AET Diagnostic and Testing Tools 3 — Questions and Answers
Question 1: When measuring resistance in an installed aircraft circuit with a digital multimeter, the technician must first:
- Set the meter to the highest resistance range
- De-energize the circuit and discharge any capacitors (Correct answer)
- Short the test leads and zero the meter as in an analog meter
- Switch the meter to DC voltage mode
Correct answer: De-energize the circuit and discharge any capacitors
Measuring resistance in a live circuit will give inaccurate readings and can damage the meter; the circuit must be de-energized first.
Question 2: Which of the following best describes the function of a BITE (Built-In Test Equipment) system?
- An external bench tester used to qualify LRUs before installation
- Onboard self-test circuitry that monitors and reports system faults automatically (Correct answer)
- A handheld tool used to inject test signals into avionics buses
- A dedicated oscilloscope built into the aircraft instrument panel
Correct answer: Onboard self-test circuitry that monitors and reports system faults automatically
BITE is embedded within avionics units to continuously or on-command perform self-tests and store fault codes for maintenance retrieval.
Question 3: A technician uses an oscilloscope to observe a 400 Hz aircraft power bus waveform and notices the top of the sine wave is flattened. This condition is called:
- Phase shift
- Clipping or waveform clipping (Correct answer)
- Frequency modulation
- Hysteresis
Correct answer: Clipping or waveform clipping
Clipping occurs when a signal exceeds the linear range of a component or supply, causing the peaks to be cut flat.
Question 4: The correct procedure when using a megohmmeter (megger) to test aircraft wire insulation is to:
- Apply test voltage with all other wires in the bundle connected to their normal loads
- Disconnect components from the wire under test to avoid applying high voltage to sensitive avionics (Correct answer)
- Test only with the aircraft master power switch ON
- Leave the wire bundle shield grounded throughout the test
Correct answer: Disconnect components from the wire under test to avoid applying high voltage to sensitive avionics
Megohmmeter test voltages (typically 500–1000 V) can permanently damage solid-state avionics connected to the wire under test.
Question 5: When calibrating a pressure transducer used in an aircraft fuel quantity system, which test equipment is most appropriate?
- Dead-weight tester or precision pneumatic pressure calibrator (Correct answer)
- Digital multimeter with diode check function
- RF signal generator
- Hipot tester
Correct answer: Dead-weight tester or precision pneumatic pressure calibrator
A dead-weight tester or precision pneumatic calibrator provides traceable reference pressures needed to verify and adjust pressure transducer accuracy.
Question 6: An aircraft COM radio's audio output is distorted. The most effective first step using a digital storage oscilloscope is to:
- Measure supply voltage at the radio's power connector
- Capture the audio waveform and compare it to a known-good waveform for clipping or noise (Correct answer)
- Perform an insulation resistance test on the antenna feedline
- Check the SWR of the antenna with an antenna analyzer
Correct answer: Capture the audio waveform and compare it to a known-good waveform for clipping or noise
Capturing and visually comparing the audio waveform reveals the nature of distortion (clipping, noise, hum) to guide further diagnosis.
Question 7: What does a high VSWR reading on an antenna analyzer indicate about an aircraft antenna installation?
- The antenna is properly matched to the transmission line
- There is a significant impedance mismatch causing reflected power (Correct answer)
- The antenna gain is above specification
- The coaxial feedline has excessive capacitance
Correct answer: There is a significant impedance mismatch causing reflected power
A high Voltage Standing Wave Ratio means reflected power is returning to the transmitter, indicating an impedance mismatch in the antenna system.
AET Diagnostic and Testing Tools 3
When measuring resistance in an installed aircraft circuit with a digital multimeter, the technician must first: