One of the most valuable diagnostic tools available to an aircraft maintenance technician (AMT) is the magnetic chip detector (MCD). Installed in the engine lubrication system, these deceptively simple devices use a permanent magnet to capture ferrous (iron-based) metal particles suspended in the engine oil. Because internal engine components — gears, bearings, shafts, and other steel or iron parts — shed microscopic particles as they wear, the chip detector acts as a continuous sentinel, quietly collecting evidence of what is happening deep inside the engine where no direct visual inspection is possible.
Understanding how to properly inspect, interpret, and service magnetic chip detectors is a fundamental competency for any powerplant technician. The FAA and aircraft manufacturers treat chip detector findings with the utmost seriousness, because what you find — or don't find — on that magnet can be the difference between catching a developing failure early and experiencing a catastrophic in-flight engine loss.
How Magnetic Chip Detectors Work
A magnetic chip detector is typically a threaded plug assembly that screws into a boss located at or near the lowest point of an engine sump, gearbox, or accessory section. The device consists of an outer housing that threads into the engine and an inner probe tipped with a strong permanent magnet. As engine oil circulates through the lubrication system, it flows past — and in some designs, through — the detector. Any ferrous particles suspended in the oil are attracted to the magnet and adhere to it, building up a deposit that the technician can examine during scheduled maintenance.
Some chip detectors are simple passive plug-type devices that must be physically removed and visually inspected. More sophisticated designs incorporate an electrical circuit: two electrodes are separated by a small gap around the magnet tip. When enough conductive metallic particles bridge that gap, the circuit closes and illuminates a cockpit warning light or triggers a maintenance alert. These are called electric chip detectors or chip detector warning systems. In either case, the underlying detection principle is the same — magnetism attracts ferrous debris from the oil stream.
In turbine engines, chip detectors are commonly found in the main engine gearbox, accessory gearbox, and sometimes the oil tank sump. In reciprocating engines, the oil sump drain plug is often a magnetic chip detector, and additional detectors may be installed in oil filter housings or remote oil sumps. The exact locations are always specified in the aircraft and engine manufacturer's maintenance manuals, which the technician must consult for each specific installation.
Inspection Procedures Step by Step
Before Removal
Before disturbing any chip detector, the technician should review the aircraft logbooks and maintenance records for previous findings. Knowing what was found on the last inspection provides critical context. Next, consult the applicable maintenance manual — specifically the inspection procedure section — because torque values, safety-wiring requirements, and acceptable particle descriptions vary by engine model. Ensure the engine has been shut down and cooled sufficiently, and that all appropriate safety precautions (hot surface warnings, rotating component lockout) are observed. Place absorbent rags or a drain pan below the detector to catch oil spillage.
Removal
Using the correct tool — typically a specialized chip detector wrench or a standard open-end wrench of the correct size — carefully break the detector loose. Move slowly; residual oil pressure or hot oil may still be present. As the detector clears the housing, tip it so the magnet-tipped probe faces upward to prevent particles from sliding off before examination. Cap or plug the open port if the inspection will take more than a moment, preventing oil loss and contamination ingestion.
Visual Examination
This is the most critical step. Hold the detector probe in good lighting — a magnifying glass is recommended — and examine the magnet tip carefully. Findings fall into several general categories:
- No debris (clean): A clean detector with no metallic particles is the expected and desired finding. It indicates no detectable ferrous wear is occurring.
- Fuzzy gray coating or very fine metallic fuzz: A light, fuzzy, gray-black coating that resembles a thin layer of velvet is generally considered normal. This fine magnetic fuzz consists of extremely small particles generated by normal wear of ferrous components over time. Most manufacturer manuals describe an acceptable appearance threshold — the technician must compare findings to that description.
- Small flakes or chips: Discrete metallic flakes or chips that are clearly separated particles, rather than fuzzy coating, indicate abnormal wear. The size, quantity, and appearance of these chips are significant. Bright, shiny flakes suggest fresh material removal. Larger pieces indicate more aggressive wear or a developing failure.
- Large chunks or pieces: Any sizeable metallic fragments are an immediate red flag. This finding typically grounds the aircraft and triggers further investigation — including an oil filter inspection, oil analysis, borescope inspection, and possible engine removal for teardown.
The technician should note findings in writing and, where possible, photograph the detector before cleaning for the maintenance record. The manufacturer's maintenance manual will specify exactly what constitutes an acceptable finding versus a finding that requires additional action. When in doubt, the conservative course of action is always to escalate the finding to engineering or a supervisor before returning the aircraft to service.
Cleaning and Reinstallation
After examination and documentation, clean the detector using an approved solvent and lint-free cloths or swabs. Ensure all particles are fully removed from the magnet — particles left behind will contaminate the next inspection's findings. Inspect the detector's O-ring or gasket for condition; replace if there is any sign of hardening, cracking, or damage. Lubricate the new O-ring with clean engine oil before installation.
Thread the detector in by hand to avoid cross-threading, then torque to the value specified in the maintenance manual — never guess at torque. After torquing, safety-wire the detector according to the manufacturer's requirements. Re-check the installation after the first engine run for any oil seepage.
Why Chip Detector Inspection Matters
The magnetic chip detector inspection is one of the few non-invasive windows into the internal mechanical health of a running engine. Unlike a borescope inspection — which requires access ports and significant setup — the chip detector is accessible during routine oil changes and scheduled inspections. Catching a developing bearing failure or gear tooth fracture through chip detector findings can allow a planned, controlled engine removal instead of a forced landing. The economic and safety stakes are immense: reciprocating and turbine aircraft engines are extraordinarily expensive to overhaul or replace, and an in-flight engine failure can have fatal consequences.
Chip detectors are also valuable trend monitoring tools. A single inspection finding of light fuzz means little in isolation; the same finding compared against several previous inspections showing a clear upward trend in debris quantity tells a compelling story of accelerating wear. This is why meticulous documentation of every chip detector inspection — including description of findings, not just a simple pass/fail notation — is so important.
Key Numbers and Rules
- Inspection intervals: Chip detector inspection intervals are set by the engine or aircraft manufacturer and vary widely by engine type, installation, and applicable maintenance program — always follow the applicable maintenance manual and Instructions for Continued Airworthiness, as there is no universal FAA-mandated interval that overrides manufacturer specifications.
- Torque values: Always torque chip detectors to the manufacturer's specified value — typically found in the maintenance manual's installation section. Over-torquing can crack the housing boss; under-torquing risks oil leakage in flight.
- Safety wire: Most chip detectors require safety wiring after installation to prevent loosening from vibration. Use the wire gauge and method specified in the maintenance manual.
- O-ring replacement: Replace the O-ring or crush washer at every removal unless the maintenance manual explicitly states it is reusable and it passes inspection.
- Escalation threshold: Any finding of discrete chips, flakes, or particles beyond the fuzzy normal wear coating requires follow-up action per the maintenance manual — do not simply clean and reinstall without proper investigation and documentation.
- Oil analysis pairing: Chip detector inspection is most powerful when combined with spectrographic oil analysis (SOAP), which detects both ferrous and non-ferrous wear metals. Neither method alone catches everything.
Common Test Traps
- Confusing normal fuzz with abnormal wear: The FAA knowledge test may ask a technician to distinguish between the fine, velvet-like magnetic fuzz of normal wear and the discrete metallic chips of abnormal wear. Fine fuzz is generally acceptable; chips and flakes are not, and always require further investigation.
- Assuming a clean detector means a healthy engine: A clean chip detector is reassuring, but does not guarantee engine health. Non-ferrous components (aluminum, brass, titanium, carbon) are not magnetically attracted and generally will not build up on the magnet, though fine non-ferrous debris can sometimes be visually noted on or near the probe if comingled with other material. This is why chip detection alone is never the sole diagnostic method.
- Skipping O-ring replacement: Re-using an old O-ring to save time is a common error that can cause in-flight oil leaks. The correct procedure is to inspect and replace as specified.
- Applying incorrect torque: The torque value for chip detectors is specified in the maintenance manual for a reason. Using a general fastener torque chart instead of the manufacturer's specified value is incorrect procedure.
- Failing to document findings accurately: Simply writing "chip detector inspected — clean" when particles were found and cleaned off is a falsification of records and an airworthiness violation. Every finding must be accurately documented, including description and disposition.