C4701

For the following discuss items see C4202 Discuss Items

Loss of Tail Rotor Effectiveness
Ref: N 11-7

Four aircraft characteristics during low-speed flight have been identified through extensive flight and wind tunnel tests as contributing factors in unanticipated right yaw.

For this occurrence, certain relative wind velocities and azimuth (direction of relative wind) must be present. The aircraft characteristics and relative wind azimuth regions are:

1. Weathercock stability (120 to 240°)

2. Tail rotor vortex ring state (210 to 330°)

3. Main rotor vortex disk interference (285 to 315°)

4. Loss of translational lift (all azimuths).

5. Angle of Attack Reduction (060-120°). (out of Aero book pg. 9-29)

The aircraft can be operated safely in the above relative wind regions if proper attention is given to controlling the aircraft. However, if the pilot is inattentive for some reason and a right yaw is initiated in one of the above relative wind regions, the yaw rate may increase unless suitable corrective action is taken.

Weathercock Stability (120 to 240°)

Winds within this region will attempt to weathervane the nose of the aircraft into the relative wind. This characteristic comes from the fuselage and vertical fin. The helicopter will make an uncommanded turn either to the right or left depending upon the exact wind direction unless a resisting pedal input is made. If a yawrate has been established in either direction, it will be accelerated in the same direction when the relative wind enters the 120 to 240° shaded area of Figure 11-3 unless corrective pedal action is made. The importance of timely corrective action by the pilot to prevent high yaw rates from occurring cannot be overstressed.

Tail Rotor Vortex Ring State (210 to 330°)

Winds within this region, as shown in Figure 11-4, will result in the development of the vortex ring state of the tail rotor. The tail rotor vortex ring state causes tail rotor thrust variations that result in yaw rates. Since these tail rotor thrust variations do not have a specific period, the pilot must make corrective pedal inputs as the changes in yaw acceleration are recognized. The resulting high pedal workload in tail rotor vortex ring state is well known and helicopters are operated routinely in this region. This characteristic presents no significant problems unless corrective action is not timely. If a right yaw rate is allowed to build, the helicopter can rotate into the wind azimuth region where weathercock stability will then accelerate the right turn rate. Pilot workload during tail rotor vortex ring state will be high; therefore, the pilot must concentrate fully on flying the aircraft and not allow a right yaw rate to build.

Main Rotor Disk Vortex (285 to 315°)

Winds within this region, as shown in Figure 11-5, can cause the main rotor vortex to be directed onto the tail rotor. The effect of this main rotor disk vortex is to change the tail rotor angle of attack. Initially, as the tail rotor comes into the area of the main rotor disk vortex during a right turn, the angle of attack of the tail rotor is increased. This increase in angle of attack requires the pilot to add right pedal (reduce thrust) to maintain the same rate of turn. As the main rotor vortex passes the tail rotor, the tail rotor angle of attack is reduced. The reduction in angle of attack causes a reduction in thrust and a right yaw acceleration begins. This acceleration can be surprising, since the pilot was previously adding right pedal to maintain the right turn rate. Analysis of flight test data during this time verifies that the tail rotor does not stall. The helicopter will exhibit a tendency to make a sudden, uncommanded right yaw which, if uncorrected, will develop into a high right turn rate. When operating in this region, the pilot must anticipate the need for sudden left pedal inputs.

Loss of Translational Lift

The loss of translational lift results in increased power demand and additional anti-torque requirements. If the loss of translational lift occurs when the aircraft is experiencing a right turn rate, the right turn rate will be accelerated as power is increased, unless corrective action is taken by the pilot. When operating at or near maximum power, this increased power demand could result in rotor rpm decay. This characteristic is most significant when operating at or near maximum power and is associated with unanticipated right yaw for two reasons. First, if the pilot’s attention is diverted as a result of the increasing right yaw rate, he may not recognize that he is losing relative wind and, hence, losing translational lift. Second, if the pilot does not maintain airspeed while making a right downwind turn, the aircraft can experience an increasing right yaw rate as the power demand increases and the aircraft develops a sink rate. Insufficient pilot attention to wind direction and velocity can lead to an unexpected loss of translational lift. The pilot must continually consider aircraft heading, groundtrack, and apparent groundspeed, all of which contribute to wind drift and airspeed sensations. Allowing the helicopter to drift over the ground with the wind results in a loss of relative windspeed and a corresponding decrease in the translational lift produced by the wind. Any reduction in translational lift will result in an increase in power demand and anti-torque requirements.

Procedures: Ref: N 14-20

*1. Pedals - Maintain full left pedal *2. Collective - Reduce (as altitude permits) *3. Cyclic - Forward to increase airspeed

If spin cannot be stopped:

*4. Autorotative landing - Execute

Angle of Attack Reduction (060 to 120°).
In a right crosswind, the relative wind shifts toward the rail rotor blade's chordline because of effectively increased induced velocity. The shifted relative wind impacts at a lower angle of attack, which develops lower lift and results in less thrust. The pilot will automatically compensate by adding more left pedal, but in some cases can reach pedal travel limits before adequate thrust can be generated.

=Complete loss of tail rotor thrust= Ref: N 14-17

Probable causes: tail rotor drive shaft severed; loss of tail rotor blades

Helo reaction: nose of helo will swing rapidly to the right in a hover with an accompanying sideslip in forward flight.

Procedures:

In a hover: During transition to forward flight: At altitude:
 * 1. Twist grip - closed.
 * 2. Cyclic - eliminate drift.
 * 3. Collective - increase to cushion landing.
 * 1. Twist grip - closed.
 * 2. Cyclic - eliminate sideward drift.
 * 3. Collective - increase to cushion landing.


 * 1. Collective - Reduce to minimize yaw
 * 2. Cyclic - Adjust for best airspeed to control yaw

WARNING

- Airspeed indications during a sideslip are unreliable. At airspeeds below approximately 50 its, the sideslip may suddenly become uncontrollable, and the helicopter will begin an unrecoverable vertical axis "flat spin."

- If attempting to achieve higher airspeeds, care must be taken to avoid excessive cyclic input coupled with large power settings that could lead to mast bumping or rapid nose tucking.

Note:

- Depending on the nature of the failure and degree of damage, airspeeds between 50-72 KIAS may provide the best opportunity to maintain level flight. Due to yaw stiffness provided by the vertical fin at higher airspeeds, it may be possible to continue at faster airspeeds in cruise or descent depending on power requirements.

- A non-typical nose down attitude may be required to achieve a desired airspeed, due to increased drag on the tail.

- Turns to the right may provide greater controllability of airspeed and potentially minimize altitude loss.

- Banking to the left will aid in counteracting torque.

If yaw is not controllable:
 * 3. Autorotate.
 * 4. Twist grip - closed prior to flare

If yaw is controllable:
 * 5. Continue powered flight and set up to a suitable landing area at or above minimum rate of descent auto-rotational airspeed.
 * 6. Autorotate.
 * 6. Twist grip - closed prior to flare

WARNING:

- In the autorotation, maintain airspeed above minimum rate of descent airspeed until flare to avoid loss of yaw control.

- Once the engine is secured, in the absence of torque, the lift produced by the vertical fin may tend to yaw the nose left at faster airspeeds. As the airspeed slows and Nr decays, the decelerating rotorhead and swashplate friction will create additional left yaw, increasing the chance for roller over. Depending on landing profile, consideration should be given to leaving the twist grip open until pulling collective at the bottom of the autorotation to allow conrol of yaw with twist grip.

=Fixed Pitch Left Pedal Applied (High Power)= Ref: N 14-19

Probable causes: pedals locked in fixed position because of FOD; control linkage failure during a left pedal applied situation.

Helo reaction: Pilot will be unable to control left yaw with pedal input. Decrease in power will aggravate the yaw or sideslip.

Procedures:

In a hover:

If rate of rotation is not excessive and landing surface is smooth and firm: If rate of rotation is excessive or landing surface is unsuitable for a power on landing:
 * 1. Collective - decrease to effect a power on landing.
 * 2. Twist grip - slowly reduce while increasing collective to stop rotation.
 * 3. Collective - coordinate with twist grip to maintain heading and allow aircraft to settle.

At altitude:

1. Maintain airspeed and engine rpm to streamline the aircraft.

2. Plan an approach to a smooth, level surface into the wind or with a slight left crosswind if possible.

3. Establish a normal approach and maintain 60 KIAS during the initial part of the approach.

4. On final approach, maintain engine rpm within limits and begin a slow deceleration in order to arrive at a point about 2 ft above the intended touchdown area as effective translational lift is lost.

5. Apply collective pitch to slow the rate of descent and align the helicopter with the intended landing path. If the aircraft is not aligned after pitch application, adjust the twist grip to further help with alignment. Allow the aircraft to touch down at near zero groundspeed maintaining alignment with the twist grip.

Note: In a fixed pitch left pedal situation, it is possible for the pilot to slow the aircraft to a hover and effect such a recovery.

=Fixed Pitch Right Pedal Applied (Low Power)= Ref: N 14-18

Probable causes: pedals locked in fixed position due to FOD; control linkage failure during a right pedal applied situation.

Helo reaction: Pilot will be unable to control right yaw with pedals. Increase in power will aggravate yaw or sideslip.

Procedure:

In a hover:

If rate of rotation is not excessive and landing surface is smooth and firm: If rate of rotation is excessive or landing surface is unsuitable for a power on landing:
 * 1. Collective - decrease to effect a power on landing.
 * 2. Twist grip - reduce as nose approaches windline.
 * 3. Cyclic - eliminate drift.
 * 4. Collective - increase to cushion landing.

At altitude:

1. Maintain airspeed and engine rpm to streamline the aircraft.

2. Plan an approach to a smooth level surface into the wind or with a slight left crosswind if possible.

3. Establish a shallow approach, maintaining 60 KIAS until on final.

Note: In such an approach profile, it is not unusual for the nose to be yawed slightly to the left.

4. At 50 to 75 ft AGL and when the landing area can be made, start a slow deceleration to arrive over the intended point with minimum forward speed required for directional control.

5. At approximately 2 to 3 ft skid height, increase collective to slow the rate of descent and coordinate twist grip to maintain nose alignment.

Warning: If necessary, a waveoff should be made early in the approach, using cyclic to increase forward airspeed. If it becomes necessary to use large collective inputs to wave off near the deck, the nose will yaw right and possibly enter uncontrolled flight.

Note: If nose swings right after touchdown, follow the turn with cyclic to prevent the aircraft from rolling over.