Reliability Starts with One Skill: Set the Standards, Catch the Deviations

Following our previous article on the Reliability Cycle—where the continuous process of data collection, analysis, corrective action and decision-making was outlined—this article focuses on one of the essential elements of that cycle: identifying deviations from performance standards.

A Performance Standard is an operational goal defined by the operator as an acceptable level of reliability, e.g., number, ratio, or percentage. It may be calculated by the number of events occurring in a specified operating period, expressed in Flight Hours, Flight Cycles, Operating Hours, or calendar time. The Reliability Program should include procedures for the periodic review and adjustment of these standards based on in-service experience.

Deviations from these standards, if not promptly identified and addressed, can impact not only operational efficiency and fleet utilization but also safety. To manage such deviations, operators rely on structured reliability methods, which often combine alert-based, event-based, index-based, and trend monitoring programs.

Alert-Based Programs

Alert-based programs detect deviations from normal performance by comparing current data against historical patterns. An Alert Level is triggered when failure rates or operational findings exceed expected variation—serving as an early warning, not an airworthiness limit.

These programs rely on several categories of aircraft and component reliability metrics applied at aircraft, fleet, or subfleet level:

• Utilization Metrics – Show how the aircraft is being used (e.g., daily or monthly Flight Hours or Cycles). They help confirm that the aircraft operates within the utilization envelope assumed by the maintenance program.

• Dispatch & Operational Reliability Metrics – Measure how reliably the aircraft departs and completes flights. This includes Technical Dispatch Reliability (TDR) and Operational Reliability (OR), which also accounts for in-flight technical events. Variants exist for ETOPS/EDTO, RNAV, and major components such as engines and APUs. Supporting indicators include Operational Availability, Cancellation Rate, and Schedule Completion Rate.

• Capability Metrics (Special Operations) – Track performance in environments requiring enhanced capability, such as CAT III, RNAV, or EDTO/ETOPS. These metrics help identify degradations in operational readiness.

• Logbook and MEL Metrics – Monitor the volume and management of technical defects (PIREPs, MAREPs, CAREPs, etc.). Complementary MEL metrics—MEL Items Rate, Deferred Defects Rate, and MEL Usage—highlight trends in defect resolution and operational limitations.

• Component Reliability Metrics – Identify underperforming components using parameters such as MTBF, MTBUR, and Unscheduled Removal Rate.

Index-Based Programs

Index-based programs expand the alert-based approach by combining multiple data sources into a single performance index. For example, pilot, maintenance, cabin, and cargo reports, routine task findings, delays, cancellations, and MEL/CDL items. To ensure meaningful analysis, a common coding system—often based on ATA codes—is essential to correlate all reliability sources.

The resulting performance index ranks systems, components, or maintenance tasks, highlighting those with the worst performance. This ranking enables operators to prioritize investigations and corrective actions, ensuring that persistent reliability issues are addressed effectively. Index-based programs provide a comprehensive view, capturing interdependencies across systems and operational functions that might be overlooked in single-metric analyses.

Event-Based Programs

Event-based programs focus on individual technical or operational events with significant safety or operational impact. These include interruptions, such as diversions, and also rejected takeoffs, hard landings, or in-flight shutdowns. Because many of these events fall under mandatory occurrence reporting requirements, they must be documented, investigated, and addressed. Each event is tracked to facilitate investigation and corrective action, helping operators maintain compliance with safety standards while identifying potential reliability issues before they escalate. Metrics such as the In-Flight Shutdown (IFSD) Rate and Occurrence Report Rate are commonly used to quantify these deviations.

Trend Monitoring Programs

Trend monitoring programs complement alert-, index-, and event-based approaches by tracking the current performance of systems, structures, or capabilities and identify out of limit conditions or deterioration tendencies. For example, Flight Data Monitoring (FDM) / Flight Operations Quality Assurance (FOQA) programs, Engine Condition Monitoring and Oil Consumption Monitoring (ETOPS twin-engine), and Aircraft Health Monitoring.

Note: Trend monitoring is the main enabler of Condition-Based Maintenance, where maintenance actions are triggered by the actual condition of systems and components rather than fixed intervals. CBM represents the future of aircraft maintenance, supporting smarter, more efficient, and more proactive reliability strategies.

Conclusion

Deviations from performance standards are inevitable in complex aviation operations. Effective Reliability Programs—combining alert-based, index-based, event-based, and trend monitoring programs—provide operators with the tools to detect, analyze, and mitigate these deviations. By defining clear parameters, assigning alert levels, tracking component and operational performance, and consolidating multiple data sources for a comprehensive assessment, operators can maintain reliable operations.

Interested in learning more?

If you want to deepen your understanding of Reliability Programs and learn how to build, audit, or optimize them in real operations, explore our Reliability Programs Course — designed for aviation professionals seeking practical, industry-ready skills.