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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 (that's why the
CDP is about 40 feet high). Things
happen in reverse on landing. This
procedure is not without its critics,
since prolonged hovering at high
engine power outputs is not good
engine handling.
Anyhow, whatever you're flying, you
will find the data needed to check
your performance in the Flight
Manual, which will have a supplement
if your aircraft is foreign made, or
you are using non-standard
equipment—these override any
information in the standard manuals.
General principles concerning
distances for take-off and landing are
similar for aeroplanes and
helicopters; for example, take-off
distances for both will increase by
10% for each 1,000-foot increase in
Pressure Altitude.
Accuracy with charts is essential –
very often you have to interpolate
between figures or lines, and it's a
good idea to get used to paralleling
lines between the several graphs that
may be on one chart. Study the
examples carefully and always read
the conditions on which the chart is
based – helicopter ones, for
example, often need the generator
switched off.
Since you can't find out V1 until you
know the maximum weight, for large
fixed wing aircraft, the general
procedure for using charts is:
·  Find the lesser of the all-engines
and one-engine-out distances
(using a zero clearway, zero
slope and zero wind length that
allows the same weight as the
runway you are trying to use,
otherwise known as Distance D).
·  Find the maximum takeoff
weights for the balanced field
length*, WAT limits, tyre speed
limits, or anything else peculiar
to your aircraft, such as brakes.
In some cases, if the runway
length is way over that required,
but you are otherwise stopped
by WAT limits, you can increase
V2 to increase 2nd segment
performance at the expense of
extra runway.
·  If there are no obstacles, take
the lesser weight, determine V1,
VR & V2, reduce weight as
applicable to suit the VMBE (max
brake energy) to get your max
takeoff weight.
·  If there are obstacles, reduce the
gross weight until the gradient
available matches that required,
then add it to the selection of
weights to choose from in the
above paragraph.
Flight Planning 343
*Balanced Field Length V1 means that
the distance to abort and stop is the
same as it is to carry on, assuming
engine failure occurs exactly at V1. In
other words, accelerate-stop is the
same as accelerate-go. Since it is
often safer to carry on, however,
many people use a lower V1 instead.
The term V1 Cut refers to simulating
an engine failure at V1.
Factors affecting performance are:
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
will be in the Flight Manual,
although humidity is usually ignored
in the average performance chart,
because it has more to do with
engine power than aerodynamic
efficiency, and high air density and
humidity do not often go hand in
hand. However, if the air is humid,
say after a good shower, you would
be wise to be careful.
Anyhow, the idea is that the more
the density of the air decreases for
any reason, the higher your aircraft
thinks it is. If you look at the lift
formula, you will see that the lift
from a wing or thrust from a
propeller is directly dependent on air
density, as is drag, of course. The
effects are as valid at sea level as they
are in mountainous areas when
temperatures are high – for example,
90° (F) at sea level is really 1900' as
far as your machine is concerned. In
extreme circumstances, you may
have to restrict your operations to
early morning or late afternoon.
Here is a handy chart:
°F/C 60/15.6 70/21.1 80/26.7
1,000’ 1300 2000 2700
2000’ 2350 3100 3800
3000’ 3600 4300 5000
4000’ 4650 5600 6300
　
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