直升机飞行手册Rotorcraft flying handbook

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presence of obstructions, natural or manmade. For

example, a clearing in the woods, a city street, a road, a

building roof, etc., can each be regarded as a confined

area. Generally, takeoffs and landings should be made

into the wind to obtain maximum airspeed with minimum groundspeed.

There are several things to consider when operating in

confined areas. One of the most important is maintaining

a clearance between the rotors and obstacles forming the

confined area. The tail rotor deserves special consideration because, in some helicopters, you cannot always see

it from the cabin. This not only applies while making the

approach, but while hovering as well. Another consideration is that wires are especially difficult to see;

however, their supporting devices, such as poles or

towers, serve as an indication of their presence and

approximate height. If any wind is present, you should

also expect some turbulence. [Figure 10-8]

Something else for you to consider is the availability of

forced landing areas during the planned approach. You

should think about the possibility of flying from one

alternate landing area to another throughout the

approach, while avoiding unfavorable areas. Always

leave yourself a way out in case the landing cannot be

completed or a go-around is necessary.

APPROACH

A high reconnaissance should be completed before initiating the confined area approach. Start the approach

phase using the wind and speed to the best possible

advantage. Keep in mind areas suitable for forced landing. It may be necessary to choose between an

Figure 10-7. Slope takeoff.

Wind

Figure 10-8. If the wind velocity is 10 knots or greater, you

should expect updrafts on the windward side and downdrafts

on the lee side of obstacles. You should plan the approach

with these factors in mind, but be ready to alter your plans if

the wind speed or direction changes.

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approach that is crosswind, but over an open area, and

one directly into the wind, but over heavily wooded or

extremely rough terrain where a safe forced landing

would be impossible. If these conditions exist, consider

the possibility of making the initial phase of the

approach crosswind over the open area and then turning into the wind for the final portion of the approach.

Always operate the helicopter as close to its normal

capabilities as possible, taking into consideration the

situation at hand. In all confined area operations, with

the exception of the pinnacle operation, the angle of

descent should be no steeper than necessary to clear

any barrier in the approach path and still land on the

selected spot. The angle of climb on takeoff should be

normal, or not steeper than necessary to clear any barrier. Clearing a barrier by a few feet and maintaining

normal operating r.p.m., with perhaps a reserve of

power, is better than clearing a barrier by a wide margin but with a dangerously low r.p.m. and no power

reserve.

Always make the landing to a specific point and not to

some general area. This point should be located well

forward, away from the approach end of the area. The

more confined the area, the more essential it is that you

land the helicopter precisely at a definite point. Keep

this point in sight during the entire final approach.

When flying a helicopter near obstructions, always

consider the tail rotor. A safe angle of descent over barriers must be established to ensure tail rotor clearance

of all obstructions. After coming to a hover, take care

to avoid turning the tail into obstructions.

TAKEOFF

A confined area takeoff is considered an altitude over

airspeed maneuver. Before takeoff, make a ground

reconnaissance to determine the type of takeoff to be

performed, to determine the point from which the takeoff should be initiated to ensure the maximum amount

of available area, and finally, how to best maneuver the

helicopter from the landing point to the proposed takeoff position.

If wind conditions and available area permit, the helicopter should be brought to a hover, turned around, and

hovered forward from the landing position to the takeoff position. Under certain conditions, sideward flight

to the takeoff position may be necessary. If rearward

flight is required to reach the takeoff position, place

reference markers in front of the helicopter in such a

way that a ground track can be safely followed to the

takeoff position. In addition, the takeoff marker should

be located so that it can be seen without hovering

beyond it.

When planning the takeoff, consider the direction of

the wind, obstructions, and forced landing areas. To

help you fly up and over an obstacle, you should form

an imaginary line from a point on the leading edge of

the helicopter to the highest obstacle to be cleared. Fly

this line of ascent with enough power to clear the

obstacle by a safe distance. After clearing the obstacle,

maintain the power setting and accelerate to the normal

climb speed. Then, reduce power to the normal climb

power setting.

COMMON ERRORS

1. Failure to perform, or improper performance of, a

high or low reconnaissance.

2. Flying the approach angle at too steep or too shallow an approach for the existing conditions.

3. Failing to maintain proper r.p.m.

4. Failure to consider emergency landing areas.

5. Failure to select a specific landing spot.

6. Failure to consider how wind and turbulence

could affect the approach.

7. Improper takeoff and climb technique for existing conditions.

PINNACLE AND RIDGELINE

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OPERATIONS

A pinnacle is an area from which the surface drops

away steeply on all sides. A ridgeline is a long area

from which the surface drops away steeply on one or

two sides, such as a bluff or precipice. The absence of

obstacles does not necessarily lessen the difficulty of

pinnacle or ridgeline operations. Updrafts, downdrafts,

and turbulence, together with unsuitable terrain in

which to make a forced landing, may still present

extreme hazards.

APPROACH AND LANDING

If you need to climb to a pinnacle or ridgeline, do it on

the upwind side, when practicable, to take advantage of

any updrafts. The approach flight path should be parallel to the ridgeline and into the wind as much as possible. [Figure 10-9]

Load, altitude, wind conditions, and terrain features

determine the angle to use in the final part of an

approach. As a general rule, the greater the winds, the

steeper the approach needs to be to avoid turbulent air

and downdrafts. Groundspeed during the approach is

Altitude over Airspeed—In this type of maneuver, it is more important

to gain altitude than airspeed. However, unless operational considerations dictate otherwise, the crosshatched or shaded areas of the

height/velocity diagram should be avoided.

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more difficult to judge because visual references are

farther away than during approaches over trees or flat

terrain. If a crosswind exists, remain clear of downdrafts on the leeward or downwind side of the

ridgeline. If the wind velocity makes the crosswind

landing hazardous, you may be able to make a low,

coordinated turn into the wind just prior to terminating

the approach. When making an approach to a pinnacle,

avoid leeward turbulence and keep the helicopter

within reach of a forced landing area as long as

possible.

On landing, take advantage of the long axis of the area

when wind conditions permit. Touchdown should be

made in the forward portion of the area. Always perform a stability check, prior to reducing r.p.m., to

ensure the landing gear is on firm terrain that can safely

support the weight of the helicopter.

TAKEOFF

A pinnacle takeoff is an airspeed over altitude maneuver made from the ground or from a hover. Since

pinnacles and ridgelines are generally higher than the

immediate surrounding terrain, gaining airspeed on the

takeoff is more important than gaining altitude. The

higher the airspeed, the more rapid the departure from

slopes of the pinnacle. In addition to covering unfavorable terrain rapidly, a higher airspeed affords a more

favorable glide angle and thus contributes to the

chances of reaching a safe area in the event of a forced

landing. If a suitable forced landing area is not available, a higher airspeed also permits a more effective

flare prior to making an autorotative landing.

On takeoff, as the helicopter moves out of ground

effect, maintain altitude and accelerate to normal climb

airspeed. When normal climb speed is attained, establish a normal climb attitude. Never dive the helicopter

down the slope after clearing the pinnacle.

COMMON ERRORS

1. Failure to perform, or improper performance of, a

high or low reconnaissance.

2. Flying the approach angle at too steep or too shallow an approach for the existing conditions.

3. Failure to maintain proper r.p.m.

4. Failure to consider emergency landing areas.

5. Failure to consider how wind and turbulence

could affect the approach and takeoff.

Figure 10-9. When flying an approach to a pinnacle or ridgeline, avoid the areas where downdrafts are present, especially when excess power is limited. If you encounter

downdrafts, it may become necessary to make an immediate

turn away from the pinnacle to avoid being forced into the

rising terrain.

Airspeed over Altitude—This means that in this maneuver, obstacles

are not a factor, and it is more important to gain airspeed than altitude.

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11-1

Today helicopters are quite reliable. However

emergencies do occur, whether a result of mechanical

failure or pilot error. By having a thorough knowledge

of the helicopter and its systems, you will be able to

more readily handle the situation. In addition, by

knowing the conditions that can lead to an

emergency, many potential accidents can be avoided.

AUTOROTATION

In a helicopter, an autorotation is a descending maneuver where the engine is disengaged from the main rotor

system and the rotor blades are driven solely by the

upward flow of air through the rotor. In other words, the

engine is no longer supplying power to the main rotor.

The most common reason for an autorotation is an

engine failure, but autorotations can also be performed

in the event of a complete tail rotor failure, since there

is virtually no torque produced in an autorotation. If

altitude permits, they can also be used to recover from

settling with power. If the engine fails, the freewheeling unit automatically disengages the engine from the

main rotor allowing the main rotor to rotate freely.

Essentially, the freewheeling unit disengages anytime

the engine r.p.m. is less than the rotor r.p.m.

At the instant of engine failure, the main rotor blades

are producing lift and thrust from their angle of attack

and velocity. By immediately lowering collective pitch,

which must be done 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 system. This upward flow of air

through the rotor provides sufficient thrust to maintain

rotor r.p.m. throughout the descent. Since the tail rotor

is driven by the main rotor transmission during autorotation, heading control is maintained as in normal flight.

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Several factors affect the rate of descent in autorotation; density altitude, gross weight, rotor r.p.m., and

airspeed. Your primary control of the rate of descent is

airspeed. Higher or lower airspeeds are obtained with

the cyclic pitch control just as in normal flight.

In theory, you have a choice in the angle of descent

varying from a vertical descent to maximum range,

which is the minimum angle of descent. Rate of descent

is high at zero airspeed and decreases to a minimum at

approximately 50 to 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 energy stored

in the rotating blades is used to decrease the rate of

descent and make a soft landing. 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.

Each type of helicopter has a specific airspeed at which

a power-off glide is most efficient. The best airspeed is

the one which combines the greatest glide range with

the slowest rate of descent. The specific airspeed is

somewhat different for each type of helicopter, yet

certain factors affect all configurations in the same

manner. For specific autorotation airspeeds for a particular helicopter, refer to the FAA-approved rotorcraft

flight manual.

The specific airspeed for autorotations is established

for each type of helicopter on the basis of average

weather and wind conditions and normal loading.

When the helicopter is operated with heavy loads in

high density altitude or gusty wind conditions, best

performance is achieved from a slightly increased airspeed in the descent. For autorotations at low density

altitude and light loading, best performance is achieved

from a slight decrease in normal airspeed. Following

this general procedure of fitting airspeed to existing

conditions, you can achieve approximately the same

glide angle in any set of circumstances and estimate the

touchdown point.

When making turns during an autorotation, generally

use cyclic control only. Use of antitorque pedals to

assist or speed the turn causes loss of airspeed and

downward pitching of the nose. When an autorotation

is initiated, sufficient antitorque pedal pressure should

be used to maintain straight flight and prevent yawing.

This pressure should not be changed to assist the turn.

Use collective pitch control to manage rotor r.p.m. If

rotor r.p.m. builds too high during an autorotation, raise

the collective sufficiently to decrease r.p.m. back to the

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normal operating range. If the r.p.m. begins decreasing,

you have to again lower the collective. Always keep

the rotor r.p.m. within the established range for your

helicopter. During a turn, rotor r.p.m. increases due to

the increased back cyclic control pressure, which

induces a greater airflow through the rotor system. The

r.p.m. builds rapidly and can easily exceed the maximum limit if not controlled by use of collective. The

tighter the turn and the heavier the gross weight, the

higher the r.p.m.

To initiate an autorotation, other than in a low hover,

lower the collective pitch control. This holds true

whether performing a practice autorotation or in the

event of an in-flight engine failure. This reduces the

pitch of the main rotor blades and allows them to

continue turning at normal r.p.m. During practice

autorotations, maintain the r.p.m. in the green arc

with the throttle while lowering collective. Once the

collective is fully lowered, reduce engine r.p.m. by

decreasing the throttle. This causes a split of the

engine and rotor r.p.m. needles.

STRAIGHT-IN AUTOROTATION

A straight-in autorotation implies an autorotation from

altitude with no turns. The speed at touchdown and the

resulting ground run depends on the rate and amount of

flare. The greater the degree of flare and the longer it is

held, the slower the touchdown speed and the shorter

the ground run. The slower the speed desired at touchdown, the more accurate the timing and speed of the

flare must be, especially in helicopters with low inertia

rotor systems.

TECHNIQUE

Refer to figure 11-1 (position 1). From level flight at

the manufacturer’s recommended airspeed, between

500 to 700 feet AGL, and heading into the wind,

smoothly, but firmly lower the collective pitch control

to the full down position, maintaining r.p.m. in the

green arc with throttle. Coordinate the collective movement with proper antitorque pedal for trim, and apply

aft cyclic control to maintain proper airspeed. Once the

collective is fully lowered, decrease throttle to ensure a

clean split of the needles. After splitting the needles,

readjust the throttle to keep engine r.p.m. above

normal idling speed, but not high enough to cause

rejoining of the needles. The manufacturer often

recommends the proper r.p.m.

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 r.p.m. in the green arc. Aft

cyclic movements cause an increase in rotor r.p.m.,

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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 r.p.m.,

and leads to “chasing the r.p.m.” Avoid looking straight

down in front of the aircraft. Continually cross-check

attitude, trim, rotor r.p.m., and airspeed.

At approximately 40 to 100 feet above the surface, or

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. Care must

be taken in the execution of the flare so that the cyclic

control is not moved rearward so abruptly as to cause

the helicopter to climb, nor should it be moved so

slowly as to not arrest the descent, which may allow

the helicopter to settle so rapidly that the tail rotor

strikes the ground. When forward motion decreases to

the desired groundspeed, which is usually the slowest

possible speed (position 4), move the cyclic control

forward to place the helicopter in the proper attitude

for landing.

The altitude at this time should be approximately 8 to

15 feet AGL, depending on the altitude recommended

by the manufacturer. Extreme caution should be used

to avoid an excessive nose high and tail low attitude

below 10 feet. 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

check the descent and cushion the landing. Additional

antitorque pedal is required to maintain heading as collective pitch is raised due to the reduction in rotor

r.p.m. and the resulting reduced effect of the tail rotor.

Touch down in a level flight attitude.

A power recovery can be made during training in lieu

of a full touchdown landing. Refer to the section on

power recoveries for the correct technique.

Figure 11-1. Straight-in autorotation.

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After touchdown and after the helicopter has come to a

complete stop, lower the collective pitch to the fulldown 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.

COMMON ERRORS

1. Failing to use sufficient antitorque pedal when

power is reduced.

2. Lowering the nose too abruptly when power is

reduced, thus placing the helicopter in a dive.

3. Failing to maintain proper rotor r.p.m. during

the descent.

4. Application of up-collective pitch at an excessive

altitude resulting in a hard landing, loss of

heading control, and possible damage to the tail

rotor and to the main rotor blade stops.

5. Failing to level the helicopter.

POWER RECOVERY FROM PRACTICE

AUTOROTATION

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

At approximately 8 to 15 feet above the ground,

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 r.p.m. 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

r.p.m. results. Use sufficient collective pitch to stop the

descent and 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 at normal hovering altitude and then

descend to a landing.

If a go-around is to be made, the cyclic control should

be moved forward to resume forward flight. In transitioning from a practice autorotation to a go-around,

exercise care to avoid an altitude-airspeed combination

that would place the helicopter in an unsafe area of its

height-velocity diagram.

COMMON ERRORS

1. Initiating recovery too late, requiring a rapid application of controls, resulting in overcontrolling.

2. Failing to obtain and maintain a level attitude

near the surface.

3. Failing to coordinate throttle and collective pitch

properly, resulting in either an engine overspeed

or a loss of r.p.m.

4. Failing to coordinate proper antitorque pedal with

the increase in power

AUTOROTATIONS WITH TURNS

A turn, or a series of turns, can be made during an

autorotation in order to land into the wind or avoid

obstacles. The turn is usually made early so that the

remainder of the autorotation is the same as a straight

in autorotation. The most common types are 90° and

180° autorotations. The technique below describes a

180° autorotation.

TECHNIQUE

Establish the aircraft on downwind at recommended

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airspeed at 700 feet AGL, parallel to the touchdown area.

In a no wind or headwind condition, establish the ground

track approximately 200 feet away from the touchdown

point. If a strong crosswind exists, it will be necessary to

move your downwind leg closer or farther out. When

abeam the intended touchdown point, reduce

collective, and then split the needles. Apply proper

antitorque pedal and cyclic to maintain proper attitude.

Cross check attitude, trim, rotor r.p.m., and airspeed.

After the descent and airspeed is established, roll into a

180° turn. For training, you should initially roll into a

bank of a least 30°, but no more than 40°. Check your

airspeed and rotor r.p.m. Throughout the turn, it is

important to maintain the proper airspeed and keep the

aircraft in trim. Changes in the aircraft’s attitude and

the angle of bank cause a corresponding change in rotor

r.p.m. Adjust the collective, as necessary, in the turn to

maintain rotor r.p.m. in the green arc.

At the 90° point, check the progress of your turn by

glancing toward your landing area. Plan the second

90 degrees of turn to roll out on the centerline. If you are

too close, decrease the bank angle; if too far out, increase

the bank angle. Keep the helicopter in trim with antitorque pedals.

The turn should be completed and the helicopter

aligned with the intended touchdown area prior to passing through 100 feet AGL. If the collective pitch was

increased to control the r.p.m., it may have to be

lowered on roll out to prevent a decay in r.p.m. Make

an immediate power recovery if the aircraft is not

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aligned with the touchdown point, and if the rotor

r.p.m. and/or airspeed is not within proper limits.

From this point, complete the procedure as if it were a

straight-in autorotation.

POWER FAILURE IN A HOVER

Power failures in a hover, also called hovering autorotations, are practiced so that you automatically make

the correct response when confronted with engine

stoppage or certain other emergencies while hovering.

The techniques discussed in this section refer to helicopters with a counter-clockwise rotor system and an

antitorque rotor.

TECHNIQUE

To practice hovering autorotations, establish a normal

hovering altitude for the particular helicopter being

used, considering load and atmospheric conditions.

Keep the helicopter headed into the wind and hold

maximum allowable r.p.m.

To simulate a power failure, firmly roll the throttle into

the spring loaded override position, if applicable. 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. Leave the collective pitch where it is on entry.

Helicopters with low inertia rotor systems will begin to

settle immediately. Keep a level attitude and ensure a

vertical descent with cyclic control while maintaining

heading with the pedals. At approximately 1 foot above

the surface, apply upward collective pitch control, as

necessary, to slow the descent and cushion the landing.

Usually the full amount of collective pitch is required.

As upward collective pitch control is applied, the throttle has to be held in the closed position to prevent the

rotor from re-engaging.

Helicopters with high inertia rotor systems will maintain

altitude momentarily after the throttle is closed. Then, as

the rotor r.p.m. decreases, the helicopter starts to settle.

When the helicopter has settled to approximately 1 foot

above the surface, apply upward collective pitch control

while holding the throttle in the closed position to slow

the descent and cushion the landing. The timing of collective pitch control application, and the rate at which it

is applied, depends 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 on the

skids, cease the 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 pitch is a most important

consideration. If it is applied too soon, the remaining

r.p.m. may not be sufficient to make a soft landing. On

the other hand, if collective pitch control is applied too

late, surface contact may be made before sufficient

blade pitch is available to cushion the landing.

COMMON ERRORS

1. Failing to use sufficient proper antitorque pedal

when power is reduced.

2. Failing to stop all sideward or backward movement prior to touchdown.

3. Failing to apply up-collective pitch properly,

resulting in a hard touchdown.

4. Failing to touch down in a level attitude.

5. Not rolling the throttle completely to idle.

HEIGHT/VELOCITY DIAGRAM

A height/velocity (H/V) diagram, published by the

manufacturer for each model of helicopter, depicts the

critical combinations of airspeed and altitude should an

engine failure occur. Operating at the altitudes and airspeeds shown within the crosshatched or shaded areas

of the H/V diagram may not allow enough time for the

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critical transition from powered flight to autorotation.

[Figure 11-2]

An engine failure in a climb after takeoff occurring in

section A of the diagram is most critical. During a

climb, a helicopter is operating at higher power settings

and blade angle of attack. An engine failure at this point

causes a rapid rotor r.p.m. decay because the upward

movement of the helicopter must be stopped, then a

descent established in order to drive the rotor. Time is

also needed to stabilize, then increase the r.p.m. to the

normal operating range. The rate of descent must reach

a value that is normal for the airspeed at the moment.

Since altitude is insufficient for this sequence, you end

up with decaying r.p.m., an increasing sink rate, no

deceleration lift, little translational lift, and little

response to the application of collective pitch to cushion the landing.

It should be noted that, once a steady state autorotation

has been established, the H/V diagram no longer

applies. An engine failure while descending through

section A of the diagram, is less critical, provided a safe

landing area is available.

11-5

You should avoid the low altitude, high airspeed portion

of the diagram (section B), because your recognition of an

engine failure will most likely coincide with, or shortly

occur after, ground contact. Even if you detect an engine

failure, there may not be sufficient time to rotate the

helicopter from a nose low, high airspeed attitude to one

suitable for slowing, then landing. Additionally, the

altitude loss that occurs during recognition of engine failure and rotation to a landing attitude, may not leave

enough altitude to prevent the tail skid from hitting the

ground during the landing maneuver.

Basically, if the helicopter represented by this H/V diagram is above 445 feet AGL, you have enough time and

altitude to enter a steady state autorotation, regardless

of your airspeed. If the helicopter is hovering at 5 feet

AGL (or less) in normal conditions and the engine fails,

a safe hovering autorotation can be made. Between

approximately 5 feet and 445 feet AGL, however, the

transition to autorotation depends on the altitude and

airspeed of the helicopter. Therefore, you should

always be familiar with the height/velocity diagram for

the particular model of helicopter you are flying.

THE EFFECT OF WEIGHT VERSUS

DENSITY ALTITUDE

The height/velocity diagram depicts altitude and airspeed situations from which a successful autorotation

can be made. The time required, and therefore, altitude

necessary to attain a steady state autorotative descent,

is dependent on the weight of the helicopter and the

density altitude. For this reason, the H/V diagram for

some helicopter models is valid only when the helicopter is operated in accordance with the gross weight vs.

density altitude chart. Where appropriate, this chart is

found in the rotorcraft flight manual for the particular

helicopter. [Figure 11-3]

Figure 11-3. Assuming a density altitude of 5,500 feet, the

height/velocity diagram in figure 11-2 would be valid up to a

gross weight of approximately 1,700 pounds. This is found by

entering the graph at a density altitude of 5,500 feet (point A),

then moving horizontally to the solid line (point B). Moving vertically to the bottom of the graph (point C), you find that with the

existing density altitude, the maximum gross weight under

which the height/velocity diagram is applicable is 1,700 pounds.

The gross weight vs. density altitude chart is not

intended as a restriction to gross weight, but as an advisory to the autorotative capability of the helicopter

during takeoff and climb. You must realize, however,

that at gross weights above those recommended by the

gross weight vs. density altitude chart, the H/V diagram

is not restrictive enough.

VORTEX RING STATE (SETTLING WITH

POWER)

Vortex ring state describes an aerodynamic condition

where a helicopter may be in a vertical descent with up

to maximum power applied, and little or no cyclic

authority. The term “settling with power” comes from

the fact that helicopter keeps settling even though full

engine power is applied.

In a normal out-of-ground-effect hover, the helicopter

is able to remain stationary by propelling a large mass

of air down through the main rotor. Some of the air is

recirculated near the tips of the blades, curling up from

the bottom of the rotor system and rejoining the air

500

450

400

350

300

250

200

150

100

50

0

A

B

Smooth Hard Surface.

Avoid Operation in

Shaded Areas.

INDICATED AIRSPEED KNOTS

(CORRECTED FOR INSTRUMENT ERROR)

HEIGHT ABOVE SURFACE - FEET

0 10 20 30 40 50 60 70 80 90 100 110 120

A B

C

7,000

6,000

5,000

4,000

3,000

1,500 1,600 1,700 1,800 1,900

GROSS WEIGHT – POUNDS

DENSITY ALTITUDE – FEET

Figure 11-2. By carefully studying the height/velocity

diagram, you will be able to avoid the combinations of altitude and airspeed that may not allow you sufficient time or

altitude to enter a stabilized autorotative descent. You might

want to refer to this diagram during the remainder of the

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discussion on the height/velocity diagram.

11-6

entering the rotor from the top. This phenomenon is

common to all airfoils and is known as tip vortices. Tip

vortices consume engine power but produce no useful

lift. As long as the tip vortices are small, their only

effect is a small loss in rotor efficiency. However, when

the helicopter begins to descend vertically, it settles

into its own downwash, which greatly enlarges the tip

vortices. In this vortex ring state, most of the power

developed by the engine is wasted in accelerating the

air in a doughnut pattern around the rotor.

In addition, the helicopter may descend at a rate that

exceeds the normal downward induced-flow rate of the

inner blade sections. As a result, the airflow of the inner

blade sections is upward relative to the disc. This produces a secondary vortex ring in addition to the normal

tip-vortices. The secondary vortex ring is generated

about the point on the blade where the airflow changes

from up to down. The result is an unsteady turbulent

flow over a large area of the disc. Rotor efficiency is

lost even though power is still being supplied from the

engine. [Figure 11-4]

A fully developed vortex ring state is characterized by

an unstable condition where the helicopter experiences

uncommanded pitch and roll oscillations, has little or

no cyclic authority, and achieves a descent rate, which,

if allowed to develop, may approach 6,000 feet per

minute. It is accompanied by increased levels of

vibration.

A vortex ring state may be entered during any maneuver that places the main rotor in a condition of high

upflow and low forward airspeed. This condition is

sometimes seen during quick-stop type maneuvers or

during recoveries from autorotations. The following

combination of conditions are likely to cause settling in

a vortex ring state:

1. A vertical or nearly vertical descent of at least

300 feet per minute. (Actual critical rate depends

on the gross weight, r.p.m., density altitude, and

other pertinent factors.)

2. The rotor system must be using some of the available engine power (from 20 to 100 percent).

3. The horizontal velocity must be slower than

effective translational lift.

Some of the situations that are conducive to a settling

with power condition are: attempting to hover out of

ground effect at altitudes above the hovering ceiling of

the helicopter; attempting to hover out of ground effect

without maintaining precise altitude control; or downwind and steep power approaches in which airspeed is

permitted to drop to nearly zero.

When recovering from a settling with power condition,

the tendency on the part of the pilot is to first try to stop

the descent by increasing collective pitch. However,

this only results in increasing the stalled area of the

rotor, thus increasing the rate of descent. Since inboard

portions of the blades are stalled, cyclic control is

limited. Recovery is accomplished by increasing

forward speed, and/or partially lowering collective

pitch. In a fully developed vortex ring state, the only

recovery may be to enter autorotation to break the

vortex ring state. When cyclic authority is regained,

you can then increase forward airspeed.

For settling with power demonstrations and training in

recognition of vortex ring state conditions, all maneuvers should be performed at an elevation of at least

1,500 feet AGL.

To enter the maneuver, reduce power below hover

power. Hold altitude with aft cyclic until the

airspeed approaches 20 knots. Then allow the sink

rate to increase to 300 feet per minute or more as the

attitude is adjusted to obtain an airspeed of less than

10 knots. When the aircraft begins to shudder, the

application of additional up collective increases the

vibration and sink rate.

Recovery should be initiated at the first sign of vortex ring state by applying forward cyclic to increase

airspeed and simultaneously reducing collective.

The recovery is complete when the aircraft passes

through effective translational lift and a normal

climb is established.

RETREATING BLADE STALL

In forward flight, the relative airflow through the

main rotor disc is different on the advancing and

retreating side. The relative airflow over the advancing side is higher due to the forward speed of the

Figure 11-4. Vortex ring state.

11-7

helicopter, while the relative airflow on the retreating side is lower. This dissymmetry of lift increases

as forward speed increases.

To generate the same amount of lift across the rotor

disc, the advancing blade flaps up while the retreating blade flaps down. This causes the angle of attack

to decrease on the advancing blade, which reduces

lift, and increase on the retreating blade, which

increases lift. As the forward speed increases, at

some point the low blade speed on the retreating

blade, together with its high angle of attack, causes a

loss of lift (stall).

Retreating blade stall is a major factor in limiting a

helicopter’s top forward speed (VNE) and can be felt

developing by a low frequency vibration, pitching

up of the nose, and a roll in the direction of the

retreating blade. High weight, low rotor r.p.m., high

density altitude, turbulence and/or steep, abrupt

turns are all conducive to retreating blade stall at

high forward airspeeds. As altitude is increased,

higher blade angles are required to maintain lift at a

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given airspeed. Thus, retreating blade stall is

encountered at a lower forward airspeed at altitude.

Most manufacturers publish charts and graphs showing a VNE decrease with altitude.

When recovering from a retreating blade stall condition, moving the cyclic aft only worsens the stall

as aft cyclic produces a flare effect, thus increasing

angles of attack. Pushing forward on the cyclic

also deepens the stall as the angle of attack on the

retreating blade is increased. Correct recovery from

retreating blade stall requires the collective to be

lowered first, which reduces blade angles and thus

angle of attack. Aft cyclic can then be used to slow

the helicopter.

GROUND RESONANCE

Ground resonance is an aerodynamic phenomenon

associated with fully-articulated rotor systems. It

develops when the rotor blades move out of phase

with each other and cause the rotor disc to become

unbalanced. This condition can cause a helicopter to

self-destruct in a matter of seconds. However, for

this condition to occur, the helicopter must be in

contact with the ground.

If you allow your helicopter to touch down firmly on

one corner (wheel type landing gear is most

conducive for this) the shock is transmitted to the

main rotor system. This may cause the blades to

move out of their normal relationship with each

other. This movement occurs along the drag hinge.

[Figure 11-5]

Figure 11-5. Hard contact with the ground can send a shock

wave to the main rotor head, resulting in the blades of a

three-bladed rotor system moving from their normal 120°

relationship to each other. This could result in something like

122°, 122°, and 116° between blades. When one of the other

landing gear strikes the surface, the unbalanced condition

could be further aggravated.

If the r.p.m. is low, the corrective action to stop ground

resonance is to close the throttle immediately and fully

lower the collective to place the blades in low pitch. If the

r.p.m. is in the normal operating range, you should fly the

helicopter off the ground, and allow the blades to automatically realign themselves. You can then make a normal

touchdown. If you lift off and allow the helicopter to

firmly re-contact the surface before the blades are

realigned, a second shock could move the blades again

and aggravate the already unbalanced condition. This

could lead to a violent, uncontrollable oscillation.

This situation does not occur in rigid or semirigid rotor

systems, because there is no drag hinge. In addition,

skid type landing gear are not as prone to ground

resonance as wheel type gear.

DYNAMIC ROLLOVER

A helicopter is susceptible to a lateral rolling tendency,

called dynamic rollover, when lifting off the surface.

For dynamic rollover to occur, some factor has to first

cause the helicopter to roll or pivot around a skid, or

landing gear wheel, until its critical rollover angle is

reached. Then, beyond this point, main rotor thrust continues the roll and recovery is impossible. If the critical

rollover angle is exceeded, the helicopter rolls on its

side regardless of the cyclic corrections made.

Dynamic rollover begins when the helicopter starts to

pivot around its skid or wheel. This can occur for a

variety of reasons, including the failure to remove a

tiedown or skid securing device, or if the skid or wheel

122° 116°

122°

11-8

contacts a fixed object while hovering sideward, or if

the gear is stuck in ice, soft asphalt, or mud. Dynamic

rollover may also occur if you do not use the proper

landing or takeoff technique or while performing slope

operations. Whatever the cause, if the gear or skid

becomes a pivot point, dynamic rollover is possible if

you do not use the proper corrective technique.

Once started, dynamic rollover cannot be stopped by

application of opposite cyclic control alone. For example, the right skid contacts an object and becomes the

pivot point while the helicopter starts rolling to the

right. Even with full left cyclic applied, the main rotor

thrust vector and its moment follows the aircraft as it

continues rolling to the right. Quickly applying down

collective is the most effective way to stop dynamic

rollover from developing. Dynamic rollover can occur

in both skid and wheel equipped helicopters, and all

types of rotor systems.

CRITICAL CONDITIONS

Certain conditions reduce the critical rollover angle,

thus increasing the possibility for dynamic rollover and

reducing the chance for recovery. The rate of rolling

motion is also a consideration, because as the roll rate

increases, the critical rollover angle at which recovery

is still possible, is reduced. Other critical conditions

include operating at high gross weights with thrust (lift)

approximately equal to the weight.

Refer to figure 11-6. The following conditions are

most critical for helicopters with counter-clockwise

rotor rotation:

1. right side skid/wheel down, since translating tendency adds to the rollover force.

2. right lateral center of gravity.

3. crosswinds from the left.

4. left yaw inputs.

For helicopters with clockwise rotor rotation, the opposite would be true.

CYCLIC TRIM

When maneuvering with one skid or wheel on the

ground, care must be taken to keep the helicopter cyclic

control properly trimmed. For example, if a slow takeoff is attempted and the cyclic is not positioned and

trimmed to account for translating tendency, the critical

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recovery angle may be exceeded in less than two seconds. Control can be maintained if you maintain proper

cyclic position and trim, and not allow the helicopter’s

roll and pitch rates to become too great. You should fly

your helicopter into the air smoothly while keeping

movements of pitch, roll, and yaw small, and not allow

any untrimmed cyclic pressures.

NORMAL TAKEOFFS AND LANDINGS

Dynamic rollover is possible even during normal takeoffs and landings on relative level ground, if one wheel

or skid is on the ground and thrust (lift) is approximately equal to the weight of the helicopter. If the

takeoff or landing is not performed properly, a roll rate

could develop around the wheel or skid that is on the

ground. When taking off or landing, perform the

maneuver smoothly and trim the cyclic so that no pitch

or roll movement rates build up, especially the roll rate.

If the bank angle starts to increase to an angle of

approximately 5 to 8°, and full corrective cyclic does

not reduce the angle, the collective should be reduced

to diminish the unstable rolling condition.

SLOPE TAKEOFFS AND LANDINGS

During slope operations, excessive application of cyclic

control into the slope, together with excessive collective

pitch control, can result in the downslope skid rising

sufficiently to exceed lateral cyclic control limits, and an

upslope rolling motion can occur. [Figure 11-7]

Pivot Point

Bank Angle

Weight

TipPathPlaneNeutralCyclic

TipPathPlaneFullLeftCyclic

Crosswind

TailRotorThrust

Main

Rotor

Thrust

Figure 11-6. Forces acting on a helicopter with right skid on

the ground.

TailRotorThrust

Slope

Horizontal

Areaof

CriticalRollover

Full Opposite Cyclic Limit

to Prevent Rolling Motion

Figure 11-7. Upslope rolling motion.

11-9

When performing slope takeoff and landing maneuvers, follow the published procedures and keep the roll

rates small. Slowly raise the downslope skid or wheel

to bring the helicopter level, and then lift off. During

landing, first touch down on the upslope skid or wheel,

then slowly lower the downslope skid or wheel using

combined movements of cyclic and collective. If the

helicopter rolls approximately 5 to 8° to the upslope

side, decrease collective to correct the bank angle and

return to level attitude, then start the landing procedure

again.

USE OF COLLECTIVE

The collective is more effective in controlling the rolling

motion than lateral cyclic, because it reduces the main

rotor thrust (lift). A smooth, moderate collective reduction, at a rate less than approximately full up to full down

in two seconds, is adequate to stop the rolling motion.

Take care, however, not to dump collective at too high a

rate, as this may cause a main rotor blade to strike the

fuselage. Additionally, if the helicopter is on a slope and

the roll starts to the upslope side, reducing collective too

fast may create a high roll rate in the opposite direction.

When the upslope skid/wheel hits the ground, the

dynamics of the motion can cause the helicopter to

bounce off the upslope skid/wheel, and the inertia can

cause the helicopter to roll about the downslope ground

contact point and over on its side. [Figure 11-8]

The collective should not be pulled suddenly to get airborne, as a large and abrupt rolling moment in the

opposite direction could occur. Excessive application

of collective can result in the upslope skid rising sufficiently to exceed lateral cyclic control limits. This

movement may be uncontrollable. If the helicopter

develops a roll rate with one skid/wheel on the ground,

the helicopter can roll over on its side.

PRECAUTIONS

The following lists several areas to help you avoid

dynamic rollover.

1. Always practice hovering autorotations into the

wind, but never when the wind is gusty or over

10 knots.

2. When hovering close to fences, sprinklers,

bushes, runway/taxi lights, or other obstacles that

could catch a skid, use extreme caution.

3. Always use a two-step liftoff. Pull in just enough

collective pitch control to be light on the skids

and feel for equilibrium, then gently lift the

helicopter into the air.

4. When practicing hovering maneuvers close to

the ground, make sure you hover high enough to

have adequate skid clearance with any obstacles, especially when practicing sideways or

rearward flight.

5. When the wind is coming from the upslope direction, less lateral cyclic control will be available.

6. Tailwind conditions should be avoided when

conducting slope operations.

7. When the left skid/wheel is upslope, less lateral

cyclic control is available due to the translating

tendency of the tail rotor. (This is true for

counter-rotating rotor systems)

8. If passengers or cargo are loaded or unloaded, the

lateral cyclic requirement changes.

9. If the helicopter utilizes interconnecting fuel lines

that allow fuel to automatically transfer from one

side of the helicopter to the other, the gravitational

flow of fuel to the downslope tank could change

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the center of gravity, resulting in a different

amount of cyclic control application to obtain the

same lateral result.

10. Do not allow the cyclic limits to be reached. If the

cyclic control limit is reached, further lowering of

the collective may cause mast bumping. If this

occurs, return to a hover and select a landing point

with a lesser degree of slope.

11. During a takeoff from a slope, if the upslope

skid/wheel starts to leave the ground before the

downslope skid/wheel, smoothly and gently

TailRotorThrust

Slope

Horizontal

Areaof

CriticalRollover

Full Opposite Cyclic Limit

to Prevent Rolling Motion

F

Figure 11-8. Downslope rolling motion.

11-10

lower the collective and check to see if the

downslope skid/wheel is caught on something.

Under these conditions vertical ascent is the only

acceptable method of liftoff.

12. During flight operations on a floating platform, if

the platform is pitching/rolling while attempting to

land or takeoff, the result could be dynamic rollover.

LOW G CONDITIONS AND MAST

BUMPING

For cyclic control, small helicopters depend primarily

on tilting the main rotor thrust vector to produce

control moments about the aircraft center of gravity

(CG), causing the helicopter to roll or pitch in the

desired direction. Pushing the cyclic control forward

abruptly from either straight-and-level flight or after a

climb can put the helicopter into a low G (weightless)

flight condition. In forward flight, when a push-over is

performed, the angle of attack and thrust of the rotor is

reduced, causing a low G or weightless flight condition. During the low G condition, the lateral cyclic has

little, if any, effect because the rotor thrust has been

reduced. Also, in a counter-clockwise rotor system (a

clockwise system would be the reverse), there is no

main rotor thrust component to the left to counteract

the tail rotor thrust to the right, and since the tail rotor

is above the CG, the tail rotor thrust causes the helicopter to roll rapidly to the right, If you attempt to stop the

right roll by applying full left cyclic before regaining

main rotor thrust, the rotor can exceed its flapping

limits and cause structural failure of the rotor shaft due

to mast bumping, or it may allow a blade to contact the

airframe. [Figure 11-9]

Since a low G condition could have disastrous results,

the best way to prevent it from happening is to avoid the

conditions where it might occur. This means avoiding

turbulence as much as possible. If you do encounter

turbulence, slow your forward airspeed and make small

control inputs. If turbulence becomes excessive,

consider making a precautionary landing. To help prevent turbulence induced inputs, make sure your cyclic

arm is properly supported. One way to accomplish this

is to brace your arm against your leg. Even if you are

not in turbulent conditions, you should avoid abrupt

movement of the cyclic and collective.

If you do find yourself in a low G condition, which

can be recognized by a feeling of weightlessness

and an uncontrolled roll to the right, you should immediately and smoothly apply aft cyclic. Do not attempt

to correct the rolling action with lateral cyclic. By

applying aft cyclic, you will load the rotor system,

which in turn produces thrust. Once thrust is restored,

left cyclic control becomes effective, and you can roll

the helicopter to a level attitude.

LOW ROTOR RPM AND BLADE STALL

As mentioned earlier, low rotor r.p.m. during an

autorotation might result in a less than successful

maneuver. However, if you let rotor r.p.m. decay to the

point where all the rotor blades stall, the result is usually fatal, especially when it occurs at altitude. The

danger of low rotor r.p.m. and blade stall is greatest in

small helicopters with low blade inertia. It can occur

in a number of ways, such as simply rolling the throttle the wrong way, pulling more collective pitch than

power available, or when operating at a high density

altitude.

When the rotor r.p.m. drops, the blades try to maintain

the same amount of lift by increasing pitch. As the

pitch increases, drag increases, which requires more

power to keep the blades turning at the proper r.p.m.

When power is no longer available to maintain r.p.m.,

and therefore lift, the helicopter begins to descend.

This changes the relative wind and further increases

the angle of attack. At some point the blades will stall

unless r.p.m. is restored. If all blades stall, it is almost

impossible to get smooth air flowing across the

blades.

Even though there is a safety factor built into most helicopters, anytime your rotor r.p.m. falls below the green

arc, and you have power, simultaneously add throttle

and lower the collective. If you are in forward flight,

gently applying aft cyclic loads up the rotor system and

helps increase rotor r.p.m. If you are without power,

immediately lower the collective and apply aft cyclic.

RECOVERY FROM LOW ROTOR RPM

Under certain conditions of high weight, high temperature, or high density altitude, you might get into a