01 March 2005
These electrons interact with the earth's magnetic
occur. First, if the total aircraft structure is charged,
field, radiating high intensity fields. Field strengths can
electric fields at its extremities can become sufficient
be on the order of 50,000 volts per meter with pulse
to cause arcing. Second, if insulated dielectric surfaces
durations of hundreds of nanoseconds. The large skin
are charged, such as a windshield or radome, spark-like
currents generated on the surface of an enclosure
discharges across the dielectric surface to the
require that joint impedances are low. This prevents
surrounding metal structure can result. Third, if isolated
large voltage drops at these discontinuities in the
(unbonded) metal sections of the aircraft become
shield. The characteristics of EMP appear to be very
charged, arcing to adjacent metal structures can occur.
similar to that of the lightning transient. The differences
Finally, slowly varying induction pulses can be produced
are in pulse risetime, rate-of-rise, and pulse duration.
in antennas moving through clouds of charged particles.
T h e s e processes create an uncontrolled
electromagnetic environment that generally impacts
receiver inputs via antennas. This degrades the
7-4.1. EMI SHIELDING PRACTICES. Some form of
o p e r a t i o n a l performance of navigation and
EMI shielding is required for avionics aboard all modem
communication equipment. Broadband digital systems
military aircraft. Shielding is used to protect avionics
can also be affected because discharges occur as
from the electromagnetic environment present in field
pulses that may be misinterpreted as control signals.
service. Shielding is accomplished by enclosing
Chapter 8 provides more in depth theory on electrostatic
electronic equipment in a conductive shell or enclosure
discharge (ESD) as it relates to military avionic
(Faraday cage). The enclosure reduces the incident
equipment, modules, and components.
electromagnetic energy to tolerable levels by reflection
and absorption. Radiation incident on the interior and
7-3.5. LIGHTNING. Lightning is probably the most
exterior surfaces does not escape or penetrate the
enclosure. Openings in the shield required for power/
severe uncontrolled electromagnetic environment to
signal cables, covers and access doors, ventilation,
which aircraft are exposed. The severity of the lightning
and windows allow EMI leakage. Shielding for these
threat depends on the probability of a lightning strike to
openings usually takes the form of gaskets, seals, or
the aircraft. Lightning strike is a function of weather,
joints that provide electrical continuity across voids to
temperature, geographical region, aircraft altitude,
ensure a continuous protective shell. In addition, this
aircraft type, and other parameters. Statistics show
shielding also aids lightning strike protection by
that an aircraft experiences less than one strike every
providing a conductive path. Performance of an EMI
3000 hours of flight operation. The damage from a
shielding system is a function of the weakest
lightning strike to the aircraft depends on its
component. Shielding effectiveness of the various
characteristics, such as peak current, rate-of-rise,
components (wall material, access doors, vents,
risetime, charge, action integral, and number of
cabling, connectors, joint seals) should be maintained.
subsequent strikes. The direct effects of a lightning
Changes in the shielding effectiveness of individual
strike are burning, eroding, blasting and structural
components over time should be correctable through
deformation caused by arcing. Also, high pressure
routine maintenance. Conditions that can inhibit the
shock waves and magnetic forces are produced by the
life and shielding effectiveness of EMI gaskets are:
associated high currents. These effects pose great
dangers to flight safety in addition to aborted missions.
a. The environment in which a gasket is stored.
The indirect (induced) effects result mostly from the
interaction of electromagnetic fields with electrical/
b. The environment in which the gasketed joint is to
electronic circuits. These effects also can be hazardous
since lightning may generate high voltages and currents
in circuits through coupling mechanisms.
c. The force on the gasketed joint.
7-3.6. ELECTROMAGNETIC PULSE. EMP is a threat
to military electronics which arises primarily from high
d. The unevenness of the joint.
altitude nuclear detonations. Gamma rays produced in
the explosion interact with air molecules and
7-4.2. EMI SHIELDING EFFECTIVENESS. When
particulates with sufficient energy to eject electrons.
electromagnetic energy impinges on a conductive
barrier, a portion of the energy is reflected/transmitted.