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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 JAR 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.
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.
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
5000’ 6350 6900 7600
6000’ 7400 8100 8800
7000’ 8600 9300 1,0000
8000’ 9700 10400 11100
9000’ 11,000 11600 12400
1,0000’ 12250 13000 13600
11,000’ 13600 14300 15000
12000’ 14750 15400 16000
Flight Planning 163
It shows that, at 6,000 feet and 21°C,
for example, you should enter
performance charts at 8100 feet.
If you want to work it out for
yourself, try this formula:
DA = 145,366[1 - (X0.235)]
where X is the station pressure in
inches divided by the temperature in
Rankin degrees, which are found by
adding 459.69 to Fahrenheit totals.
Altitude
Air density drops off by .002 lbs per
cubic foot (i.e. 2 ½ %) for every
1000 feet in the lower layers of the
atmosphere.
Humidity
Adding water vapour to air makes it
less dense because the molecular
weight is lower (dry air is 29 –water
vapour is 18). On cold days,
humidity is less of a problem simply
because cold air holds less vapour. A
relative humidity of 90% at 70°F
means twice as much than at 50°F.
Temperature
As heat expands air, it becomes
thinner. Thinner air is less dense
(Boyles Law). On the surface, an
increase in temperature will decrease
density and increase volume, with
pressure remaining constant. At
altitude, however, pressure reduces
more than temperature does, and
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