01 March 2005
7-1. OVERVIEW. Electromagnetic energy that is
a. Devices that are part of a peripheral to a system/
generated/absorbed aboard military aircraft can
sub system within the aircraft (intrasystem EMI).
interfere with avionic systems and cause malfunctions.
b. Radiated emissions from ground-based or other
(EMI), is produced by radar antennas, electric motors,
external emitters (intersystem EMI).
and inadequately shielded avionics units. Lightning
and other natural sources can also affect normal
c. Electrostatic discharges on the airframe or inside
performance of aircraft electrical/electronic systems.
Some examples of EMI-related system malfunctions
false indicators (i.e., alarms, lights, readouts), and
power loss. The results of such malfunctions can
e. Electromagnetic Pulse (EMP) generated from a
cause a flight safety situation, aborted mission, or
high-altitude nuclear detonation.
unacceptable system/subsystem operation. In order
to minimize EMI-induced failures, avionic housings
7-3.2. EMI THREAT CATEGORIES. Military aircraft
must absorb and reflect incident electromagnetic
often operate in severe electromagnetic environments
energy. Usually the shielding system consists of a
while carrying out intended missions. This presents a
conductive gasket sandwiched between an aluminum
potential threat to digital flight controls and mission
housing and an aluminum lid. This gasket provides
essential avionics due to single/multiple EMI sources.
The following are examples of the kinds of problems
gasket/lid junction per military grounding/EMI shielding
that can result from EMI-induced malfunctions. Aircraft
requirements. It also prevents fluid intrusion into the
can jettison bombs while taking off from an aircraft
carrier due to the response of bomb release circuits to
7-2. FACTORS INFLUENCING EMI. EMI involves
wings can inadvertently fold when illuminated by radar.
the interaction of electromagnetic energy (electric or
EMI can cause aircraft computers to "dump" programs
magnetic fields) with the circuitry of an electronic
necessary for the operation of mission essential
device. Currents or voltages caused by EMI can
equipment. Table 7-1 compares several typical
couple with digital signal lines and produce erroneous
data. Analog devices can also malfunction as a result
of EMI. Corrosion is a major cause of EMI shielding
7-3.3. INTRASYSTEM/INTERSYSTEM EMI. Aircraft
avionic subsystems operate over a frequency range
oxides, organic coatings, and nonconductive films.
from a few kilohertz to tens of gigahertz. Ensuring that
Corrosion produces nonconductive films allowing EMI
these pieces of equipment operate compatibly is a
intrusion and degrades the load bearing capabilities of
primary concern. Intrasystern EMI principally involves
a structure. Conversely, EMI protection requires a
receivers responding to undesired signals and
conductive path. A typical low electrical resistance
transmissions causing undesired responses in
joint of silver and aluminum is a dissimilar metal
receivers and other equipment. Other examples of
couple that is highly susceptible to galvanic corrosion.
EMI are interphone noise due to magnetic coupling
a n d equipment malfunction via relay/solenoid
7-3. SOURCES OF AVIONIC EMI.
generated transients. Intersystem EMI is caused by
t r a n s m i s s i o n from external sources such as
7-3.1. G E N E R A L . EMI can affect avionics by
ground-based emitters (radar, TV, radio) through
introducing garbled/false signals, disrupting memory,
and destroying circuits. This is because modern circuits
operate at low power levels which can be disrupted
7-3.4. STATIC ELECTRIFICATION. The three ways
with low level noise. Sources of avionic EMI include
that static electrification of an aircraft can occur are
frictional charging, engine charging, and induction
charging. If the charge accumulation is sufficient, a