飞行员操作飞行手册Pilot Operational Flying Manual

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concerned, low-level operations

(below about 5000 feet) probably

won't need you to get too

concerned, apart from taking notice

of airspeed placards and power

limitations, because some of the

power lost with altitude is regained

with cooler temperatures. You will

find that at least 75% power is

available to a fair height, but be

careful (some pilots report no real

difference up to 8500 feet).

Power available is reduced with

height (and temperature), and rotors

turn at the same speed, so, as you

increase altitude, higher pitch and

power settings will be required (in

some helicopters, like the 500C, the

rotor blades will stall before you

reach engine limits). The dynamic

pressure applied to the ASI is also

Special Use Of Aircraft 223

reduced, so IAS will read less in

relation to TAS, so, if you maintain a

particular airspeed, your

groundspeed will increase

accordingly, and you will be going

faster than you think. The ASI will

also be slower to react.

Density Altitude is your real altitude

resulting from height, temperature

and humidity. The more the density

of the air decreases for any of those

reasons, the higher your machine

thinks it is. The effects are found at

sea level, as well as in mountains,

when temperatures are high – for

example, 90° (F) at sea level is really

1900 feet as far as your aircraft is

concerned. In extreme situations,

you may have to restrict your

operations to early morning or late

afternoon in some areas.

Larger control movements will be

needed, with more lag, so controls

must be moved smoothly and

gradually, or the effects may well

cancel each other out – you may be

on the ground well before that large

handful of collective pitch even takes

effect! Rotor RPM will rise very

quickly with the least excuse.

Your maximum weight for a given

altitude (and vice versa), as well as

cruising speed in relation to them

both should be known, at least

approximately, in advance. You also

need to know the Hover ceiling In

and Out of Ground Effect

(HIGE/HOGE) for any weight, so

you know you can come to a low

hover properly, however briefly, and

recover from an unsuitable landing

site (hovering should actually be

minimised, partly because you can’t

rely on ground effect being present,

and you have less power anyway, but

also because you need to keep a little

up your sleeve if the wind shifts, or

you begin to lose tail rotor authority.

Having said that, no-hover landings

are not recommended, because of

the chances of snagging the skids on

something). Check the performance

charts in the back of the Flight

Manual, and start practising hovers

about 1-2 feet off the ground,

bearing in mind, of course, that the

said charts were established by test

pilots, in controlled situations.

If you allow for these effects as part

of your flight planning, fine, but it's

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easy to get used to a particular place

and air density and a corresponding

take-off run, base leg, etc., and you

may get caught out one day when

things change.

Illusions

There is a psychological aspect to

mountain flying. In the initial stages,

it requires a good deal of selfcontrol, as you overcome a certain

amount of fear and tension, which is

not good when you really need to be

relaxed on the controls. You will

also have to cope with some optical

illusions.

Almost the first thing you will notice

is the lack of a natural horizon, and

will maybe want to use the mountain

tops or sides as a substitute. This,

however, will probably cause a

climb, or other exaggerated attitudes,

and make it difficult to estimate the

height of distant ground, either from

a cockpit or on the ground itself, so

you will find it best to superimpose a

horizon of your own below the

peaks. This is where using your

instruments will help, both to keep

attitude and give you a good idea of

your height and speed (however,

224 Operational Flying

you’re not supposed to be

instrument flying!).

Close to the ground, you will get an

impression of increased speed,

especially near to a ridge. For

example, climbing along a long

shallow slope is often coupled with

an unconscious attempt to maintain

height without increasing power so,

unless you keep an eye on the ASI,

you will be in danger of gradually

reducing speed—if your airspeed is

reducing, then either the nose has

been lifted or you're in a

downdraught (downdraughts will be

associated with a loss of height or

airspeed for the same power). You

can also tell if you're in a

downdraught by watching the

position of the nose—if it yaws into

the slope, the air is flowing

downwards and vice versa. A lack of

cloud above, i.e. descending air, is

also a possible indication.

Downdraughts can frequently

exceed your climbing capabilities.

Strong updraughts can suspend you

in mid-air with zero power – if the

air subsides suddenly, you will be

going down faster than you can

apply it. Do not fight it, but guide

the aircraft towards a lifting slope, or

try for a cleaner column of air. You

might get help from the ground

cushion, but the effect will be less on

a slope or grass. When valley flying,

upslopes or slopes exposed to the

sun can produce updraughts, so

place yourself on a converging

course to the line of the ridge and

positioned to obtain a straight flight

path two thirds up the slope and one

across, which is generally the area of

smoothest flight. However, local

conditions could vary this.

You could climb on a lee slope (that

is, the other side from where the

wind is coming from), taking

advantage of the updraught formed

by stronger wind returning on itself

(i.e. riding the backlash, which tends

to occur with abrupt surfaces).

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Also, there is so little room to

manoeuvre if something goes wrong,

or you meet someone coming the

other way. If you have to do this,

converging on the ridge line at 45o

gives you the best chance of an

escape route.

Similarly, try and avoid flight along

lee slopes, but if you need to

(because life's sometimes like that),

smoothest flight will be obtained by

flying as close as possible to the

ground, say about six inches, so

you’re in the boundary layer, which

is a steady movement of air close to

the surface, with a vertical element.

This gives even less room for error,

though. A good illustration of the

boundary layer comes from your car

after it's just been washed—water

left on the bodywork will not be

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affected by the air flowing over it,

because it's in a layer all to itself. Air

next to mountains behaves in the

same way and, when landing at least,

will be mostly what you work with.

Winds

Winds can increase your operational

ceiling, payload, rate of climb, range

and cruise speed. They can also do

the opposite, and be very difficult to

predict, with formidable up and

downdraughts associated with them.

When cruising downwind, along a

lee slope or not, sudden wind

reversals could make you exceed

VNE or even take away your airspeed

completely.

There are several types of wind,

loosely be grouped into prevailing or

local, with the latter subdivided into

other types, such as anabatic,

katabatic, etc., and which are

infinitely variable (glacier winds

descend 24 hours a day). The

prevailing wind is steady and fairly

reliable, and starts to affect you from

about 6000 feet upwards. Smoke

from local fires may be used to

detect direction, as can water, but

this may only give half the story. For

instance, it's not uncommon for the

windsocks at each end of Banff

airstrip in the Rockies to be 180° out

with each other! Indeed, upper

winds can come in many directions

at different levels, and are usually the

opposite of lower winds. Where

mountains are concerned, they also

acquire a vertical element, which is

actually where the boundary layer

comes from.

As a guide to speed, whitecaps on

water foam at 10 mph. Dark

depressed puddles on water are

called bearpaws (or catpaws) and are

caused by downbursts. The most

important thing to watch out for is

the funnelling of wind as it

progresses down a valley, so

although the mean windspeed may

be reported as 5 knots or so, you

may find it as high as 30 in some

places, and not necessarily coming

from the expected direction.

In fact, understanding how air

moves around terrain is one of the

keys to good mountain flying,

particularly the demarcation lines

between smooth and turbulent air.

In general, that moving up is

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smooth, and that moving down is

turbulent. You can visualise the

difference if you think of a waterfall,

and the state of the water before and

after dropping over the edge. Close

to the ground, the air moves in

laminar fashion (the boundary layer),

but the depth of the layer and the

gust spread will vary considerably,

depending on the nature of the

surface and its heating. The flow will

be broken if the ground becomes

rough, or there are trees, and the

wind is strong. Turbulence will occur

on both sides, resulting in an

updraught close to the leeward side

and a downdraught close to the

windward side as air is made to curl.

The movement of air over a crest

line has a venturi effect, giving an

increased windspeed over the

summit and a corresponding

reduction of pressure, which could

cause your altimeter to over-read.

On passing over or round an

obstacle, the air may become

turbulent or have formed into rolls

with a vertical or horizontal axis.

The general effect of a series of

ridges is to form rolls between the

crest lines, possibly causing a

226 Operational Flying

dangerous situation where a

downdraught can exist on an

upslope where an updraught would

normally be expected:

As a result, on top of steep ridges

there may be an area of nil or reverse

winds which is difficult to locate on

the first recce. The vertical distance

to which a mountain chain will

influence the movement of air is

about 3-5 times its height, changing

with the windspeed.

Horizontally, the effect is variable

and most noticeable in stable

conditions with more than 20 knots

of wind, when standing waves will

form. As you probably know, you

can recognise the existence of these

by lenticular clouds, but you will also

see ragged cloud around the peak.

These should be avoided at all costs

due to their turbulence, especially at

the wind speeds that lead to their

creation. As well as shockloading,

momentary loss of control may

occur, not to mention coffee all over

the place.

A couple of thoughts for when

you’re very high up; how much time

it takes to get down if you have a

problem, and meeting anyone else at

that height on an airway who doesn’t

expect you. And oxygen.

Landing Sites

Those on peaks or crests usually

present you with more escape routes

than any on flanks or valley bottoms

so, wherever possible, landings

should be made on ground higher

than the immediate surroundings, so

you can vary the approach according

to the wind and have a clear

overshoot path. Customers, though,

have this annoying habit of wanting

to land on the most obscure sites!

Use the windward sides of a slope;

leeward sides should only be used in

operational necessity, because wind

flowing down the slope can increase

its apparent angle (you need more

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lateral cyclic to hold the helicopter in

place, and you could run out when

you reduce power to lower the

downwind skid). Don't forget you

will not have the full effects of a

ground cushion, if at all. Where

conditions allow, go as far to the

windward edge as possible, to avoid

suddenly finding yourself in dead or

reversed airflow (as if on a leeslope)

and make overshooting easier. The

wind coming over the peak will have

increased in speed, due to Venturi

effects, so a 15 knot wind can easily

become double that, aside from your

altimeter misreading.

Finding the wind direction can be

interesting if the site is bare and

gives you no information, and it

doesn’t help that mountain flying

tends to take place in high pressure

conditions, that is, where the winds

are light and variable. We are now

talking about local winds, caused by

convection, for instance, or katabatic

effects, combined with the prevailing

wind influenced by the ground, or

even a mixture of them all. Even a

cloud shadow can increase the speed

of a downflowing wind from a cold

surface. You could judge its effects

on the machine itself, flying round

the site with a constant speed and

Special Use Of Aircraft 227

power setting, or a constant altitude,

which is otherwise known as a contour

crawl, because you use one contour

all the way round.

Look at your power settings,

whether the air is turbulent, your

groundspeed varies, whether you

drift or whether the nose yaws into

or out from the slope. How much

pedal you use to keep straight is a

good help – a lot of right pedal

means the wind is from the left, for

example, and a fair amount of

vibration means it is behind you, but

it may be a good idea, if you can’t

have it at the front, to get the wind

off to the side that requires the use

of the power pedal (the left, in a

206), in case tail rotor authority

becomes a problem. Aft cyclic would

indicate a tailwind as well.

So, with constant power and

airspeed (say 40-50 kts), when you

rise, you will be on the windward

side, and vice versa. On the other

hand, you would use less power on

the windward side if you kept a

constant height. However, use

turbulence as a guide only in lighter

winds – any found in updraughts will

be from mechanical effects, such as

trees. Smoke grenades are often used

if there’s nothing else.

Aside from picking a speed slow

enough to detect changes and yet

give enough for a margin of safety

(and cope with any turbulence),

when testing for wind, you should

also fly about 50-100 feet below the

top of the peak you want to land on,

to keep yourself away from the

demarcation line and reduce the

chances of getting the rotors in an

updraught on the leeward side. Also,

keep tight in to the side, to stay

inside the boundary layer.

The demarcation line is the point at

which smooth air is separated from

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turbulent air around a peak, rather

similar to that over an aerofoil.

Above or to the side of the line, air

is relatively smooth and upflowing –

below, it is downflowing. It steepens

as wind velocity increases (and the

severity of the slope), as does the

area of downflow, and moves

toward the top of the hill. One tip:

you don't have to keep the whole

helicopter one side or the other—

many pilots work the line by keeping

only the blades on one side of it. The

fuselage by itself is not affected

much by turbulence.

Having decided on wind direction in

general, you now need to look more

closely at your proposed landing site.

In strong wind conditions, you won't

need the contour crawl at all,

because it's obvious where the main

body of wind is coming from, but it

may have very little influence over

your final approach anyway.

The basic manoeuvre is a figure-ofeight type inspection, making all

turns away from rising ground

(returning towards the site) to give

you a good view all the time. You

could go round in a circle, but the

landing point would be out of sight

most of the time.

228 Operational Flying

As with any other potentially dodgy

landing site, you need to check for

Size, Shape, Surroundings, Slope,

Surface and Sun (you don’t want it in

your eyes). The most important,

however, in this case, is Slope, as

there's no point trying at all if you

can't land. You will get little idea of

ground conditions if you overfly the

site, so what you must do is have a

look at eye-level, which results in the

aptly named Eye-Level Pass (if the

site isn't surrounded by trees).

The most economical way is to start

with a downwind pass, turn round

and land, which is entirely possible if

you know the wind direction before

you start. Sometimes, though, this is

not obvious at all, so just make an

educated guess and fly at about 40

kts in the direction you think is

downwind very close to the site, level

with your eyes. This point is crucial. As

you do so, note the reading on the

altimeter (those people used to QFE

may want to set it to zero), and

climb up an extra hundred feet as

you increase speed to about 60

knots, using the collective.

At 100 feet, turn round for another

approach and repeat the process,

taking note of the new groundspeed

and deciding which way the wind is

coming now you are closer. If there

is no real difference in speed, check

for vibration through the pedals, aft

cyclic, etc, or anything that might

indicate the wind is from behind.

The next step is an initial approach

and overshoot, but if you have to

make a circuit, you may as well do

another eye-level pass and get as

much information as you can.

Turn in at around 50-60 knots (at

the 100 feet), taking particular note

of escape routes, up and down

draughts and turbulent areas.

Maintain a constant angle, aiming

directly for the point you wish to

land on, controlling your speed with

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collective and avoiding any last-minute

corrections. The idea is to keep the

fuselage as level as possible, so don't

move the cyclic at all, if you can help

it. One reason for using the

collective in this way is to minimise

large control movements in the final

stages, as this is a shallow approach.

Approaches

There are several schools of thought

about these, but no real standard –

as with many other activities

involving helicopters, there is more

than one “right” answer to this one.

A fairly flat, disc-loaded (shallow)

one will (in theory, anyway)

minimise collective for the hover,

and give you the most control as you

keep translational lift as long as

possible, but there’s very little up

your sleeve at the end, and you need

to be very aware of your winds, as

forward speed will mask the effects

right to the last minute, although it

does give you a good idea of the

level of your site. This assumes you

Special Use Of Aircraft 229

remember your training and keep

going forward and down, so the

cyclic is ahead of the game and

operating in the cleaner air in front

of the machine that helps with

translation. In other words, keep the

rotor disc forward, so the flow of air

is from front to back, especially

where snow is concerned, but you

shouldn’t use the shallow approach

with powdered snow anyway,

because you will lose sight of your

landing point at the critical moment

in the resulting white cloud. The

other thing to bear in mind is that

you are trying to land at probably the

only spot available, and if you have a

problem in a shallow approach, you

aren't going to get there.

You could, on the other hand, use a

steeper angle, particularly if you're

going into a clearing surrounded by

tall trees, increasing with the wind

strength, but this requires large

handfuls of power and attitude

changes in the final stages if you

don’t get ground effect, so you

wouldn’t try this in an underpowered

piston-engined machine that really

shouldn’t be there in the first

place—the engine may be able to

cope with it, but can your tail rotor?

(leading with the pedals will help).

Anyway, since ground effect reduces

your torque requirement for the

hover by up to 15%, if you approach

in such a way that you need no more

than that amount to stop, you

should find your descent stopping

nicely in the right place, assuming

the surface is conducive to it, and

whether you have high skids or not.

You also have some potential energy

available for an escape.

I guess you could use whatever

works—I generally turn in steep

around 60 kts with the disc loaded as

much as possible, consistent with

descending at about 250 fpm – if the

blades have some tension on them,

they are less likely to be overstressed.

Not only that, the controls are more

responsive. The power used will give

you a good idea of what you need in

the hover, so you have an early

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chance to abort if you are using too

much (you get to know with

experience). This works, because 250

fpm reduces the thrust required to

transition into the hover by about

15%, i.e. much the same as for

ground effect. 250 fpm is about 20

feet every 5 seconds, if you haven't

got a VSI (altimeters usually have 20-

foot segments).

Whichever you choose, if the

machine wants to weather cock, let

it—there's no point in using power

or making a lot of effort to keep

straight if you're going in the right

direction anyway. Keeping the whole

of the windward side in view over

the crest will keep you forward of

the demarcation line and in the

upflowing air. Coming at 45° will

help with escape routes.

When you make a final approach to

land, remember that you may not be

able to hover when you get there. If

you do manage it, make it low,

somewhere between 1-2 feet, and

brief, one or two seconds. No-hover

landings are not recommended.

In a confined area, there will be a

point beyond which you're

committed, so don't go beyond it

until you’re sure. Pick a point to aim

for where you know your tail will be

clear, not too far towards the end,

and fly the machine in, in as smooth

a movement as possible, going over

the lip to the clearing at around a

230 Operational Flying

walking pace. As for power checks,

you will know very early on if you're

running out (keep an eye on the

torque). The size of any surrounding

trees will give you a false illusion as

to the size of the clearing, in that big

trees will make it look smaller and

vice versa. A typical clearing will have

stumps and slash all over the surface

– if you don’t have logs to land on

(and these produce their own

problems when they are slippery),

take off a cleanly as possible, to

avoid your skids getting caught in

something (also be aware that tall

trees will sway from your

downwash). When landing, if there's

room, try to move forward slightly,

as this will bring the tail up, away

from the garbage.

So as not to use pedals too much,

you can use the cyclic to turn the

machine if need be. It is always a

good idea to do a clearing turn

before taking off, but often you

cannot, so exercise extra caution if

you think someone may be behind

you. In a Bell, as you go out of a

clearing, a little aft cyclic will

produce a little extra lift, but don't

expect the same from an AS 350, or

you will clip the trees.

Anyway, always be prepared to break

off at any time, even if only seconds

from success. Never commit

yourself till the very last moment.

Short cuts don't exist with

mountains—they've been around a

lot longer than you have!

Landing sites on the bottoms of

valleys often have difficult access,

and frequently leave no escape route

once an approach has started. In this

case, it's important to have safe

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power reserves before committing

yourself. In any case, placing the

aircraft downwind near to ground

should be avoided, but if you have

to, go low and slow when

approaching downwind with a last

minute turn into wind.

In snow, try landing with the sun

behind you, as the aircraft shadow

will give you a useful guide to the

ground slope and surface and

provide a focus for a sight picture

approach. Some people use the

landing light. For takeoff, try not to

hover too much. A jump takeoff is

useful if little power is available,

where you get light on the skids,

proceed to the edge with full RPM

and tip yourself over the edge. Good

fun, but you should be able to hover

at least for a moment, just in case

your C of G is out. In a confined

area, for a JetRanger, at least, you

need about 15% torque in hand to

do a proper vertical takeoff, so

you’re probably OK if you’re

hovering at about 80%.

Near the end of a cliff (or the deck

of a ship), try and keep the tips of

the rotors as far as possible over the

edge, to avoid a vortex formed by

wind movement over the edge (a

backlash) mixing with your

downwash, which will affect the

airflow so much that you will need

more power in a low hover. It’s a

similar effect to the recirculation

found when hovering near a

building, where the accelerated air

going through the disc actually pulls

your machine towards the wall, and

more power is used to stop it.

Log Pads and Platforms

Log pads are used when slopes are

steep, on rough ground. The quick

and easy one is a single log across

the slope for your rear skid to a solid

Special Use Of Aircraft 231

mat of smaller ones. They can be

slippery! Platforms are still made

from logs, but are much more

refined. The problem with them all

is, you can mostly only land one way,

and there may be no room to turn

once you get there, so approaching

with the wind in totally the wrong

direction is often the only choice. In

such cases, you need much more

anticipation than normal, and the

willingness to throw things away

much earlier. Of course, you don’t

actually have to land, but it’s often

worth a try. As with landing on rigs

or ships, it may be possible to

approach to the hover next to it and

move sideways on.

Here is a typical log arrangement

(note the larger one at the back):

Here is a typical forestry landing site

(look at the slash on the ground):

Summary

· Mountains take no sh*t from

nobody.

· Make turns away from rising

ground.

· Use the eye-level pass as much

as possible.

· Use controls for different

functions, collective for speed,

etc.

· Take off as cleanly as possible

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to avoid getting snagged

Night Flying

Night flying can be pleasant—there's

less traffic, you tend not to go in bad

weather and the air is denser, so the

engine and flying controls are more

responsive (if the controls become

heavier than normal, your instructor

has his hands on as well!).

Searching for an overdue aircraft in

low light conditions causes lots of

problems, and route planning should

take account of this. Otherwise, it’s

much the same as for day, though

there are some aspects that demand

some thought. Plot your route on

the chart in the normal way, but

navigate with electronic aids or

features that are prominent at night,

such as town lighting, lighted masts

or chimneys, large stretches of water

(big black holes), aerodromes,

highways, etc.

Apart from reducing visibility, rain

on th windscreen is a particular

threat when fixing your position by a

single light source. When little or no

light is on the surface and a

prominent one comes into view, it

may seem that the light is above the

232 Operational Flying

horizon, which could lead you to

pitch into a steep attitude in keeping

with the resulting false horizon.

Sometimes the effect is not much

more than an uncomfortable

climbing sensation even when you're

straight and level, but an obscured

windscreen could make objects

appear lower than they really are.

This will be more apparent with high

intensity runway lighting, which may

also give you the same effect that

actors have on stage, where they

can't see the audience through the

bright lighting. The lack of normal

contrast will also upset your altitude

perception, making you feel further

away and higher than you are. As a

result, on a final approach you could

find yourself too low and fast. The

solution is to use every piece of

sensory information you can,

including landing lights and

instruments (look ahead and slightly

to the side of the light beam).

Problems will arise if several of the

above factors affect you at once,

especially if the landing point is

sloping—this is where frequent

cross-checking of altimeters is

important. The illusions you might

get with sloping ground include:

Problem Illusion Risk

Downslope Too low High approach

Upslope Too high Low approach

Rain Closer Low approach

Narrow Too high Low approach

Wide Too low High approach & flare

Bright lts Too low High approach

The trick with landing is to get to the

point where you think the wheels are

going to touch the ground – then go

down another 30 feet.

Helicopter landing sites must be

checked out in daylight on the same

day as they are to be used at night.

Preflight checks should allow for

night flying—carry a torch, and 2

landing lights are preferred.

Permission to enter the rotor disc is