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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. 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.
344 Canadian Professional Pilot Studies
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
Getting the wheels off the runway is
only part of the story. You must also
clear an imaginary screen (usually 35
feet) at the end of the TODA
(TORA + Clearway). The distance
to do this is the Take-off Distance
Required. Obviously, the TODR must
not be more than the 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 (e.g. V1 - see V-speeds,
below). V1 is a fixed speed based on
weight and flap settings (nothing to
do with runway length), and is
supposed to give you a safe full stop
or a successful engine-out takeoff if
something happens, that is, if an
engine fails below it, you slam on the
anchors, and if it fails afterwards,
you get airborne and try to fix things
in the air. However, high speed
rejects are among the top three
causes of accidents. Unless you feel
things will be uncontrollable, your
chances may better in the air.
The Balanced Field Length is where the
abort distance is the same as the
continue distance, but because of the
dangers of stopping in a hurry, V1 is
reduced by many operators for
safety, so there is more reject
distance to play with.
In any case, you cannot determine
V1 accurately until you know your
takeoff weight, which is sometimes
not found until you are taxying out
and the information transmitted to
you over a datalink, so for flight
planning purposes, you work with an
approximation, based on standard
weights. Various other factors also
affect it, such as the wind, runway
slope, temperature, etc., discussed in
detail below. When asked whether a
condition increases or decreases V1,
think about whether you can commit
to flight at a higher or lower speed –
for example, if stopping distance has
to be increased for any reason
(antiskid doesn't work, or you have a
tailwind), decrease it, and vice versa.
The Landing Distance Available must
similarly not be less than the Landing
Distance Required. Unless the Flight
Manual states otherwise, for
propeller driven large aircraft, the
LDR must be factored by 1.43 to
Flight Planning 345
make it 70% of distance available,
with no allowance for wet runways.
For turbojets, the maximum you can
plan to use is 60% (the LDR must
be increased by 15% if the runway is
wet, not to exceed 60% of the
LDA). However, these limits only
apply up to the start of the takeoff
roll, that is, they are for flight
planning, so you have something up
　
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