Avionics News July 2020 - 77
Figure 4: FAA Figure 11-6: Altitude derating curve, which is used to determine the derating factor for wires based on altitude.
formula. We need to work this formula, because it will
adjust the chart away from 20 degrees Celsius to the
actual ambient temperature. The hardest part of this
formula isn't shown on the chart. We need to find the
value for T2. Figure 2A shows the formula necessary
to determine T2. T1 is the ambient temperature, which
could be as high as 50 degrees Celsius. We also found
the difference between the temperature rating of the
wire and the ambient temperature, which is 100 degrees.
We add this difference to T1, get an answer of 150, and
move on to the other set of parentheses. I2 is the actual
current load, which we know is 10 amps. Imax is the
maximum current with our selected wire (now 16 AWG)
could handle at 10-feet length. Going back to the graphic
and starting at the bottom at the 16 AWG wire mark, we
move up to the 10-feet length line. These are marked with
blue. The angle line we find is 20 amps. So we take 10
amps divided by 20, which gives us 0.5. Now we take the
square root of 0.5, which is 0.7071. With the parentheses
on both sides complete, we can multiply and get T2,
which is 106.07.
Now we go back to Figure 2 in order to determine L1.
We need to answer this question: Using 16 AWG wire,
how far could we go when drawing 10 amps? To get
the answer, we start again at the bottom of the chart and
move upward to the 10-amp line, this time highlighted
green. Then we move back to the left and find the 28-volt
column. We read 20 feet. We multiply 20 feet by the
constant we are given, which is 254.5. Our answer is
5,090. Now for the bottom part of the formula, we add
the actual current load T2 to another constant, 234.5,
and that gives us an answer of 244.5. We are ready now
to compute L2. We take 5,090 and divide it by 244.5.
Our answer is 20.82 feet. If our answer is higher than
the actual length we are using, we are safe. In this case,
we can still use the 16 AWG wire, because 20.82 feet
is a higher number and longer than 10 feet if L2. If this
number is lower than our actual length, we will need to
move to a larger wire and try again.
We need to do this only in rare occurrences. Still, we
need to know how to use these charts. Often, avionics
technicians have a reputation for being the smart guys
in the shop. For that reason, you may have a mechanic
coming to you for help. Those of us who learned in the
1970s and 1980s used the old version of AC 43.13 with
only one chart, so some technicians might not be familiar
with the current methodology. In closing, let me remind
you AC 43-13-1B helps you also determine derating
factors for switches and relays, which is important for
safety and the longevity of parts. q
avionics news
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july
2020
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Avionics News July 2020
Table of Contents for the Digital Edition of Avionics News July 2020
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Avionics News July 2020 - Intro
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Avionics News July 2020 - Cover2
Avionics News July 2020 - 1
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Avionics News July 2020 - Cover3
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