Environmental testing
40 JUNE \\ AEROSPACETESTINGINTERNATIONAL.COM
5 // Staff at Lilium’s
test-flight planning and
operations center
monitoring progress
traditionally run over the metalwork of the airframe,”
Hayes says. “This has the effect of reducing cabling’s
effectiveness as an antenna and hence improving its EMI
performance. Composite airframes complicate this and
increase the challenge for design teams as the traditional
solutions to EMI may no longer work.”
This makes it even more important to route cables
properly to avoid cross-coupling, keeping power cables
away from signal cables and using the proper braid or
foil shield. In addition, placement in the airframe
becomes critical to withstand handling and
environmental stresses. Hayes says, “Cables remain the
most reliable way of connecting two or more devices
together. However, this increases weight.
“Cables are also subject to mechanical
stresses. Wireless technologies overcome
this problem but equally increase the
probability of electrical interference and
hence impact reliability.
“Cable routing around assemblies can
result in chaffing and fatigue, but can also
cause unplanned loading on printed
wiring boards and connectors,” Justus
from ETS says.
Military systems have added wire
length for service loops due to an
expected long life. How the system and
cabling are tested is very important to
get a representative test. Justus says,
“Cables in an EMI test are the
coupling mechanism for emissions
that cause problems during test. The
cables used during test should
simulate the actual
installation in length and
in the construction type.”
Over time, cables and
all connections can
become fatigued and
discontinuities creep into
the system leading to
intermittent faults. “In
short,” Cockram from
Copernicus says, “The
failure mechanisms that
cause intermittency are
found most often in the
weakest points of the
system, the interconnections. Rapid
isolation of faults in the interconnections
with IFD has a profound effect on the
mean time between repair for the
system, which dramatically reduces the
wear and tear caused by frequent,
avoidable repair interventions.”
Most intermittent faults are
time-consuming to track down. Cockram
describes an incident where it took
18 months to solve an intermittent fault:
“The surprising aspect about these fault
mechanisms: dry and cracked joints,
charred, damaged or mis-wired cabling
and oily residue in edge connectors’
contacts, is that they are all uncovered by
using IFD testing, but few are identified
with automatic testing equipment.”
COMPLEXITY CONTINUES
While Justus says that, “Vibration, temp,
and EMC” continue to be the most
commonly requested tests, he believes
that the levels these are being tested at are
constantly increasing. “There are higher
test levels for radiated susceptibility,
6 // A bomb rack used to
actuate a group switches
during temperature testing
(Photo: Engineered Testing
Systems)
7 // EMC testing of
operating electromechanical
components
higher frequencies, with additional
capabilities for testing RF applications
such as Bluetooth and wi-fi, because
everything is connected to the internet.”
Hayes said that it is important to
coordinate with the testing facility on the
modes of operation and how the testing
process will run. “Let’s take a headrest
display that a passenger interfaces with…
one minute we want to use it as a video/
movie display. Making sure the image
doesn’t pixelate or freeze is important to
the user whilst in this mode.
“Then we want to use the map
function and see where the flight is – in
this mode we are taking inputs from the
flight navigation system and reliant on
the input from the flight deck via cables
that are not active whilst in movie mode.
“The pilot then has an announcement
to make and we use the system as an
audio device. The audio quality needs to
be monitored and checked. We also need
to check that the video stops playing and
blanks the display during this time.”
To make sure the testing is timely and
performed in a manner appropriate to the
airworthiness authorities, Hayes says it is
important to plan this out.
“However, it may be that the audio
function doesn’t need to be tested as the
video function already has audio and
can be assessed at the same time. We
would typically work with manufacturers
to see if this type of rationalization
is possible.
“If it is, we assist the manufacturer to
provide justification to the airworthiness
assessors through the certification plan
that the manufacturer submits for
approval prior to the start of testing.” \\
“Failure mechanisms
are most often found in
the interconnections”
5
7
6
/AEROSPACETESTINGINTERNATIONAL.COM