Avionics News May 2016 - 20

A BRIGHTER IDEA ALL AROUND
Continued from page 19

By the time pilots could benefit from instruments to help
them stay on the straight and level, those instruments' marking
typically glowed in the dark, thanks to radium-laced paint on
the flight instrument faces.
But soon, lighting came into use, with a cockpit flood light
or two to illuminate the instruments and exterior navigation
lights, then the new radios. The electrical systems had to support them all.
From there, the level of panel and cabin lighting vacillated
between flood lights, post lights and internally lit instruments.
It took precious few post lights or internally illuminated instruments to ratchet up power demand beyond the capacity of
old generators.
Then there are the bulbs that light up individual instruments; a 5 watt bulb consumes more than 0.4 amps of electricity at 12 volts.
Multiply that by six flight instruments, a couple of amps
each for four radios with internal lights, post lights for the
unlighted navigation indicators, and the intermittent demands
of landing gear motors, landing and taxi lights and it isn't long
before the load taxes the electrical system's generating source
in excess of its capacity - particularly older aircraft with a
35-amp generator.
And these demands don't get into the loads imposed by oldstyle incandescent exterior lights.
Swapping an alternator alone might not resolve the amperage deficit that incandescent illumination can set up - at least,
unless the alternator is significantly more powerful than the
generator's rated output.
Going glass helps ... LEDs there, too
In this dilemma, the plan to change over to glass flight instruments may provide a bit of power relief all by itself.
Yes, they all use electricity - but the display screens use less
than a couple of instrument lights, and the radios within benefit
from needing no separate illumination; the LED display screen
handles that job by graphically displaying radio frequencies,
flight plans and navigation routes.
The primary flight display alone uses less current than the
previous six-pack of flight instruments consumed - particularly
if any of the three gyro instruments used electrical power to
spin the gyro and light an internal light.
The attitude sensors take previous little current themselves,
and radios in an all-in-one or stand-alone models using LEDs to
illuminate the front faces typically consume less electricity than
the old radios with their display-lighting systems.
But ... throw in multiple navigation and communications
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avionics news

may

2016

radios, an autopilot or flight management system, and interior
accessory lights (reading lights, map lights, etc.) and the current
demand quickly increases.
Unless, of course, the aircraft owner chooses to make over
all the interior lights to use LEDs.
An LED for all roles: Floods to posts and spots
Just as we've covered in the past, pilots and aircraft owners
today enjoy options unavailable until recent years - options to
convert from heat-generating, life-limited resistance-based or
chemically driven lighting. Today, incandescent lighting holds
an ever-shrinking share of the lighting business; it's true in
homes, businesses, factories, cars, trucks, motorcycles, marine
applications - and aircraft.
They all share in the inherent benefits of the light-emitting
diode: light weight; vibration resistance; color stability; and
long life when put to work.
Not bad for a tiny diode once so expensive to manufacture
that their use was limited to expensive applications where
changing bulbs was a hassle or the current demands high
enough to justify the switch.
Today, the expense of manufacturing LEDs is so low that we
put millions of them into single smartphones, on the displays of
portable computing devices and in the big-screen high-definition televisions we watch.
That proliferation helped drive down the costs to the point
that solutions are available for every aviation application.
If the LED has one drawback of concern, it stems from one
of the traits considered beneficial: color stability.
Engineers design LEDs for the color of light desired for the
application, and the output is generally limited to that designed
spectrum.
So an LED's radiated light falls strictly within the frequency
range - or color spectrum if you prefer - for which it was designed.
Incandescent lights, conversely, emit light all across the
spectrum, visible and invisible, while also generating heat out
of the excess power applied. That heat is wasted energy and is
lost to the visible spectrum.
That makes LEDs designed for the human eye's sensitivity
spectrum ill-suited to be visible to night-vision gear - a factor the Federal Aviation Administration discovered when some
airports started converting to LED runway and taxiway lights
from incandescent.
Today, avionics and LED makers recognize the complication
and offer displays with the option to be visible via night-vision
gear.
Fortunately, the spectrum of products available for aviation
applications also includes LED products designed specifically
to show up to night-vision sensors.

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