AOPA Turbine Pilot Magazine - September 2024 - T-7

of atmospheric pressure to work with. At
Aspen, which is almost 8,000 feet above
sea level, there is just a little over 10.5
pounds of air pressure. That's at standard
air temperature, which is 15 degrees
Celsius at Teterboro but below freezing
at Aspen. Raise the Aspen temperature to
a summer day and you can see how little
air density is available. That's why airport
elevation, thus lower available air pressure,
is so devastating to takeoff performance.
The FAA and many safety advocates
have been on a crusade to remind pilots
to " check density altitude " before takeoff.
Many automated weather reporting
systems will include the density altitude
in their broadcasts. That's essential
advice, but not something we need to do
in turbine airplanes because the FAAapproved
airplane flight manual (AFM)
has already done that for us. And so do the
flight management systems; nearly all can
calculate performance.
So, in a jet I really don't know what
the density altitude is, and there's no
place in the AFM to look for it or apply
that information. Instead, we consult
a chart or FMS that considers airport
elevation-pressure altitude, more
specifically-and air temperature and tells
us how much runway is required. We are,
in fact, considering all the ramifications of
density altitude without ever knowing or
applying that number.
It's really ISA, or deviations above
or below ISA temperature, that affect
our flying. That's why quality flight
planning services provide a cruise
altitude temperature as plus or minus
ISA. And the digital electronic air data
computers nearly all of us fly with show
air temperature as plus or minus ISA.
In the bad old days, the only outside
air temperature we had in the cockpit
was ram air temperature (RAT) which
is warmer than the actual temperature
depending on speed and altitude. Charts
in the AFM converted RAT into static
air temperature (SAT) or what was often
called TAT for true air temperature, and
then another chart that compared that
value to ISA. Or you could perform the
calculation with one of those little circular
whiz wheels. Or you could look at a cruise
chart that showed performance for a given
RAT and not consider ISA. Crude.
Now, with air data computers, we are
constantly aware of ISA and the actual
conditions compared to it. We also
see how jumbled the real atmosphere
is. Instead of decreasing at a constant
rate with altitude as ISA models, the air
temperature can jump all over the place.
That's because ISA doesn't consider
wind or convection, the two big factors
that stir up the atmosphere and are the
source of turbulence.
Also, there is the height of the
tropopause, the level where the
atmosphere changes from the troposphere
to the stratosphere. ISA places the " trop "
level at about 36,000 feet. Above that
level ISA temperature remains constant
at minus-56.5 degrees C until climbing
to about 70,000 feet where the air
temperature begins to decline again.
But the height of the " trop " varies with
latitude and season, and movement of air
masses. The trop is typically highest at the
equator and lowest over the poles. The trop
is lower in winter and higher in summer.
At typical business jet altitudes, it's not
uncommon to see the air temperature
decrease during the early segment of
descent because we were cruising above
the trop level. And a low altitude trop can
put air temp 10 degrees C or more above
ISA at flight level 400 and above, which is
a real killer on cruise performance.
Lower levels of the atmosphere
often mimic what's going on at the
surface. We usually see colder than
standard temperatures for the first
several thousand feet during winter,
or warmer during summer. But at
jet cruise levels those often reverse
making it impossible to generalize about
temperature at cruise levels based on
surface conditions. Way below standard
conditions on the runway can give you
terrific performance margins for takeoff,
but in the higher flight levels abovestandard
temperatures can rob speed
and range.
For all these reasons and more, ISA
governs how we plan trips and fly. It also
sets the standards for aircraft design and
certification and is key to comparing
airplane capabilities. Without ISA we'd
be wondering how our airplanes were
going to perform in real-life conditions.
It's the " standard " in ISA that gives
us the information we need to adjust
to the atmosphere as we find it.
J. Mac McClellan is a corporate pilot with
more than 12,000 hours, and a retired
aviation magazine editor living in Grand
Haven, Michigan.
AOPA PILOT / September 2024
T-7
http://www.aerox.com
AOPA Turbine Pilot Magazine - September 2024

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