Audit sampling in a large line maintenance station
An ANAC surveillance team conducts a scheduled audit of a line maintenance station at a busy airport. The station performs transit checks, daily checks, and defect rectification for three operators, with 42 technicians across three shifts. The station's records show 1200 task cards completed in the last quarter. The audit plan allocates two days and a sample of 25 task cards, selected by the station manager in advance. The team finds all 25 cards correctly signed. However, one auditor notices that the torque wrench calibration stickers on two wrenches show expiry dates three months past, and that the tool control log does not record which technician used which wrench. The station manager says the wrenches are 'rarely used' and that the sample proves the records are good. The team also notes that the last internal audit was 14 months ago, although the organisation's procedure requires annual internal audits.
1. Critique the audit sampling approach and explain how the sample selection affects the validity of the conclusion.
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The sample is invalid as evidence of conformity because the station manager selected it. That introduces selection bias: the manager can choose cards known to be complete. An audit sample must be selected independently by the auditor, using a documented method such as random or stratified sampling across shifts, operators, task types, and technicians.
With 1200 cards, a sample of 25 is small but can be workable if randomly selected and if the auditor also uses other evidence. The auditor should stratify by shift and operator, and should include high-risk tasks such as torque-critical work and defect rectification. The finding that all 25 were correct therefore says little; it may simply reflect the selection.
The correct response is to re-draw the sample independently, document the method, and expand sampling in areas where the tool calibration finding indicates systemic risk. The audit conclusion should be based on the auditor's own sample plus interviews and observation, not on a manager-provided set.
2. How should the auditor treat the calibration and tool control findings relative to the record findings?
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The calibration finding is more serious than a paperwork gap. A torque wrench used past calibration expiry means any torque value applied with it is unverified, so the affected tasks must be identified and assessed for rework. The tool control log gap means the organisation cannot trace which tasks used the out-of-calibration wrenches, which compounds the problem.
The auditor should:
- Identify all tasks performed with those wrenches since expiry, using work orders and shift records.
- Assess whether re-torque or inspection is needed, and require the organisation to do it.
- Raise a finding against the tool calibration and control process, not just the individual wrenches.
- Treat the record sample as inconclusive rather than as evidence of conformity.
The 'rarely used' argument is irrelevant: calibration status is binary, and use frequency does not cure an expired calibration.
3. The internal audit is 14 months overdue against an annual requirement. What does this indicate, and how should the surveillance team respond?
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It indicates a breakdown in the organisation's own quality system, which is a more significant finding than a single overdue audit. The internal audit program is a required element of the maintenance organisation's quality system, and its failure means the organisation is not detecting its own nonconformities.
The team should:
- Raise a finding against the internal audit program and require root cause analysis.
- Check whether other stations or processes are similarly overdue.
- Review the last internal audit report for quality and follow-up of previous findings.
- Consider increased surveillance frequency until the quality system is demonstrated to work.
The combination of manager-selected samples, expired calibration, and an overdue internal audit points to a weak quality culture, so the response should address the system, not only the individual defects.