Maintenance program interval extension after repeated findings
An approved maintenance organisation holds RBAC 145 approval for line and base maintenance on a turboprop type. Over 18 months, the operator's reliability program shows a rising rate of unscheduled removals of a particular hydraulic pump, from 0.8 to 2.4 per 1000 flight hours, while the scheduled replacement interval is 3000 hours. The operator proposes to extend the interval to 4000 hours to reduce maintenance burden, citing that the pump is 'on-condition monitored'. The maintenance organisation supports the proposal because the pump manufacturer issued a service letter recommending 4000 hours, based on bench testing. The ANAC inspector notes that the reliability data has not been analysed for the effect of the extension, that the last two pump failures caused in-flight system degradation, and that the operator's maintenance program revision has not been submitted for approval. The operator argues that the manufacturer's service letter is sufficient justification.
1. Evaluate whether the manufacturer's service letter is sufficient justification for the interval extension under the applicable RBAC requirements.
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It is not sufficient on its own. A manufacturer service letter is supporting data, not an approval. The maintenance program is approved by ANAC for that operator, and any change to a scheduled task interval affecting airworthiness must go through the program revision and approval process.
The operator must show that the extension is justified for its own fleet and operating environment. That requires analysis of its reliability data, not bench test data alone. The rising removal rate is a red flag: extending the interval while the removal rate is increasing would likely increase exposure to in-flight failures. The two in-flight degradation events are occurrences that must be considered in the analysis and may require reporting.
The correct path is a formal reliability analysis, a risk assessment, a proposed program revision with justification, and ANAC approval before implementation. The manufacturer letter can be cited as one input but cannot substitute for operator-specific data and regulatory approval.
2. What reliability analysis should the operator perform, and what result would support or defeat the proposal?
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The analysis should include:
- Removal rate trend by pump part number and by aircraft, with confidence intervals, not just a single average.
- Failure mode classification: distinguish benign removals from failures causing system degradation.
- Hazard assessment for the in-flight degradation events, including whether the remaining system redundancy was adequate.
- Comparison with the manufacturer's bench test assumptions, especially duty cycle and environmental factors.
- A projection of expected failures at 4000 hours using the observed failure distribution.
A result supporting the extension would show a stable or declining removal rate, no hazardous failure modes, and a projected failure probability within the operator's safety criteria. A result defeating it would show an increasing hazard rate with age, any failure mode with catastrophic potential, or insufficient data to estimate the risk. Here the trend and the two degradation events point against approval without further mitigation.
3. The organisation supports the proposal. What does this reveal about the maintenance organisation's obligations, and what should the inspector do?
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The maintenance organisation has an obligation not to support a change that is not substantiated, and to raise safety concerns through its own safety management system. Endorsing the extension because the manufacturer suggested it, without challenging the reliability data, suggests a weak safety culture and inadequate technical review.
The inspector should:
- Reject the program revision as submitted and require a proper analysis.
- Record a finding against the maintenance organisation's technical review and SMS processes, since it endorsed an unsubstantiated change.
- Check whether the organisation has a process for assessing manufacturer recommendations against operator data.
- Verify that the two in-flight events were reported and investigated.
- Consider whether the organisation's approval scope or audit frequency needs review.
The broader lesson is that approval status does not transfer responsibility: the organisation must exercise independent technical judgement.