AOPA Pilot Magazine - June 2021 - 104

NEWS & NOTES

AOPA AIR SAFETY INSTITUTE

Such great
heights

NEW FROM ASI
THERE I WAS...:
ENGINE FAILURE

Density altitude
and performance

In this podcast episode, GA pilot
Kelly Keller, who was flying back
from a fishing resort in Arkansas
with her husband while seven
months pregnant, experienced an
engine failure in a Cessna 172.

BY ALICIA HERRON

airsafetyinstitute.org/thereiwas

AS SEASONS CHANGE, those perfect long
summer days bring with them higher temperatures and higher density altitudes.
Density altitude is pressure altitude corrected for nonstandard temperature, and
technical definition aside, density altitude
is the altitude at which the airplane " feels "
like it's flying.
Density altitude directly affects an aircraft's performance. A wing's lift depends
on the mass of air molecules it displaces. At
higher density altitudes, the air is less dense
and fewer air molecules are available to provide the needed lift. A propeller's (or a jet's
fan, compressor, and turbine blades) thrust
also depends on the mass of air molecules it
can hurl backward, and similarly, the engine's
power output depends on the amount of air
available for combustion. All these performance characteristics can be drastically
affected by a high density altitude.
Changes in density altitude will also
change your true airspeed even though
your indicated airspeed will stay the
same. You'll fly the same numbers, but
the change in true airspeed will alter the
way your approaches look, and at a high
density altitude you might feel too fast.
Instinct may dictate to pull back on the
controls and slow yourself down, and if
you aren't careful, that could lead to a traffic pattern stall.
A higher true airspeed also changes
the radius of turns and your turns will be
wider. This can be a minor issue, but if
you're flying in the mountains, it can mean
the difference between a successful canyon turn and an accident. Make sure you

104 | AOPA PILOT June 2021

know and understand how your aircraft
performs in changing density altitudes
before you head into the backcountry.
Your aircraft's climb performance
will also suffer, as will the airplane's service ceiling. As you zip along at a faster
true airspeed and have reduced climb
rates, you might not make it over terrain
you think you can or otherwise could on
a cooler day. Don't guess. Give yourself a
backup plan and equip the airplane to perform well. This might mean half tanks or
fewer passengers, but taking multiple trips
or making multiple fuel stops is far better
than overloading the airplane.
It sounds like all bad news. But if you fly
during a cooler time of the day, like morning,
density altitude will be less of a factor. Or
why not get night current and go for moonlit flights this summer? The key to dealing
with density altitude is understanding how
it affects your airplane's performance, and if
you know and respect your limits, you'll be
able to mitigate the risks. Fly safe this summer and enjoy all that extra daylight.
Learn more about density altitude
and mountain flying with this safety spotlight and video from the AOPA Air Safety
Institute archives.
airsafetyinstitute.org/spotlight/
mountainflying
EMAIL alicia.herron@aopa.org

Tip: Density altitude is reported
on an AWOS when it exceeds field
elevation by more than 1,000 feet.

ASK THE DPES
Q: Should the CFI be available on
the scheduled checkride day?
A: Absolutely! It shows his or her
commitment to the student, which
is important. Also, it allows the
designated pilot examiner to contact the CFI if there's a need to fill
out missing paperwork or provide
needed information. Ideally, the CFI
should be at the checkride to support his or her student. A CFI who
is present on the day of a test can
remedy documentation hiccups
that would otherwise cause a test
to be unable to proceed. The CFI
doesn't need to be there sitting
around during the whole test but
being there at the beginning is certainly a best practice. Learn more
tips from pilot examiners, submit
your own questions, and discover
tips for proficiency with the AOPA
Air Safety Institute's CFI to CFI
quarterly newsletter.
airsafetyinstitute.org/cfi-to-cfi

http://www.airsafetyinstitute.org/thereiwas http://www.airsafetyinstitute.org/spotlight/mountainflying http://www.airsafetyinstitute.org/spotlight/mountainflying http://www.airsafetyinstitute.org/cfi-to-cfi

AOPA Pilot Magazine - June 2021

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