What is Repeatability?

Repeatability – Aviation and Metrology Glossary

Repeatability is a cornerstone concept in aviation, aerospace manufacturing, and measurement science (metrology). It defines a process’s ability to produce consistent results when the same measurement is repeated under identical conditions, same operator, equipment, location, method, and within a short timeframe. In safety-critical industries like aviation, repeatability is not optional: it underpins everything from flight instrument calibration and aircraft maintenance to regulatory compliance and quality control.

Formal Definition and Reference Standards

According to the International Vocabulary of Metrology (VIM, JCGM 200:2012), repeatability is:

Measurement repeatability: Precision under repeatability conditions of measurement.

Repeatability conditions: Same procedure, operator, measuring system, operating conditions, location, and repeated measurements on identical or similar items within a short time.

Aviation reference standards:

• ISO/IEC 17025:2017: Requires laboratories to demonstrate repeatability for accreditation.
• ICAO Doc 9839 / ICAO Annex 5: Defines measurement requirements for aviation, emphasizing repeatability for safety and traceability.
• FAA, EASA, and national authorities: Mandate repeatability in maintenance, calibration, and manufacturing quality systems.

In summary, repeatability is rigorously defined and enforced by industry and regulatory standards to guarantee measurement integrity in aviation and beyond.

Why is Repeatability Important?

Repeatability is crucial because it determines whether measurement data can be trusted, a non-negotiable in aviation, where errors can threaten safety or regulatory compliance.

Key reasons include:

• Safety & Compliance: Ensures maintenance, calibration, and operational checks meet FAA, EASA, and ICAO standards.
• Measurement Uncertainty: Forms the basis of uncertainty budgets, poor repeatability increases overall measurement uncertainty.
• Traceability: Required for calibration labs to claim traceability to national/international standards.
• Process Validation: Underpins process control (Six Sigma, SPC), defect detection, and continuous improvement.
• Efficient Troubleshooting: Helps identify if issues are due to human error, equipment instability, or external factors.
• Cost & Efficiency: Poor repeatability leads to rework, downtime, and regulatory audit failures.

Without high repeatability, every critical decision based on measurement, from maintenance releases to safety audits, is subject to doubt.

Repeatability vs Reproducibility, Precision, and Accuracy

These terms are related but distinct:

• Repeatability: Agreement under identical conditions.
• Reproducibility: Agreement under changed conditions (different operators, instruments, or labs).
• Precision: General closeness of repeated results (encompasses repeatability + reproducibility).
• Accuracy: Closeness to the true value, high repeatability does not guarantee accuracy if there is systematic error.

Example: A technician calibrates an altimeter, all readings are tightly clustered (repeatable) but offset from the true value (not accurate) due to a calibration error.

How to Measure Repeatability

A formal protocol, often required by ISO/IEC 17025 and aviation authorities:

1. Define the measurand (e.g., pressure, torque, weight).
2. Select a reference/test point (e.g., 1000 hPa for pitot-static calibration).
3. Document the method (use validated SOPs).
4. Use the same instrument (no recalibration or swapping).
5. Same operator for all repeats.
6. Control the environment (temperature, humidity, etc.).
7. Conduct measurements in a short timeframe.
8. Repeat the measurement 5–20 times (10+ preferred for statistical reliability).
9. Record results and calculate the mean and standard deviation.
10. Document/report for audits and quality systems.

Standard deviation (s) quantifies repeatability: [ s = \sqrt{\frac{1}{n-1} \sum_{i=1}^{n} (x_i - \overline{x})^2} ] A smaller ( s ) means higher repeatability.

Repeatability studies are a core part of Gage R&R, calibration certificates, and uncertainty budgets in aviation and laboratory environments.

Factors Affecting Repeatability

Mitigation: Strict SOPs, environmental control, equipment maintenance, operator training, and validated procedures.

Practical Examples

1. Pitot-Static Tester Calibration
A technician applies 1000 hPa five times: 1000.2, 1000.1, 1000.1, 1000.2, 1000.2 hPa, small spread shows high repeatability.

2. Aircraft Weighing
Five measurements: 12,345, 12,346, 12,344, 12,345, 12,345 kg. Low standard deviation confirms repeatable weighing.

3. Runway Friction Testing
Friction coefficients: 0.62, 0.61, 0.62, 0.61, 0.62, indicates repeatable runway measurement for safe aircraft operations.

4. Temperature Sensor Calibration
Readings: 0.001, -0.001, 0.000, 0.001, 0.000°C in a controlled bath, demonstrates excellent repeatability.

Industry Use Cases and Applications

Repeatability is not just a technical term, it is a daily operational reality in aviation and metrology. It is the reason why aircraft can fly safely, maintenance is trusted, and regulatory bodies can certify operations and products. Ensuring high repeatability means every measurement, maintenance action, and calibration is a step towards safer skies.

References

• International Vocabulary of Metrology (VIM), JCGM 200:2012
• ISO/IEC 17025:2017 – General requirements for the competence of testing and calibration laboratories
• ICAO Doc 9839 – Manual on the Use of the Aeronautical Mobile Service
• FAA Order 8110.4C – Type Certification
• EASA Part 145 – Maintenance Organisation Approvals

For further guidance on improving repeatability in your aviation or laboratory processes, contact our experts or schedule a demonstration .