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rate of climb.
The CDP is the only point where
you have a choice of action – before
then, you reject. Afterwards, you
carry on. The LDP is a similar point
for landing, where the idea is to hit a
speed and height combination from
where you can make an approach
250 Operational Flying
that will allow you to land safely
(most people get to one first, then
creep up on the other).
There can be many variations on the
Cat A theme:
·  Airfield, or Clear Area, with
distances around 1000-1500 feet
to play with, accelerating to
where you can climb even at
max weight, usually close to VY.
·  Reduced Field Length, climbing
vertically or even backwards, to
land in a much smaller area if
you have to, say 300-500 feet
long, usually at about 85-90%
max weight.
·  Vertical, which speaks for itself,
but you may have the option of
a dip below heliport height or
not (i.e. at ground level). The
former allows 90% max wt, the
latter around 80%
Class 1 helipad take-off procedures
involve climbing vertically at first,
then going upwards and backwards
to a predetermined height (the
Critical Decision Point, or CDP) before
going forward (actually, ICAO now
call it the Takeoff Decision Point, or
TDP). This could be up to 150 or
200 feet above the helipad, after
going vertically to about 50 feet first
(if you've got the power on a hot
day!). For the TwinStar, you start
going backwards from about 15 feet,
at around 200 feet per minute,
keeping the helipad in sight, and at
TDP (90 feet) select max power and
10° nose down at the same time, so
your tail doesn't hit anything.
In theory, having moved backwards,
you still have the take-off spot in
sight and it's therefore available for
landing. At CDP (or TDP), if you
elect to carry on to forward flight,
you should be able to clear the
landing spot during the steep dive
you have to make to achieve flying
speed, which is why the CDP is
about 40 feet high. Once you're
happy about the power, gently
accelerate level, to VY.
All this is not without its critics,
though, since prolonged hovering at
high engine power is not necessarily
good engine handling. Not only that,
it may be impractical on an oil rig if
there are accommodation blocks and
cranes around.
Unofficially, therefore (from a rig
anyway), one school of thought
suggests a level acceleration to best
rate of climb speed, then going up to
a safe altitude, whereas others
advocate getting to best angle of
climb speed, climbing to a safe
height and then accelerating to best
rate of climb. The first is supposed
to keep you in the H/V curve less,
but the second gets you higher
sooner, so you lessen the chances of
hitting the water if an engine fails,
especially if the deck is only 50 feet
high (most major platforms are 100
feet off the water).
One technique might be to hover
over to the front of the deck with
the rotors not overhanging (the front
is where the wind is coming from).
Check the Ts & Ps as usual, then
pull power and head upwards,
rotating while there is a positive rate
of climb to a few degrees nose down
(10 is OK initially - you might lose
height with more), to get the tail up
and clear. After rotation, maintain
the collective while looking for
takeoff safety speed and accelerate.
Techie Stuff 251
As to which technique is best, you
have to make some choices yourself,
like whether you want to hit the
water or the deck, or would rather
be low with rotor RPM, or higher
with less, and little airspeed.
To get back on to the ground, you
will not be surprised to hear there is
a landing profile as well, for the
Twinstar being something like this:
For a clear area, you arrive at the
100' point at 40 kts (it's actually
difficult to get them both at the
same time, so you would first hit
one, then the other). For a helipad
(that is, not within the definition of a
clear area), the figures are 90' and 30
kts, for a semi-vertical arrival.
Factors Affecting Performance
Density Altitude
This is the altitude at which the ISA
density is the same as that of the air
in question or, in other words, your
real altitude resulting from the
effects of height, temperature,
pressure and humidity, all of which
can make the air thinner and which
are mentioned below. The details
　
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