Supplied B1 Module 07A textbook: PDF pages 121–130.
Built-in test equipment
BITE may reside inside a line-replaceable unit or report through a central maintenance system. Continuous monitoring detects some faults during operation; initiated tests check specified functions on demand. Recorded messages help identify the affected system, fault conditions and likely causes. A BITE message is evidence, not proof that the named unit alone is defective. Wiring, power supplies, configuration and interfaces can create faults that appear to originate in an LRU. Use the fault-isolation manual to interpret messages.
What is the proper role of a BITE fault message? Evidence to be interpreted with the fault-isolation procedure.
Ground-test precautions
An initiated test can energise equipment, move controls or produce transmissions. Establish the aircraft configuration, inhibit hazardous functions as specified and coordinate with other workers. Follow ground/air restrictions and ensure power conditions are correct. Observe required warm-up, equipment connection and disconnection procedures. Do not clear fault memory before recording information needed for diagnosis. An apparent pass after resetting a system may simply mean that an intermittent fault is not present at that moment.
What should happen before clearing fault memory? Record the relevant diagnostic information.
External test sets
An approved test set may simulate aircraft inputs, monitor outputs or run a functional sequence. Check its part number, software configuration, calibration and compatible leads. Correctly keyed connectors and adapters protect equipment; do not force connections. A bench test and an installed-system test answer different questions because the installed environment includes wiring, supplies and connected systems. Where a task requires an approved external test set, an improvised signal source is not equivalent merely because its nominal output matches.
Why may a bench-tested LRU still show an installed fault? Aircraft wiring, power or interfaces may be faulty.
Oscilloscope fundamentals
An oscilloscope displays voltage against time. Vertical controls set volts per division; horizontal controls set time per division. Triggering stabilises the display around a selected event. A probe's attenuation factor must be included in a voltage reading. Period is the duration of one cycle and frequency is its reciprocal. The probe earth connection must be suitable for the circuit; an earth-referenced oscilloscope lead can create an unwanted short if connected to a live floating point.
What does the horizontal oscilloscope scale normally represent? Time per division.
Signal generators and counters
A signal generator provides a controlled waveform for a test, while a frequency counter measures repetition rate. Confirm frequency, amplitude, impedance and modulation against the procedure. Incorrect signal levels can overload an input even when the frequency is correct. Matching impedance reduces reflections in suitable circuits. A measured signal needs an acceptance limit and valid test conditions before it can be judged satisfactory. Use approved methods to connect radio-frequency equipment and control unintended radiation.
Which combination must be checked for a generator-based test? Frequency, level and connection conditions.
Fault isolation and return to service
Begin with the reported symptom and recorded conditions, then verify supplies, configuration and relevant interfaces as directed. Make one controlled change at a time so its effect can be understood. Protect electronic connectors and ESD-sensitive units during handling. After correcting a fault, perform the required operational or functional tests and restore the aircraft configuration. Record the work and evidence. A technician must remain within authorisation; knowledge of test equipment does not confer authority for unrestricted avionics bench repair.
What completes fault rectification beyond changing a component? Required verification, restoration and maintenance records.
Apply the knowledge
Work through these examples before starting the quiz. Explain why the other choices would not meet the requirement.
An oscilloscope shows a cycle lasting 2 ms. What is the frequency?
500 Hz — Frequency is 1 / 0.002 seconds = 500 Hz.
A trace spans three vertical divisions at 2 V/div with a ×10 probe. What voltage span is indicated?
60 V — 3 × 2 × 10 = 60 V, unless the display already compensates for the probe factor.
An intermittent fault disappears after a reset. What is appropriate?
Follow the fault-isolation procedure using the recorded conditions — Intermittent faults need evidence-based investigation and specified verification.
A proposed test may move a flight control. What must be established?
A safe configuration and coordination with personnel in the movement area — Test initiation can create physical hazards as well as electrical ones.