General aspects of the calibration of test and measuring equipment in Part-145

Many of us know the feeling when we step on the bathroom scale at home and look hesitantly down and check if the correct number will appear. Many of us will be disappointed, especially if the number is higher than we anticipate. Sometimes, however, we seem to be "lucky", since the number has inexplicably dropped.

Chances are that the bathroom scale is one of the most used measuring devices of the public. It is also one of the least calibrated measuring devices and the results are often very unreliable - even deviating by several kilograms.

Therefore, it is clear that the equipment used to ascertain whether a particular procedure has been followed has to be of much higher quality than our scales. There is a need to monitor the reliability of the results produced by the measuring devices every time and ensure that its equipment function as they should.

When necessary to ensure meaningful results, the measuring equipment shall be calibrated at specified intervals, or before use as a minimum. Calibration is achieved by direct comparison with measurement standards, which can be traced to national measurement standards. Where no such measurement standards exists the basis used for calibration or verification shall be recorded.
 

National measurement standards are measurement standards which are intended to be a benchmark for measurements in a particular country and the Consumer Agency plays a statutory role to provide and maintain the Icelandic national measurement standards, see detailed definition in Act No. 100/1992 on Weighing, Measurements and Accreditation. Note that this Article does not provide for surveying or map-making.

If measuring equipment is used to verify compliance with the requirements of customers or producers, it is necessary to consider how it is calibrated, controlled, stored, used, and how accuracy is maintained.

Calibration is the process of comparing the measuring equipment of the company with a reference measurement base to determine how accurate the measuring equipment is and whether it meets the required accuracy of the measurements it is used for.
Calibration should be carried out at regular intervals that could be governed by:

When scales are calibrated, weights weighing 1 kg are typically used. Manufacturer's instructions on the precision of the weights should be respected to ensure that the accuracy of the scales remains within specified limits.
Normal calibration of electronic scales may involve the placing of the weight on five predetermined spots on the platform of the scales: in each of the four corners and then as close to the centre as possible. If the scales are able to cope with these locations within the given margin of error of the instrument, the scales are considered recalibrated and fit for use.

To ensure that the weight is really 1 kg within the margin of error provided for in the certificate, it is necessary to let the calibration laboratory monitor the weight for an extended time. In Iceland, this calibration laboratory is the Consumer Agency, calibration division or equivalent foreign party that can ensure the traceability of the weight to the reference measurement base.

For the reference measurement base to be valid, it must be traceable to an authorized source. Usually, a national measuring base is used, which is then traceable to an international measurement base. As an example, the Consumer Agency is a holder of weights that form the Icelandic national measuring base for mass.
These weights are traceable to Paris, where the “prototype kilogram” is stored under defined conditions. Thus, it is ensured that the scales calibrated in Iceland and scales calibrated elsewhere in the world are all “striving” to imitate the “prototype kilogram” located in Paris.

The same procedure applies to other measurements: the company is responsible for calibrating their own measuring equipment and ensure that the measurement standards it uses are traceable to national measurement standards by only doing business with “authorised calibration laboratories”.

What is a “recognized calibration laboratory”. To ensure credibility, accuracy, impartiality and discipline in all procedures, a system for accreditation has been established for the various parties that are responsible for ensuring that products and services are in fact as specified. Accreditation involves an authorised party formally recognising a body or person as competent to carry out specific tasks.

Accreditation activities are generally based on the series of standards ÍST EN 45000, ÍST EN ISO/IEC 17025 and ÍST ISO/IEC Guide 66.

Accredited calibration laboratories shall register their accreditation in their country in accordance with a relevant international standard. In Iceland, the standard is: ÍST EN ISO/IEC 17025: 2000 General requirements for the competence of testing and calibration laboratories.

Accredited calibration laboratories registered in another European state have the right to provide their services in Iceland.

No, it is not necessary at all to calibrate all measuring equipment. If the measuring equipment is solely to be used to obtain evidence, it does not need to be calibrated. Our bathroom scale is a good example. However, in order to show compliance with requirements, the equipment needs to be calibrated. Thus, various measuring equipment can be a part of the process of an undertaking only to indicate whether it is working close to the desired values.

Within maintenance centres which have achieved EASA Part-145 approval, there are strict requirements for calibration of equipment, tools and materials (see Annex A and B).

Notice particularly what is specified in: AMC 145.B.50 (a) Findings

- If the calibration control of equipment as specified in 145.A.40 (b) had previously broken down on a particular type product line such that most “calibrated” equipment was suspect from that time then that would be a level 1 finding.

 If calibration management has completely failed, a level 1 finding is considered the only possible outcome. This is a very strict judgement.

 It is stated unequivocally in Part-145 that the company must ensure that all tools, equipment and particularly test equipment are controlled and calibrated according to an “officially recognised standard”. Where reference is made in this way to an “officially recognised standard”, a reference is being made to recognized measurement standards, such as the national measurement standard, with traceability to international measurement standards.

Accuracy of measuring equipment depends on the level of tolerance allowed with regard to the measured item.

The equipment used must usually be more accurate than the object or device being measured. Unnecessarily high accuracy in turn serves little purpose. Equipment used for machines may be accurate to one-tenth of a millimetre (+/- 0.1 mm).

 Among other things, calibration serves the purpose of tracking and correcting the measurement equipment on a regular basis. Daily use of measuring equipment, tools and utilities which have built-in measurement capabilities can easily undermine the accuracy of the measurement with time or usage. Poor treatment and accidents in the handling of sensitive equipment may impair measurement accuracy and speed. For example, a torque wrench that fell on the floor may be totally useless afterwards. Anyone can see the danger of using such a torque wrench in aircraft maintenance and repair!

For good practice in the management of calibration of measuring equipment, devices and tools, it is advisable to study the requirements of ÍST ISO 9001:2000 Quality management systems - Requirements, Section 7.6 Control of monitoring and measuring equipment.

In case it is necessary to go further and take a very scientific approach regarding this project, it is advisable to view ÍST ISO 10012:2003 Measurement management systems - Requirements for measurement processes and measuring equipment.

In the majority of cases it should be sufficient to address calibration issues as specified in Section 7.6 of ISO 9001. To help undertakings meet the requirements for calibration, here are some questions that the company should respond to and address in its quality system:

(b) The organisation shall ensure that all tools, equipment and particularly test equipment, as appropriate, are controlled and calibrated according to an officially recognised standard at a frequency to ensure serviceability and accuracy.

Records of such calibrations and traceability to the standard used shall be kept by the organisation.

AMC 145.A applies to the company
AMC 145.B applies to authorities

AMC 145.A.40(a) Equipment, tools and material

Once the applicant for approval has determined the intended scope of approval for consideration by the competent authority, it will be necessary to show that all tools and equipment as specified in the maintenance data can be made available when needed. All such tools and equipment that require to be controlled in terms of servicing or calibration by virtue of being necessary to measure specified dimensions and torque figures etc, should be clearly identified and listed in a control register including any personal tools and equipment that the organisation agrees can be used.

AMC 145.A.40(b) Equipment, tools and material

AMC 145.B.50 (a) Findings

In practical terms a level 1 finding is where a competent authority finds a significant non-compliance with Part-145. The following are example level 1 findings:

Note: A complete product line is defined as all the aircraft, engine or component of a particular type.

ISO 6789:2003 Assembly tools for screws and nuts -- Hand torque tools -- Requirements and test methods for design conformance testing, quality conformance testing and recalibration procedure.