurs at high airspeeds and is usually the determining factor in the maximum airspeed of the aircraft. In helicopter with counter-clockwise turning rotors, the retreating blade will be on the left side of the rotor disk. As the stall deepens, there will be a rapid decrease in overall left, and a rapid roll to the left. The natural impulse will be to apply right cyclic, but this is the incorrect response.

The correct response when retreating blade stall is inadvertently entered should be to simultaneously:

  • Reduce collective to reduce angle of attack on blades
  • Apply aft cyclic to reduce forward airspeed

In general, the pilot should avoid entering retreating blade stall by paying attention to the airspeed limitations of the aircraft.

4 Low Rotor RPM and Rotor Stall

Low rotor RPM blade stall differs from retreating blade stall in that it can happen at any airspeed. Low rotor RPM is usually due to pilot mismanagement of the available power, or failure to react quickly to loss of power. As collective pitch is increased, the amount of power needed to maintain sufficient rotor RPM also increases. If the available engine power does not meet the demand, the rotor RPM will begin to decay. If the rotor RPM is allowed to drop below about 80%, it may be nearly impossible to recover. For this reason, it is important for pilots to develop an awareness for their rotor RPM during flight.

In normal operation, engine power is usually indicated by a torque gauge (turbine aircraft) or a manifold pressure gauge (piston aircraft). A red line on the gauge usually shows the upper bound on the available power. The pilot should observe operational limits, as well as monitor rotor RPM during flight. In the event that a low rotor RPM condition is encounter inadvertently, the pilot should lower the collective and apply gentle aft cyclic to reduce the angle of attack of the blades and thus the power demand on the engine.

5 System Malfunctions

5.1 Anti-Torque System Failure

The anti-torque system, usually a tail rotor, applies a torque to oppose the torque generated by the main rotor system. If the tail rotor fails in flight either through direct damage to the blades, or damage to the gearboxes that drive the tail rotor, the helicopter will yaw in the direction of the torque (usually to the right). This may make the helicopter difficult to control.

When such a failure occurs in cruise flight, the pilot should enter an autorotation (see Autorotations) and apply cyclic to maintain the normal autorotation speed for the aircraft (generally around 60 knots). Entering an autorotation will eliminate the torque from the main rotor and stop the spin. Maintaining forward speed will allow weather-cock stability to keep the aircraft oriented in the direction for forward movement.

When a tail rotor failure occurs in a hover, the pilot should quickly roll the throttle to idle, then perform a hovering autorotation (see Hovering Autorotations). Idling the engine will remove the torque and allow a normal hovering autorotation to be performed.

5.2 Main Drive Shaft or Clutch Failure

The failure of the main drive shaft or clutch, will have a similar effect to an engine failure. The pilot should immediately enter an autorotation. However, depending on the type of failure, it is also possible for the engine to overspeed. Once safely established in an autorotation, the pilot should then reduce the engine to idle to prevent any additional damage.

5.3 Governor or Fuel Control Failure

The purpose of the governor or fuel controller is to regulate engine power so as to maintain a constant RPM. When the governor or fuel flow fails, automatic adjustment of the throttle will be inoperative, and manually control will be necessary. In general, manually throttle changes will be necessary when the collective changes. For this reason, any collective changes should be made as slowly as possible to reduce workload on the pilot. See the flight model for your specific version to see the procedures for manual throttle adjustment.

5.4 Hydraulic Failure

Many helicopters use a hydraulic assist system to reduce the flight control forces necessary to operate the aircraft. The hydraulic system is typically comprised of servos on each flight control, a pump (usually driven by the transmission), and a reservoir. A switch in the cockpit can be used to turn the system on or off. Furthermore, in some aircraft, there may be multiple redundant hydraulic systems.

When a hydraulic system fails, the forces necessary to control the aircraft will be greatly increased. The pilot should identify the affected system or systems and first try to restart it, and if not possible, shut down the affected system(s). Control inputs without hydraulic assist should be made as gradually as possible, and a shallow run-on landing is recommended.

6 Multi-Engine Emergency Operations

6.1 Single-Engine Failure

A single engine failure in a dual-engine helicopter is often a non-event if handled properly. It is often possible to continue flight on the remaining engine to a suitable landing area. Pilots should refer to the helicopter flight manual for any airspeed or other restrictions that mat apply under single-engine flight.

The most critical aspect of handling a single-engine failure is ensuring the pilot correctly identifies and shuts down the failed engine. Shutting down the wrong engine is likely to end in disaster. Unlike most multi-engine airplanes, there is no telltale yaw to indicate the failed engine. The pilot must use the engine instruments looking for a loss of torque or RPM.

6.2 Dual-Engine Failure

A dual engine failure results in conditions that are similar to engine failure in single-engine helicopters. The pilot must immediately enter an autorotation and select a suitable landing location. Pilots of multi-engine helicopters should avoid becoming complacent about having two engines as common-mode failures such as misfuelling can sometimes lead to dual engine failures.