Calibration for Aerospace MRO in Singapore: CAAS, EASA Part-145 & AS9100 Requirements

The Regulatory Framework for Aerospace MRO Calibration in Singapore

CAAS and Airworthiness Organisation Requirements

The Civil Aviation Authority of Singapore (CAAS) regulates approved maintenance organisations (AMOs) under the Air Navigation Order and associated Airworthiness Notices. AMOs approved by CAAS must comply with CAAS AWO requirements, which include documented maintenance procedures covering the inspection, testing, and calibration of tools and equipment used in aircraft maintenance. CAAS conducts annual surveillance audits of approved AMOs, and calibration systems are a standard audit item. Inspectors verify that tools are within calibration, that records are maintained, and that out-of-calibration tools are not in use.

The CAAS calibration requirements are principle-based rather than prescriptive. The AMO must define its own calibration intervals, recall systems, and record formats, but these must be documented in approved maintenance procedures and consistently followed. CAAS will verify that the actual calibration practice matches what is written in the AMO's approved documentation, making consistency between procedure and practice as important as the procedure itself.

EASA Part-145 Requirements

EASA Part-145 is the European regulation for approved maintenance organisations, and many Singapore-based MROs hold EASA Part-145 approval to service European-registered aircraft. Part-145.A.40 (Equipment requirements) requires that all tools, equipment, and material needed for the approved scope of maintenance be available. Part-145.A.25 (Facility requirements) and the associated GM (Guidance Material) and AMC (Acceptable Means of Compliance) require that test equipment and tools used in maintenance be calibrated at defined intervals, with traceability to national measurement standards, and that calibration records be maintained. EASA auditors apply the same calibration rigour as CAAS, often with reference to the OEM's tooling calibration requirements specified in the Aircraft Maintenance Manual (AMM).

EASA Part-145 auditors additionally verify that the calibration laboratory performing the calibration holds appropriate accreditation. Calibration certificates from non-accredited laboratories are not acceptable for EASA compliance. Where the AMM specifies that a particular special tool must be calibrated at a defined interval or to a defined accuracy, the EASA auditor will check that the calibration certificate confirms compliance with the AMM's requirement.

FAA Part 145 Requirements

Singapore MROs serving US carriers or US-registered aircraft typically hold FAA Part 145 approval under the US-Singapore Bilateral Aviation Safety Agreement (BASA). FAA §145.109 (Equipment, materials, and data requirements) requires that approved equipment be in proper condition for its intended use and, where applicable, calibrated. FAA Inspectors are particularly attentive to calibration documentation for safety-critical measurement tools. The FAA's emphasis on traceability aligns with the requirements of other frameworks.

FAA auditors also pay close attention to the currency of calibration certifications and the recall systems used to ensure that tools are withdrawn from service when calibration is due. A tool found in active use with an expired calibration certificate during an FAA surveillance audit is a serious finding that can result in corrective action requirements and, in repeated instances, approval action.

AS9100D. Quality Management for Aviation, Space, and Defense

AS9100D (the aerospace-specific quality management standard based on ISO 9001) is held by many Singapore aerospace manufacturers and some MROs. AS9100D Section 7.1.5.2 (Measurement traceability) requires that measuring equipment be calibrated or verified at specified intervals against measurement standards traceable to national or international measurement standards, and that the calibration results be retained as documented information. AS9100D goes beyond ISO 9001 with additional requirements for the identification of measuring and monitoring devices on design documents, preservation of calibration records during the useful life of the equipment, and control of jigs, fixtures, templates, and other items used as measurement aids.

AS9100D also requires that, when measuring equipment is found to be out of tolerance (OOT), the organisation determine whether the previous measurements remain valid and take appropriate corrective action. This OOT investigation and assessment requirement demands a systematic approach to understanding when the equipment went out of tolerance and what products or measurements may have been affected. A significantly more demanding response than simply sending the equipment back for re-calibration.

Critical Instrument Categories in Aerospace MRO

Torque Wrenches and Torque Multipliers

Torque wrenches are arguably the highest-stakes calibration item in aerospace maintenance. Incorrect fastener torque (whether over or under), is a direct safety risk. Aircraft maintenance manuals specify fastener torque values to OEM tolerances that are typically ±5% or ±10% of the nominal value. A torque wrench that is 15% or 20% out of calibration can produce fasteners that are either dangerously loose or damaged by over-torquing.

CAAS and EASA auditors consistently identify torque wrench calibration as a high-priority audit area. Common findings include:

• Torque wrenches in use with expired calibration dates
• Calibration not performed in the direction of use (CW vs CCW separately required)
• Calibration performed at only one set-point rather than across the full range or at specified application points
• No recall system to ensure wrenches are withdrawn from service when calibration due date is reached
• Calibration certificates from non-accredited laboratories

Torque wrench calibration must be performed to ISO 6789 (the international standard for torque tools), with calibration at a minimum of three set-points across the wrench's range (typically at 20%, 60%, and 100% of maximum capacity). The calibration certificate must show the torque accuracy at each test point and confirm the wrench meets the ±4% accuracy requirement of ISO 6789 Type II or the tighter OEM requirements.

Torque multipliers (gear-driven multipliers used to apply high torque values to large fasteners), must also be calibrated, including the multiplication ratio. This requires specialist torque calibration equipment with sufficient capacity.

Dimensional Measurement Tools

Micrometers, vernier calipers, bore gauges, depth gauges, thread gauges, and height gauges are used throughout aerospace MRO for inspection of component dimensions against aircraft maintenance manual tolerances. Component dimensional inspection is critical for: engine component wear measurement (turbine blade tip clearance, bearing bore diameters), airframe structure inspection (hole diameter, skin thickness, gap measurement), and tooling verification.

Dimensional tool calibration is performed against calibrated gauge blocks (slip gauges) and ring gauges traceable to SI units (the metre). Calibration intervals for dimensional tools in aerospace MRO are typically 12 months, but higher-precision or more heavily-used tools may require shorter intervals. AS9100D requires that measuring and monitoring devices used on design documents be identified; this means the specific calibrated tools used for critical dimensional measurements must be recorded.

Borescopes and Borescope Measurement Tools

Borescopes (rigid and flexible) are used for visual inspection of engine interiors, airframe structures, and difficult-to-access areas. Where borescopes incorporate measurement capability (e.g. measurement borescopes for turbine blade damage assessment), the measurement function must be calibrated. Calibration of borescope measurement systems involves verification against a calibrated reference target of known dimensions.

The calibration status of the optical or digital measurement system in a measurement borescope is distinct from a standard visual inspection borescope. Only the instruments where measurement values are relied upon for maintenance decisions require formal calibration certification; purely visual borescopes used only for observational inspection are typically controlled differently, though they must still be maintained in serviceable condition.

NDT Equipment

Non-destructive testing is fundamental to aerospace MRO, and NDT equipment calibration requirements are specified by the relevant NDT standards:

• Ultrasonic flaw detectors: Calibrated per ISO 16811 (sensitivity and range calibration), EN 12668 (performance characteristics), or OEM-specific procedures. Reference blocks (calibrated step wedges, side-drilled hole blocks) must themselves be certified.
• Eddy current instruments: Calibrated per EN 4050 or equivalent, with calibrated reference standards (notch standards) used for sensitivity setting.
• Dye penetrant process controls: Black light intensity meters (UV-A radiometers) must be calibrated; penetrant concentration and sensitivity must be monitored with calibrated test panels.
• Magnetic particle test equipment: Field strength meters (Gaussmeters) used for magnetisation verification must be calibrated.

NDT equipment calibration is typically performed annually for the instrument, with more frequent (daily or per-shift) reference standard checks as part of the NDT procedure. The calibration of NDT reference standards themselves (the step wedge blocks, notch standards, and calibration panels), is equally critical; reference standards used with uncalibrated or uncertified blocks provide no assurance of sensitivity.

Electrical Test Equipment

Electrical maintenance and testing of aircraft electrical systems requires calibrated test equipment including:

• Digital multimeters for voltage, current, and resistance measurement
• Insulation resistance testers (megohmmeters) for wiring insulation testing
• Bonding resistance testers for aircraft bonding and grounding verification
• Capacitance and inductance meters for avionics component testing
• Oscilloscopes for avionics signal analysis

Electrical test equipment is calibrated against traceable electrical standards. Calibration intervals are typically 12 months. Calibration certificates must show the calibration of all functions tested, not just one or two ranges. A multimeter calibration certificate that only verifies DC voltage on one range does not satisfy the requirement for a tool used across multiple measurement functions in aircraft electrical maintenance.

Pressure Test Equipment

Hydraulic and pneumatic test rigs used for aircraft system pressure testing (hydraulic system, landing gear, fuel system, pneumatic system) contain calibrated pressure gauges or pressure transducers that must be calibrated at regular intervals. The pressure measurement in a test rig is safety-critical; an uncalibrated pressure gauge can result in unsafe test pressures that damage components or create personnel hazards. Pressure test rig calibration involves verifying the calibrated pressure gauge or transducer against a primary or transfer standard (dead-weight tester or digital pressure calibrator with traceable reference).

On-site calibration is often the only practical approach for large hydraulic test rigs and pneumatic test stands that cannot be transported to a laboratory. A mobile calibration service with appropriate high-pressure calibration equipment is essential for these assets, and the calibration must be documented against the test rig's installed gauges in situ.

Temperature Calibration. Oven and Heat Treatment

Heat treatment of aircraft components (aluminium alloy heat treatment, composite cure cycles) requires calibrated temperature measuring systems. Temperature uniformity surveys of heat treatment ovens are required before use in qualification heat treatment. Oven temperature calibration and TUS (Temperature Uniformity Surveys) follow similar methodology to semiconductor manufacturing. Calibrated thermocouples, calibrated DAQ systems, and comparison against the temperature uniformity specification.

Composite repair curing ovens and autoclaves require documented temperature uniformity surveys to aerospace material specifications (typically AMS 2750E or equivalent OEM specification). The temperature uniformity survey must be performed with calibrated thermocouples placed at defined positions within the working zone, and the survey must demonstrate that the temperature across the zone meets the class requirement across the entire qualified temperature range.

Torque Calibration. The Most Critical Item in Aerospace MRO

Given the safety criticality and frequency of torque wrench audit findings, a more detailed examination of the requirements is warranted.

Calibration Direction

Torque wrenches must be calibrated in the direction of use. A wrench that is calibrated only in the clockwise (tightening) direction does not have a verified calibration for counter-clockwise (loosening) use. Where wrenches are used in both directions, both must be calibrated. The calibration certificate must state the direction(s) calibrated. Many calibration laboratories default to clockwise-only calibration unless specifically requested otherwise; aerospace MROs should confirm with their calibration provider that the calibration direction matches the operational use of each wrench.

Calibration at Application Set-Points

For wrenches used at specific set-point torque values (e.g. always set to 45 Nm for a specific fastener), calibration verification should include the actual application set-point. A calibration performed only at the extremes of the range may not verify the accuracy at the set-point that matters. Best practice is to provide the calibration laboratory with the application torque values so these can be included as calibration points on the certificate. This is particularly important for wrenches used at low set-points relative to their maximum capacity, where accuracy at low-range settings may be different from full-range performance.

Recall Systems

A robust calibration recall system is essential. When a torque wrench reaches its calibration due date, it must be automatically flagged for withdrawal from service. Manual systems (diary reminders, spreadsheets) are error-prone. CMMS (Computerised Maintenance Management Systems) with calibration due-date alerts are the preferred solution for MROs with large tool inventories. Calibration service providers who provide an online portal or recall service (automatic notification when calibration is approaching due date) add value beyond the calibration itself.

The recall system must also trigger when a wrench has been dropped, subjected to overload, or damaged. Even if the calibration date has not expired. Physical impact events can shift calibration without any visible damage, and the wrench must be re-calibrated before return to service. Calibration status labels on tools should include not only the next calibration due date but also a unique tool identifier that ties the physical wrench to its calibration record in the management system.

AS9100D vs ISO 9001. Key Calibration Differences

Identification on Design Documents

AS9100D requires that measuring and monitoring devices used in verification activities be identified on the design documentation or work instructions. This means that where a specific calibrated tool is required for a measurement (e.g. a specific micrometer for a dimensional check), that tool must be referenced in the work instruction, and the calibration status of the specific tool used must be recorded. This traceability requirement (linking a specific calibrated instrument to a specific measurement event on a specific component), is more demanding than simply demonstrating that calibrated tools are available. AS9100D-certified manufacturers supplying aerospace MRO organisations typically encounter this requirement in their first AS9100D third-party audit.

Preservation During Useful Life

AS9100D requires that calibration records be preserved during the useful life of the equipment being verified. For long-life aircraft structures and components, this can mean calibration records must be retained for the life of the aircraft, potentially 25–30 years. This is a significant records management requirement. Digital records management systems with appropriate backup, disaster recovery, and long-term retention capability are the practical solution. Paper-based calibration records are difficult to preserve reliably over multi-decade timescales and are increasingly unacceptable to AS9100D auditors.

Jigs, Fixtures, and Templates

AS9100D extends calibration-like controls to jigs, fixtures, templates, and other items used as measurement aids. These items must be controlled, verified to be fit for purpose, and records maintained. This is a common AS9100D audit finding in MRO facilities where fixtures are used to position components for measurement but are not included in the formal calibration register. The control requirements for jigs and fixtures as measurement aids do not necessarily require formal calibration to a standard (verification of dimensional correctness and fitness for purpose may be sufficient), but the verification must be documented and the jig/fixture must be included in the facility's controlled equipment register.

First Article Inspection (FAI) and Calibration

First Article Inspection (FAI) is required under AS9100D and aerospace OEM requirements (e.g. AS9102) for new production of aerospace parts. All measurement tools used in FAI must have current calibration certificates at the time of the FAI. The FAI package submitted to the customer must include:

• A list of all measurement instruments used
• The calibration certificate number, calibration date, and expiry date for each instrument
• Confirmation that all instruments were within their calibration interval during the FAI measurement activities

Expired calibration certificates at the time of FAI measurement invalidate the FAI data, requiring re-measurement with calibrated tools. This is a common cause of FAI delays and customer rejection of FAI packages. Planning calibration renewals to ensure all instruments are within calibration well before planned FAI dates is an important part of production planning for AS9100D-certified suppliers.

Where FAI measurements are performed by a third-party inspection service (e.g. coordinate measuring machine services at an external metrology laboratory), the CMM calibration certificates and the calibration certificates for reference standards used by the CMM must also be included in the FAI package documentation.

Aerospace MRO Instrument Calibration Summary