14.5 Troubleshooting, Defect Assessment & Repair Control

14.5 Troubleshooting, Defect Assessment & Repair Control

Troubleshooting is a controlled process for turning a reported symptom into an evidenced cause and an approved corrective action. A warning message, leak, vibration, failed test, or crew report may be incomplete or intermittent. Replacing the first plausible component can waste serviceable parts and leave the real fault in the aircraft.

Define and reproduce the defect

Read the technical log and speak with the reporter when possible. Clarify when the event occurred, system configuration, environmental conditions, associated messages, reset attempts, and whether the symptom is repeatable. Review maintenance history and recent disturbed areas without assuming that the latest work must be the cause.

Make the aircraft safe and establish the test configuration in approved data. Confirm required power, fluid quantity, temperature, software or configuration state, and test-equipment setup. A test performed under the wrong conditions can create a false fault or false pass.

Follow evidence

Use the fault-isolation manual, wiring diagrams, schematics, maintenance messages, built-in test information, and system description. Understand what each test separates. Work from shared inputs and simple external causes toward intrusive action when the procedure supports that order.

Check power and grounds, connectors, pins, wiring, bonding, pipes, mechanical linkages, sensors, data buses, software state, and environmental effects as applicable. Never defeat a protection or substitute an uncontrolled jumper merely to make the symptom disappear. Record readings with units, conditions, instrument identity where required, and the expected range.

An intermittent fault needs evidence. Inspect for chafe, fretting, moisture, heat, vibration, poor support, disturbed connectors, and load-dependent behaviour. Do not shake or stress wiring outside approved limits. If a fault cannot be reproduced, follow the specified no-fault-found process rather than declaring the aircraft serviceable without completing the required assessment.

Visual inspection and damage description

Good defect assessment is precise. State component or zone, side, station or reference, orientation, dimensions, depth if measurable by an approved method, distance from holes or edges, finish condition, and related leakage or distortion. Photographs should include scale and orientation.

Distinguish a crack from a scratch only through the prescribed inspection method. Clean using an authorised method before inspection. Do not blend corrosion, drill-stop a crack, straighten structure, or remove material simply to improve appearance before the limits and repair route are known.

Structural limits and repair

Use the current Structural Repair Manual or other approved data applicable to the exact aircraft, material, location, and configuration. Damage limits often depend on size, spacing, edge distance, adjacent repairs, load path, and remaining material. A repair from another model, a previous work order, or an online illustration is not automatically applicable.

If damage is outside published limits, if required data are ambiguous, or if the configuration differs, obtain an approved engineering disposition through the organisation’s process. Temporary and permanent repairs must be identified as such, with any inspection interval, life, or operating limitation recorded and controlled.

Prove the correction

After rectification, inspect the work, restore all disturbed systems and protections, and perform the specified operational or functional tests. Confirm that the original symptom is cleared and that the intervention did not create a new fault. Repeatedly clearing a message without finding or addressing the cause is not proof.

Document the original defect, findings, measurements, data used, parts or repair installed, test results, and work status. Communicate unresolved or intermittent conditions at handover. Systematic troubleshooting is disciplined uncertainty reduction: verify, isolate, measure, compare, correct under authority, and prove.

Building and testing a fault hypothesis

A fault tree is useful when several symptoms share a source. If multiple indications fail together, first consider a common power supply, ground, data source, or environmental cause before replacing separate receivers. If only one channel fails, compare channels only where the approved procedure permits and where swapping will not transfer configuration or create an unrecorded change. Every test should answer a defined question and have a safe next step for either result.

Test equipment can also create symptoms. Verify calibration or serviceability, correct adapters, loading, software version, and reference connection. A meter with the wrong input impedance or an oscilloscope grounded at an unsuitable point can alter or damage the circuit. Built-in test messages are evidence, not necessarily the failed-part name.

When a repair removes the symptom, examine whether the proposed cause explains all observations. A connector reseated during diagnosis may temporarily clear fretting or moisture without establishing a durable rectification. Inspect, clean, repair, or replace it only under the approved instructions, then repeat the required test under representative conditions. Where a removed component is sent to a shop, communicate the recorded symptoms and test conditions so the workshop can reproduce the problem rather than returning it as no fault found without context.

Evidence Loop

1. Verify the symptom and test configuration.
2. Predict what each possible cause would make the next measurement show.
3. Test through the approved fault-isolation procedure with suitable equipment.
4. Compare the result with the stated value or decision point.
5. Correct only under applicable maintenance or repair authority.
6. Prove the rectification, restore configuration and record the outcome.