AEA Pilot's Guide 2021-2022 - 54

TEMPERATURE TORTURE
Continued from page 53
sound of failing bearings is even more pronounced
thanks to the higher temperatures. "
Temperature-sensitive bearings may not be an
issue for digital flight instruments using MEMS -
microelectromechanical sensors. But temperature
extremes can still impact the operation of glass
cockpits, indicators, switches, relays and batteries
when cold affects the tiny sensors.
Observed issues
Pull out the instruction manual or spec sheet of
that new tablet or smartphone common with pilots
today. Down in the specifications: a list showing
the temperature range within which the device
manufacturer knows the device works.
Different components may each come with their own
temperature limits, but it's the sum of the parts that
dictate the known temperature range within which the
device has been shown - through testing - to function
normally.
Today's advanced avionics systems suffer less from
extreme cold - and only slightly more from insufficient
cooling in hot conditions. Heat, it seems, is a problem
for digital electronics in consumer, industrial and
commercial products. But compared to a few decades
ago, temperature extremes must truly be extreme
before today's avionics systems begin to balk from the
effect of environmental extremes in both directions.
To prove the temperature tolerance of modern
avionics, manufacturers engage in testing across an
extreme range of conditions. That means exposing
system hardware to temperatures down in the range
of -30 Fahrenheit to -40 - where the Fahrenheit and
Celsius scales merge. Minus 40 on one scale is also
-40 on the other.
Avionics makers today subject their systems to
temperatures in the 150-degree range, according to
several manufacturers. It's truly a change from decades
past. At least for aircraft electronics systems.
Cold causes few problems for modern digital
electronics compared to the electronics used 30 years
ago and more, according to input from several avionics
makers. Heat, however, is an enemy of electronics
in general - and more so for the fine connections
embedded in the tiny digital circuitry inside chips.
For example, the MEMS used in solid-state gyro
- 54 -
systems - AHRS and ADAHRS - have a temperature
sensitivity that can be offset with proper calibration,
a step common to the production of modern attitudesensing
systems. Extreme temperatures can also cause
failure of the fine wire connections within some MEMS.
And letting some electronics sit in a heat-soaking
environment can cause issues, as well. Excessive
temperatures are simply an anathema to electronics.
Just ask an electronic flight bag user about leaving
their tablet computer on the glareshield or seat for the
sun to heat it to the point that it shuts itself down.
Ask any iPad, iPhone or Android tablet user about
their experiences using their devices in the cockpit.
Chances are you'll hear a few experiences that
occurred from the pilot leaving the device up on the
glareshield, sandwiched between two unhealthy heat
sources: the sunlight streaming through the windshield
and the electronics in the panel, just below the
glareshield.
These tablets are prone to overheating in such
conditions since their mere operation generates heat
within their tightly packed shells and fan cooling is not
usually a feature. Moving these devices, popular for
running electronic flight bag software, into the shade
and, even better, into the flow of cool air typically brings
them back to life.
Several avionics companies noted that heat can
cause display screens to seemingly fade and lose the
rich colors as the mercury rises above 120 degrees
Fahrenheit.
Keeping cool
Channeling cool air around installed avionics long
ago became a standard solution for many aircraft -
particularly avionics-heaving aircraft predominantly
used in business-turbine aircraft. The cooling became
a significant issue when avionics makers started
fielding new integrated avionics systems using CRTs for
displays of everything from flight data to engine status
and multifunction displays.
CRTs require space, a lot of electrical power and
plenty of cooling to keep them operating within their
safe temperature range. Cooling sources varied, with
scoops in some aircraft, cooling fans in others, but both
installed to circulate cooling air through the avionics
installed in the panel. After all, it's typically hotter
behind the panel, where all the electronics work.
But not all instrument panels provide a path for the
free flow of air to cool avionics, especially with many
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