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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
Getting the wheels off the runway is
only part of the story. You must also
clear an imaginary screen (usually 35
or 50 feet) at the end of the TODA
(TORA + Clearway). The distance
for this is Take-off Distance Required,
which must not be over TODA.
The Stopway may be added to the
TORA to form the Emergency Distance
Available (EDA), or the ground run
distance available to abort a take-off
and come to rest safely. EDA is
sometimes also referred to as the
Emergency Distance or Accelerate-Stop
Distance. The greater the EDA, the
higher the speed you can accelerate
to before the point at which you
must decide to stop or go when an
engine fails.
The Landing Distance Available must
similarly not be less than the Landing
Distance Required.
Altitude and Temperature
The higher you are, the less dense
the air and the less the ability of the
wings (rotating or otherwise) and
engines to "bite" into it, thus
requiring more power and longer
take-off runs to get airborne.
Humidity has a similar effect.
TODR will increase by 10% for each
1,000' increase in aerodrome altitude
and 10% per 10°C increase in
temperature (factor by 1.1). LDR
will increase by 5% for each 1,000'
increase in PA and 10°C increase in
temperature (factor by 1.05).
164 JAR Private Pilot Studies
Aircraft weight
Greater mass means slower
acceleration or deceleration and
longer distances. TODR will
increase by 20% for each 10%
increase in weight and LDR 10% per
10% increase in weight (factor by 1.2
and 1.1).
Some manuals give take-off and
landing weights that should not be
exceeded at specific combinations of
altitude and temperature, so that
climb performance is not
compromised. These are known as
WAT limits (Weight, Altitude and
Temperature).
Runway slope
Going uphill when taking off will
delay acceleration and increase the
distance required. The converse is
true of downhill slopes and a rule of
thumb is that TODR will increase
10% for each 2% of uphill slope,
and vice versa (factor both by 1.1).
When landing, an uphill slope aids
stopping, thereby reducing LDR.
Any gains from landing upslope or
taking off downslope should not be
made use of but accepted as a bonus
(that is, don't use them as part of
your planning).
Surface winds
Headwinds will reduce the distances
required and improve the flight path
after take-off. Tailwinds have reverse
effects and crosswinds may even
exceed the ability of the tyres to grip
the runway. Aside from the handling
problem, crosswinds may also
increase the TODR if you need to
use the brakes to keep straight.
Forecast winds should be factored
by 50% for a headwind and 150%
for a tailwind. TODR and LDR will
increase by 20% for each tailwind
component of 10% of the lift-off
and landing speed (factor by 1.2).
The flight manual will state
maximum crosswinds for your
machine (try the limitations section).
A useful guide (for American
machines, anyway) is that the
maximum crosswind will be about
20% of Vso (see below). When
finding the angle between the wind
and the runway, remember that
runway headings are magnetic and
forecast winds are true.
You can also use the crosswind chart
to find a limiting wind, or the
maximum you can accept from any
given angle. Just draw a line upwards
from the maximum speed you can
accept, and stop when you reach the
line representing the wind direction.
The curved line at that point (or its
interpolation) is the maximum
windspeed you can take.
Surface
Performance information is based
on a dry, hard surface. A
"contaminated" runway has standing
water or slush more than 3mm thick,
or snow and ice anywhere along the
takeoff run or accelerate-stop
surface. However, your flight manual
may have different ideas.
The most important factors are loss
of friction when decelerating, and
displacement of (and impingement
drag when accelerating through)
whatever is on it, so it may be
difficult to steer, and take-off and
accelerate-stop distances may
increase due to slower acceleration,
as will landing distance because of
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