Avionics News October 2016 - 18

THE COLD-SOAKED COCKPIT
Continued from page 17

It didn't take long for avionics makers to move from LCD
displays to displays using LED technology - light-emitting
diodes. Instead of depending on shifting the orientation of the
liquid crystals to change the image, LEDs turn on and off as
commanded by the display driver, producing light in the colors
for which they were designed. The combination of light from
red, green and blue LEDs produces tens of thousands of colors
thanks to varying steps of LED intensity in each of the colors.
Cold has little effect on the function of LED displays, unlike
the preceding technologies. But excess heat can impact the
displays and the function of the electronics driving them.
The hot stuff
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 minus 30 degrees Fahrenheit to minus 40
F - where the Fahrenheit and Celsius scales merge. Minus 40 on
one scale is also minus 40 on the other.
Avionics makers also subject their systems to temperatures in
the 150-degree range, according to several manufacturers.
Even avionics accessory makers subject their products to
temperature extremes in testing. For example, Mid-Continent
Instruments and Avionics' MD41-series of annunciators for Garmin's GTN- and GNS-line of all-in one com/nav/GPS products.
Mid-Continent Instruments and Avionics shows the operating
range of its MD41s of minus 55 C to plus 70 C; that's minus 67
F to 158 F.
According to Dynon's president, Robert Hamilton, his
company tests its products down to minus 30 degrees Fahrenheit
to 150 degrees Fahrenheit - or minus 34 degrees Celsius to 65
degrees Celsius.
The specifications for Avidyne's IFD540 show an operatingtemperature range of minus 20 C (minus 4 F) to plus 55 C (plus
131 F) - and for a "short term," as high as 70 C (158 F).
Hamilton echoed other avionics-company executives in noting that cold temperatures pose fewer performance problems
than heat for today's electronic flight instruments and radios.
Cold, Hamilton said, has "little impact on how our products
perform" compared to heat. "The cold does little to nothing
to display performance, and the electronics aren't affected the
way older technologies - like liquid-crystal displays - were
impacted."
Heat, conversely, is an enemy of electronics in general and
more so for the fine connections embedded in the tiny digital
circuitry inside chips. And the MEMS used in solid-state gyro
systems - AHRS and ADAHRS - have a temperature sensitivity
that can be offset with proper calibration, a step common to the
production of modern attitude-sensing systems.
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avionics news

october

2016

But excess heat or cold can cause some drift in their output
until they reach the temperature range for which they were
calibrated.
Now you see it
Several avionics companies noted that heat can cause screens
to perform less robustly, with LED screens seeming to fade
and lose the rich colors as the mercury rises above 120 degrees
Fahrenheit.
At the upper ranges of the tests, Hamilton and others noted,
it's important to remember that temperatures behind the panel
can run significantly above temperatures in the cockpit.
Channeling cooling air has long been a standard for many
aircraft, with scoops and fans installed to circulate cooling
air through the avionics installed in the panel. But not all
panels allow for the free flow of air to cool avionics; and
many smaller light-sport aircraft and experimental, amateurbuilt aircraft are particularly challenging to keep cool for the
health of the avionics.
To assure their products enjoy a sufficient flow of cooling air,
many avionics and accessories makers integrate cooling fans
into their products - assuring that the product gets air flow, albeit what often may be air as hot as ambient cabin temperatures.
The benefits of extra cooling air
The solution: a fan-driven cooling system to aid circulation
behind the panel. Properly installed and operating, these systems
can help ease the chore of cooling fans integral to the avionics
boxes.
As one avionics technician consulted for this story noted, trying to cool the space behind the panel serves up something of a
conundrum - but keeping that air cool can benefit cockpit avionics and displays when they live and work in one of the hottest
spots in the aircraft.
"Pilots see the displays, the front of the avionics," the technician explained. "What they often fail to remember is that those
electronics - heat-sensitive, heat-generating electronics - are
tucked into a tight space, underneath a black glareshield that sits
right below the windshield. The sun through the windscreen acts
like an oven as it heats the black glareshield, which in turn heats
the air beneath and conducts that heat into the space beneath
where the avionics live."
The avionics themselves generate their own heat when running, and several technicians noted that on a 100-degree day
the cabin can hit 120 while the smaller, more-contained space
behind the panel quickly warms to 150 degrees.
The issue of excess heat can even exist during colder weather
when an aircraft sits on a sunlit ramp and the cabin - and avionics stack - are subjected to the warming effects of the sun on the
equivalent to a greenhouse. These kinds of extreme heat conditions can exist in an airplane on a winter day with clear dry air
and sunlight warming the cabin - and its panel - well above 100

Table of Contents for the Digital Edition of Avionics News October 2016