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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 UK
supplement if your aircraft is foreign
made—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 1000-foot increase in
Pressure Altitude.
Some factors affecting performance
include:
Density Altitude
This is 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
138 Operational Flying
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
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. 2½ %) 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
will produce an apparent
contradiction, where temperature
will decrease from the expansion.
Pressure
Air density reduces with atmospheric
pressure (Charles Law). When you
compress air, its density increases.
Runway length
Details are declared by the Airport
Authority and published in the AIP.
This declared distance is either the
Take-off Run Available (TORA) or
Landing Distance Available (LDA).
Any areas at the ends unsuitable to
run on, but nevertheless clear of
obstacles, are called Clearways, which,
with the TORA, form the Take-off
Operational Procedures 139
Distance Available (TODA), which
should not be more than 1½ x
TORA.
However, getting the wheels off the
runway is only part of the story. You
must also clear an imaginary screen
(usually 35 feet, but 50 for piston
aircraft) at the end of the TODA
(TORA + Clearway). The distance
to do this is the Take-off Distance
Required. If a single runway distance
is given, it must be used for both
TODA and TORA. The Take-off Run
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