Tool Control & Calibration

Tool control and calibration is the maintenance-organization discipline that ensures the instruments used to judge whether a part is airworthy are themselves trustworthy. Its regulatory home for U.S. repair stations is 14 CFR 145.109, which addresses the equipment, materials, and data a station must have and maintain. The provision that matters most here is the requirement that all test and inspection equipment and tools used to make airworthiness determinations on articles be calibrated to a standard acceptable to the FAA, and that the calibration be traceable to that standard. A torque wrench, a micrometer, a pressure gauge, a borescope, or an eddy-current unit is only as good as its last valid calibration; if the measuring device is out of tolerance, every airworthiness decision made with it is suspect.

An important precision point: the regulation does not name NIST. It is common in industry shorthand to say calibration must be "NIST-traceable," and traceability to the U.S. national metrology institute is one very common way to satisfy the rule, but 14 CFR 145.109 speaks in terms of a standard acceptable to the FAA and traceability to that standard. The correct phrasing for a compliance document is therefore "calibrated to an FAA-acceptable, traceable standard," not "NIST-required." This distinction matters for organizations operating internationally, where the accepted standard may trace to a different national metrology institute, and the rule accommodates that so long as the standard is acceptable to the FAA and traceability is documented.

Beyond calibration of measuring equipment, tool control is the broader practice of managing the tooling itself. A well-run maintenance organization knows which tools it has, where they are, whether they are the manufacturer-recommended tool or an acceptable equivalent, and — for anything used in airworthiness determinations — when each was last calibrated and when it is next due. Calibration is performed on a defined interval, the calibration status is marked on or associated with each tool, out-of-service and overdue tools are segregated so they cannot be used inadvertently, and a record links each tool to its calibration history. Some organizations extend tool control into foreign-object-debris prevention through shadow boards and tool-inventory checks, so that a tool is never left inside an aircraft or engine.

The airworthiness logic ties these threads together. When a mechanic torques a critical fastener to a value specified in the maintenance data, the torque wrench must be within calibration or the recorded value means nothing. When an inspector uses a borescope to accept or reject an internal engine condition, or an NDT technician relies on a calibrated instrument to find a crack, the calibration record is part of what makes the resulting sign-off defensible. Calibration is therefore not an isolated metrology chore; it is a link in the same chain of accountability that runs through the work order and the release to service.

The principle generalizes across regulatory systems. EASA Part 145 and its associated requirements impose equivalent expectations: tools and equipment used for airworthiness determinations must be controlled and calibrated to an officially recognized standard, with records retained. Whatever the jurisdiction, the underlying idea is identical — measurements that decide airworthiness must be made with instruments proven, on a recurring basis, to measure correctly.

For a flight school or flying club that maintains any of its own tooling — even a torque wrench used for tire and wheel work under preventive maintenance — the calibration principle applies whenever a measurement supports an airworthiness decision. Operators that contract maintenance out rarely manage a calibration program themselves, but they benefit from understanding it: a shop that cannot produce current calibration records for the equipment it used on your aircraft has a gap that undermines the trustworthiness of its work, and that is a fair question to ask.

For a maintenance manager or MRO, tool control and calibration is a perennial audit topic. Auditors check that every instrument used for airworthiness determinations is within its calibration interval, that calibration is traceable to an FAA-acceptable standard, that overdue or out-of-service tools are segregated, and that the records connect each tool to the work it touched. An expired calibration sticker on a torque wrench found in the hangar is a classic, avoidable finding — and it can call into question the airworthiness of everything that wrench was used on since its calibration lapsed.

Aviatize's Maintenance Control module can track calibration due dates for tools and test equipment the way it tracks other recurring maintenance intervals, flagging instruments before they fall out of calibration so they are never used on an airworthiness determination past their due date. That keeps the tool inventory aligned with the 14 CFR 145.109 expectation without relying on stickers and memory.

Through the Digital Data & Records and Compliance & Auditing modules, calibration certificates and tool histories can be stored and linked to the work they supported, so a maintenance manager can show an auditor that every measurement behind a release to service was made with equipment calibrated to a traceable, FAA-acceptable standard.