AOPA Turbine Pilot Magazine - September 2024 - T-6

/ AIR WAYS /
Imaginary yardstick
What is ISA and why does it rule our flying?
BY J. MAC MCCLELLAN
I 'M SURE YOU know that ISA is the
international standard atmosphere. But if
it's " standard, " why does it produce such
a profound impact on our flying and the
way turbine airplanes perform?
First, ISA is not " standard " in the
sense that it's normal, or average, or
expected. There's no sea level airport in
the world where the average temperature
is 15 degrees Celsius all day every day
and the barometric pressure is always
29.92 inches Hg. But that's what the ISA
airport is. Instead, ISA is an imaginary
yardstick used to predict how an airplane
will perform under the infinitely variable
conditions of the atmosphere we fly in.
Because turbine airplanes operate over
a vastly larger parcel of the atmosphere
and are larger and heavier, they are
affected by changes in that atmosphere
to a much greater extent than pistonpowered
airplanes. That's why ISA, and
the atmospheric conditions it depicts,
governs how fast, how far, how high, and
how much runway our turbine airplanes
can deliver on any given day.
The concept of standardizing the air,
and thus airplane performance, dates to
T-6
AOPA PILOT / September 2024
at least the 1920s. Very early in the era of
powered flight pilots and engineers saw
that an airplane performed differently
on a hot day than a cold one. And that
airports at higher elevations, and thus
lower atmospheric pressure, robbed
performance. What was needed is a metric
to measure how much variations in the
atmosphere impact airplane behavior.
The ISA we use now was formulated in
the 1950s and adopted by the International
Civil Aviation Organization. ISA has been
tinkered with a little since then, but those
changes are not significant to our flying.
Bottom line, ISA is a model of air
density. Because it is air density that
provides lift to a wing, and air that powers
the combustion in our engines, any change
in air density impacts performance.
The big factors in air density change are
air pressure and air temperature. Humidity
and even dust in the air can create small
changes in density, but they are so slight
ISA assumes the atmosphere is a dry and
clean gas. So, the two issues that rule our
flying are pressure and temperature.
As pilots of turbine aircraft, we care
a lot about air temperature at cruise
altitude. Because we fly at a flight level,
we are holding a constant air pressure.
Changes in the pressure relative to ISA
will place our constant flight level higher
or lower relative to the ground, but not
really do anything to performance.
But air temperature deviations from
standard at turbine cruise altitudes can
have a huge impact on all aspects of
performance. Warmer air is less dense
so at the same flight level-constant air
pressure-our wings and engines have
less air to work with. With less lift and
power, we won't be able to climb as high
at a given weight, and with less power
available cruise speed will be slower.
On the ground air temperature still
matters a lot for runway performance,
but the air pressure also assumes a
major role. Air pressure goes up and
down with the movement of weather
systems, and we certainly notice that
when setting the altimeter above or
below the 29.92 in Hg standard.
But really big air pressure changes
come with airport elevation. At Teterboro,
with its near-sea-level elevation, your
engines and wings have about 14.7 pounds
MIKE FIZER
AOPA Turbine Pilot Magazine - September 2024

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