Singapore is Southeast Asia's premier aerospace MRO hub — SIA Engineering and ST Engineering together overhaul hundreds of aircraft annually. In aviation maintenance, a calibration certificate isn't a formality. It's the documented chain of evidence that a measurement was trustworthy enough to return an aircraft to flight. Here's what MRO calibration programmes actually require.
Singapore occupies a unique position in the global aerospace maintenance, repair, and overhaul (MRO) industry. The island hosts two of the world's largest MRO organisations: SIA Engineering Company (SIAEC), the maintenance arm of Singapore Airlines, and ST Engineering Aerospace (formerly ST Aerospace), which serves over 100 airline customers globally. Together with dozens of specialist component overhaul shops, line maintenance providers, and engine test facilities, Singapore's MRO sector generates over SGD 4 billion annually and employs thousands of licensed aircraft engineers.
In aviation maintenance, the measurement environment is uniquely demanding. An aircraft component cleared for return to service based on inaccurate measurements is a potential airworthiness hazard. Unlike manufacturing, where a non-conforming part can be reworked or scrapped, a non-conforming maintenance action can only be discovered after the aircraft is in service — sometimes at altitude. This is why the calibration traceability requirements for aerospace MRO test equipment Singapore operations are among the most rigorous of any industry.
Key Stat
AS9110C Clause 7.1.5.2 requires that measuring equipment be calibrated at specified intervals against measurement standards traceable to international or national measurement standards. When no such standards exist, the basis for calibration must be documented. In CAAS-approved AMOs, this requirement is enforced through audit and surveillance, with calibration programme weaknesses routinely cited as findings in CAA inspections globally.
The Civil Aviation Authority of Singapore (CAAS) approves and regulates Approved Maintenance Organisations (AMOs) in Singapore under the Air Navigation Order (ANO) and associated regulations. CAAS's AMO approval requirements are aligned with ICAO Annex 6 and the European EASA Part-145 standard — reflecting Singapore's role as a hub for aircraft from carriers registered in multiple countries.
An AMO approval from CAAS requires the organisation to have a documented quality management system covering, among many other requirements, the control of measuring and test equipment. CAAS inspectors review calibration records, calibration procedures, and out-of-calibration handling processes during initial approval audits and regular surveillance audits. A weak calibration programme — gaps in calibration records, instruments in service with lapsed calibration, or missing traceability documentation — is a major finding that can jeopardise AMO approval.
Beyond CAAS regulatory requirements, Singapore's major MRO organisations hold AS9100D and/or AS9110C certification — the IAQG (International Aerospace Quality Group) quality management system standards for the aerospace industry. AS9110C, specifically designed for AMOs, has detailed requirements for the measurement and monitoring equipment programme.
Key AS9110C requirements include: calibration at defined intervals using measurement standards traceable to national or international standards; calibration status labelling on all controlled instruments (typically showing calibration due date and certificate number); documented procedures for what happens when an instrument is found out of calibration; records of calibration results maintained for the life of the programme; and software validation for computer-based measurement systems.
Traceability in metrology means that a measurement result can be related to a stated reference — ultimately to SI units (the international system of units) — through an unbroken chain of calibrations, each with documented uncertainty. For an MRO shop in Singapore, this chain typically looks like:
This chain is what allows a SINGLAS-accredited calibration certificate from Unitest's calibration laboratory to satisfy CAAS and AS9110C traceability requirements. The SINGLAS logo on a calibration certificate is the documented evidence that the accreditation chain has been verified by the Singapore Accreditation Council.
Pro Tip
When filing calibration certificates in your MRO quality system, maintain a traceability map — a document showing how each reference standard in your lab traces back to NMC or another NMI. This document is what CAAS and AS9110 auditors want to see when they assess your calibration programme. Without it, you have certificates but no demonstrated chain.
Aircraft electrical systems are complex: multiple AC bus systems (typically 115/200 V AC, 400 Hz for larger commercial aircraft), DC bus systems (28 V DC), and the extensive wiring harnesses connecting them. Avionics boxes (Line Replaceable Units, or LRUs) are tested for electrical performance using automated test equipment (ATE) or manual test procedures using calibrated multimeters, oscilloscopes, and signal generators.
Wiring harness testing uses insulation resistance testers to verify that wire insulation has not been compromised — critical after repairs involving drilling, sawing, or other work near wire bundles. Continuity testing of harness connectors uses low-resistance ohmmeters. Arc fault detection system testing uses specialised test equipment that must be calibrated to the manufacturer's specifications.
The Fluke Calibration range includes reference instruments and calibrators specifically designed for avionics and aerospace test applications. Fluke's calibration instruments are used in aerospace metrology laboratories worldwide, providing the accuracy and documentation required by AS9110 calibration programmes.
Torque wrenches are critical in aircraft maintenance — every fastener in an aircraft has a defined torque specification, and under- or over-torquing can cause structural failure or loosening during operation. Torque wrench calibration is a core requirement in every AMO. Calibration is typically performed at 12-month intervals or more frequently if the wrench is subjected to heavy use or dropped.
Aircraft hydraulic systems operate at high pressures — typically 3,000 psi (20,684 kPa) on older commercial aircraft, 5,000 psi on newer designs like the Boeing 787. Hydraulic test equipment — pressure test rigs, pressure gauges, and flow meters — must be calibrated to verify that hydraulic system pressure is correctly set during maintenance and that hydraulic actuators and valves operate within specification.
Watch Out
Aviation-specific automatic test equipment (ATE) often has internal calibration procedures that adjust the test set against its own internal reference. This is not a substitute for traceable external calibration. The internal reference itself must be verified against an external traceable standard at defined intervals. ATE that appears to be "self-calibrating" still requires inclusion in the AMO's external calibration programme.
Every AS9110-certified MRO will, at some point, discover an instrument that is out of calibration — either found expired or found to have failed its calibration check. What happens next is what separates a mature quality management system from a paper programme.
The out-of-calibration process requires: immediate quarantine of the instrument; determination of when it was last known to be in calibration (the recall date); review of all work performed using that instrument during the recall period; assessment of whether any measurements may have been affected and whether aircraft released to service may be impacted; and, if impact on aircraft airworthiness cannot be excluded, notification to the aircraft operator and potentially to CAAS under the occurrence reporting system.
This is why calibration interval discipline matters so much in MRO. A calibration that has slipped by one month is unlikely to have a different technical impact than if it had been done on time — but the fact that it was allowed to lapse is itself a quality system failure, and the out-of-calibration process may still require an expensive recall review.
Unitest's SAC-SINGLAS accredited calibration laboratory provides calibration services for electrical and electronic test instruments used in Singapore's aerospace MRO sector. Our calibration certificates cite the measurement standards used, the calibration results, and the measurement uncertainty — all required elements under AS9110C and CAAS AMO standards.
Explore our calibrators and the Fluke Calibration range of reference instruments used in aerospace metrology applications. Contact Unitest to discuss calibration scheduling for your AMO quality management system, and to explore how our SINGLAS-accredited services fit into your traceability chain.
Singapore's aerospace MRO sector is built on the principle that every measurement made during aircraft maintenance must be trustworthy — traceable, documented, and performed with calibrated instruments. Aerospace MRO test equipment Singapore operations that invest in rigorous calibration programmes aren't just satisfying CAAS and AS9110 auditors. They're protecting the passengers on the aircraft their work maintains. In aviation, measurement quality is safety — and there is no higher standard.
What calibration traceability is required for aerospace MRO test equipment in Singapore?
CAAS (Civil Aviation Authority of Singapore) and international standards AS9100D and AS9110C require that measuring equipment used in aircraft maintenance be calibrated with results traceable to international measurement standards — specifically to national measurement institutes (NMIs) such as NIST (USA), NPL (UK), PTB (Germany), or Singapore's A*STAR NMC (National Metrology Centre). The calibration chain must be documented: each calibration certificate references the standard used, whose traceability back to an NMI is documented in its own certificate.
What is AS9110 and how does it differ from AS9100 for aerospace MRO shops?
AS9100 is the aerospace quality management system standard for the broader aerospace industry (design, manufacturing, and aftermarket). AS9110 is the specific quality management system standard for Aviation Maintenance Organizations (AMOs) — repair and overhaul shops. AS9110C (the current revision) has specific requirements for control of monitoring and measuring equipment, including calibration with traceable certificates, labelling of calibration status on instruments, and documented procedures for handling out-of-calibration discoveries. CAAS-approved AMOs in Singapore are required to implement AS9110-equivalent quality systems.
What types of test equipment are used in aerospace MRO and need calibration?
Aerospace MRO test equipment requiring calibration includes: multimeters and clamp meters for avionics and electrical system testing; torque wrenches (calibrated at defined intervals); pressure gauges for hydraulic and pneumatic system testing; vibration analysers for engine health monitoring; oscilloscopes for avionics fault diagnosis; temperature calibrators for engine thermocouple testing; insulation resistance testers for aircraft wiring harness checks; and specialised avionic test sets (ATE — Automatic Test Equipment) for LRU testing. All must have calibration records traceable to national standards.
What happens if an aerospace MRO finds a test instrument was out of calibration?
An out-of-calibration discovery in an aerospace MRO triggers a mandatory non-conformance process under AS9110. The MRO must: quarantine the out-of-calibration instrument; assess all work performed using that instrument since the last known good calibration (the 'recall period'); determine whether any aircraft released to service may have been affected by inaccurate measurements; and if product risk cannot be ruled out, notify the aircraft operator and potentially the relevant CAA (CAAS or the aircraft's state of registry). This can trigger airworthiness directives for the affected aircraft. The consequences are severe, which is why calibration programme discipline is treated as safety-critical in MRO.
Does Unitest provide SINGLAS-accredited calibration for aerospace MRO test equipment?
Yes. Unitest's SAC-SINGLAS accredited calibration laboratory provides calibration services for a range of electrical and electronic test instruments used in aerospace MRO environments. SINGLAS accreditation means our calibration certificates are traceable to international standards through A*STAR NMC, and are recognised under ILAC MRA, satisfying CAAS and AS9110 traceability requirements. Contact us to discuss calibration programmes for your MRO quality management system.
Need expert advice or a quote?
Singapore's authorised Fluke, Rotronic & Amprobe distributor — same-day response.
