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It is your responsibility to decide
whether or not a safe takeoff (and
landing) can be made under the
prevailing conditions. This means, in
particular, that, although the
crosswind on a particular runway
may be within limits for your
aircraft, you can choose another one
if you are not happy.
Individual aircraft of a given species
will vary in performance due to such
variables as the age of the airframe
and engines, the standard of
maintenance, or the skill and
experience of the crews. What you
can do on one day under a given set
of circumstances may well be
impossible another time. The
original testing, of course, is done
with new aircraft and highly
experienced pilots. "Performance" is
therefore based on average values.
There are fudge factors applied to
unfactored figures to produce net
performance (and gross performance when
they're not), which are meant to
offset the effects of tired engines or
variations in pilot skill. Occasionally,
performance data in a flight manual
will already be factored, but you will
have to check the small print on the
chart, in case they surprise you.
Figures and graphs are based on
Standard conditions which allow for
fixed reductions in pressure and
temperature with height. As we all
know, the real world isn't like that,
so these assumptions may not always
be true and due allowance must
therefore be made for them (if your
aircraft is performing sluggishly, you
may find it's not the machine, but
the conditions it has to work under
that are at fault).
162 Canadian Private Pilot Studies
All the relevant data will be in the
graphs, but some groups have no
information at all in some areas. For
instance, an aircraft may be assumed
to have all engines working until
above 200 feet, under which height
there is no data for landing or takeoff
(which is why the take-off
minima should not be below this,
because you must be visual to avoid
any obstacles should an engine fail).
Sometimes, there can be no specific
provision for engine failure at all.
High Performance aircraft are a
special case, because they tend to be
operated much closer to their limits,
which means more planning.
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, 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.
Some 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:
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