NCATT AET & AEIT Endorsement
Questions & Answers with Rationales
Covering: EMI • Grounding • Bonding • TSO • PMA • Aircraft Wiring and More
This study guide is designed as an independent practice resource to help
candidates prepare for the National Center for Aerospace and Transportation
Technologies (NCATT) Aircraft Electronics Technician (AET) and Avionics
Electrical Installation Technician (AEIT) endorsement exams. Each question
is followed immediately by the correct answer and a rationale explaining the
underlying principle, per FAA guidance (AC 43.13-1B, AC 20-136B, 14 CFR
Parts 21/43/45) and industry practice.
,Section 1: Electromagnetic Interference (EMI) &
Shielding
Q1. What is the primary purpose of shielding a wire bundle in an aircraft
wiring installation?
A. To increase the current-carrying capacity of the conductor
B. To reduce the emission and susceptibility of the conductor to
electromagnetic interference
C. To provide additional mechanical strength to the bundle
D. To reduce the overall weight of the wiring harness
Answer: B
Rationale: Shielding is applied to a conductor primarily to control
electromagnetic interference (EMI) — it reduces the amount of
electromagnetic energy the wire radiates (emission) and the amount of
external electromagnetic energy the wire picks up (susceptibility). Shielding
has no effect on current capacity, adds weight rather than reducing it, and
offers minimal mechanical benefit.
Q2. A shield termination that is 'pigtailed' (twisted into a single lead
before connecting to a connector pin or ground point) is generally
discouraged for high-frequency EMI protection because it:
A. Increases the DC resistance of the shield to an unacceptable level
B. Creates an inductive tail that degrades high-frequency shielding
effectiveness
C. Is more expensive to install than a 360-degree termination
D. Is prohibited by all aircraft manufacturers without exception
Answer: B
Rationale: A pigtail lead adds inductance and breaks the continuous shield
coverage, which is fine at low frequencies but significantly degrades shield
effectiveness at higher frequencies (RF range). A 360-degree (full
circumferential) termination using a shield termination band or backshell
maintains lower impedance and better high-frequency performance.
Q3. For best high-frequency EMI shielding effectiveness, a shield
should be terminated:
, A. With as long a pigtail as possible to allow flexibility
B. Only at one end, always the source end
C. With a 360-degree (all-around) connection to the backshell or shield
termination device
D. Using solder directly to the aircraft skin only
Answer: C
Rationale: A 360-degree termination minimizes the impedance path and
preserves the shield's continuity around the full circumference of the cable,
which is essential for effective high-frequency EMI attenuation. This is the
industry-preferred method described in AC 43.13-1B and SAE wiring
standards.
Q4. Which type of shield termination is generally acceptable ONLY at
low frequencies (below approximately 1 MHz)?
A. 360-degree shield termination band
B. Pigtail (single-lead) termination
C. Conductive backshell termination
D. EMI gasket termination
Answer: B
Rationale: Pigtail terminations are acceptable for lower-frequency circuits
because the added inductance has a smaller relative effect at lower
frequencies. As frequency increases, the pigtail's inductive reactance grows
and shielding effectiveness drops significantly, making it unsuitable for
high-frequency or RF applications.
Q5. Twisting a pair of wires together throughout a wire run primarily
helps to reduce which type of interference coupling?
A. Electrostatic (capacitive) coupling only
B. Magnetic (inductive) coupling by canceling the loop area between
conductors
C. Conductive coupling through the airframe
D. Thermal coupling from adjacent heat sources
Answer: B
Rationale: Twisting wires causes the magnetic fields generated by current in
each conductor to alternate direction along the length of the pair, effectively
, canceling out over each twist. This greatly reduces the loop area available to
couple with external magnetic fields, minimizing inductively-coupled
(magnetic) interference.
Q6. When routing wiring, what is the recommended minimum
separation practice between EMI-sensitive signal wiring and
high-current power wiring or wiring that generates transients?
A. No separation is required if both wires are shielded
B. Route them in the same bundle to save space, regardless of shielding
C. Physically separate them per the applicable wiring diagram/manual,
using separate bundles or maximum practical spacing
D. Separation is only necessary for wires carrying DC current
Answer: C
Rationale: Aircraft wiring practices (AC 43.13-1B, Chapter 11) call for
physical segregation of sensitive signal, audio, and data wiring from power
wiring, especially wiring that switches inductive loads or carries high current,
to minimize EMI coupling. Segregation distances and bundling rules are
specified in the aircraft wiring diagrams and maintenance manuals.
Q7. Which of the following is a common source of EMI on an aircraft
that technicians should be aware of during troubleshooting?
A. Properly torqued bonding straps
B. Brush-type DC motors, relay coil switching, and static discharge from
precipitation
C. Twisted shielded pairs installed per the manufacturer's manual
D. Properly terminated coaxial connectors
Answer: B
Rationale: Common EMI sources include brush-type motors (arcing at
commutators), inductive switching transients from relay and solenoid coils,
and precipitation static (P-static) discharge from the airframe. Properly
installed shielding, bonding, and connectors are mitigation measures, not
sources, of interference.
Q8. A static discharge wick (static wick) installed on a wingtip or
control surface trailing edge is primarily used to:
Questions & Answers with Rationales
Covering: EMI • Grounding • Bonding • TSO • PMA • Aircraft Wiring and More
This study guide is designed as an independent practice resource to help
candidates prepare for the National Center for Aerospace and Transportation
Technologies (NCATT) Aircraft Electronics Technician (AET) and Avionics
Electrical Installation Technician (AEIT) endorsement exams. Each question
is followed immediately by the correct answer and a rationale explaining the
underlying principle, per FAA guidance (AC 43.13-1B, AC 20-136B, 14 CFR
Parts 21/43/45) and industry practice.
,Section 1: Electromagnetic Interference (EMI) &
Shielding
Q1. What is the primary purpose of shielding a wire bundle in an aircraft
wiring installation?
A. To increase the current-carrying capacity of the conductor
B. To reduce the emission and susceptibility of the conductor to
electromagnetic interference
C. To provide additional mechanical strength to the bundle
D. To reduce the overall weight of the wiring harness
Answer: B
Rationale: Shielding is applied to a conductor primarily to control
electromagnetic interference (EMI) — it reduces the amount of
electromagnetic energy the wire radiates (emission) and the amount of
external electromagnetic energy the wire picks up (susceptibility). Shielding
has no effect on current capacity, adds weight rather than reducing it, and
offers minimal mechanical benefit.
Q2. A shield termination that is 'pigtailed' (twisted into a single lead
before connecting to a connector pin or ground point) is generally
discouraged for high-frequency EMI protection because it:
A. Increases the DC resistance of the shield to an unacceptable level
B. Creates an inductive tail that degrades high-frequency shielding
effectiveness
C. Is more expensive to install than a 360-degree termination
D. Is prohibited by all aircraft manufacturers without exception
Answer: B
Rationale: A pigtail lead adds inductance and breaks the continuous shield
coverage, which is fine at low frequencies but significantly degrades shield
effectiveness at higher frequencies (RF range). A 360-degree (full
circumferential) termination using a shield termination band or backshell
maintains lower impedance and better high-frequency performance.
Q3. For best high-frequency EMI shielding effectiveness, a shield
should be terminated:
, A. With as long a pigtail as possible to allow flexibility
B. Only at one end, always the source end
C. With a 360-degree (all-around) connection to the backshell or shield
termination device
D. Using solder directly to the aircraft skin only
Answer: C
Rationale: A 360-degree termination minimizes the impedance path and
preserves the shield's continuity around the full circumference of the cable,
which is essential for effective high-frequency EMI attenuation. This is the
industry-preferred method described in AC 43.13-1B and SAE wiring
standards.
Q4. Which type of shield termination is generally acceptable ONLY at
low frequencies (below approximately 1 MHz)?
A. 360-degree shield termination band
B. Pigtail (single-lead) termination
C. Conductive backshell termination
D. EMI gasket termination
Answer: B
Rationale: Pigtail terminations are acceptable for lower-frequency circuits
because the added inductance has a smaller relative effect at lower
frequencies. As frequency increases, the pigtail's inductive reactance grows
and shielding effectiveness drops significantly, making it unsuitable for
high-frequency or RF applications.
Q5. Twisting a pair of wires together throughout a wire run primarily
helps to reduce which type of interference coupling?
A. Electrostatic (capacitive) coupling only
B. Magnetic (inductive) coupling by canceling the loop area between
conductors
C. Conductive coupling through the airframe
D. Thermal coupling from adjacent heat sources
Answer: B
Rationale: Twisting wires causes the magnetic fields generated by current in
each conductor to alternate direction along the length of the pair, effectively
, canceling out over each twist. This greatly reduces the loop area available to
couple with external magnetic fields, minimizing inductively-coupled
(magnetic) interference.
Q6. When routing wiring, what is the recommended minimum
separation practice between EMI-sensitive signal wiring and
high-current power wiring or wiring that generates transients?
A. No separation is required if both wires are shielded
B. Route them in the same bundle to save space, regardless of shielding
C. Physically separate them per the applicable wiring diagram/manual,
using separate bundles or maximum practical spacing
D. Separation is only necessary for wires carrying DC current
Answer: C
Rationale: Aircraft wiring practices (AC 43.13-1B, Chapter 11) call for
physical segregation of sensitive signal, audio, and data wiring from power
wiring, especially wiring that switches inductive loads or carries high current,
to minimize EMI coupling. Segregation distances and bundling rules are
specified in the aircraft wiring diagrams and maintenance manuals.
Q7. Which of the following is a common source of EMI on an aircraft
that technicians should be aware of during troubleshooting?
A. Properly torqued bonding straps
B. Brush-type DC motors, relay coil switching, and static discharge from
precipitation
C. Twisted shielded pairs installed per the manufacturer's manual
D. Properly terminated coaxial connectors
Answer: B
Rationale: Common EMI sources include brush-type motors (arcing at
commutators), inductive switching transients from relay and solenoid coils,
and precipitation static (P-static) discharge from the airframe. Properly
installed shielding, bonding, and connectors are mitigation measures, not
sources, of interference.
Q8. A static discharge wick (static wick) installed on a wingtip or
control surface trailing edge is primarily used to:
