How to gain meaningful understanding in the reliability of components installed in the aircraft?
Aircraft component reliability is an important underlying contributor to both dispatch reliability and aircraft systems reliability. Individual parts and components ultimately make up the aircraft, and understanding how those components perform in service can help Reliability Engineers identify potential reliability improvement actions and support greater aircraft and fleet availability.
Unscheduled component removals are an important part of that analysis. A removal event alone tells an engineer that a component has left the aircraft, but it does not necessarily explain whether the event represents normal behaviour or an emerging reliability issue. Meaningful component reliability analysis requires the removal to be understood in the context of component utilisation, maintenance history, aircraft configuration and comparable performance across the fleet.
Why Unscheduled Component Removals Matter
The reliability of an individual aircraft component is closely related to the amount of time or number of cycles it remains installed before removal. A component may be removed because a scheduled maintenance action is required or because an unexpected failure or performance issue requires replacement.
For Reliability Engineers, unscheduled removals are particularly valuable because recurring or abnormal removal behaviour can indicate where further technical investigation may be required. However, simply counting the number of removals is rarely sufficient. A component operating on a high-utilisation aircraft may naturally accumulate more events than the same component installed on an aircraft flying fewer hours or cycles.
The objective is therefore to understand component removals in their operational context and determine whether actual in-service behaviour differs meaningfully from expected performance.
Identifying and Tracking Serialized Aircraft Components
Within aviation, parts installed on an aircraft are identified using a Part Number (PN) and, where applicable, a Serial Number (SN). The part number identifies the generic type of part, while the serial number identifies an individual serialized unit within that part number.
For example, if 0851HT1 is the part number for a pitot probe and 23049 identifies a specific serialized unit, the complete component identification allows that individual unit to be followed throughout its operational life.
Tracking individual serial numbers makes it possible to monitor the accumulation of flying hours and cycles on specific components and to maintain the component history required for maintenance and airworthiness purposes.
Important life and utilisation parameters may include:
Time Since New (TSN): the total flying hours accumulated by a serialized component since production.
Cycles Since New (CSN): the total cycles accumulated by a serialized component since production.
Time Since Installation (TSI): the flying hours accumulated since the component was installed on its current aircraft.
Cycles Since Installation (CSI): the cycles accumulated since installation on the current aircraft.
Time Since Overhaul (TSO): the flying hours accumulated since the component was overhauled in a repair shop.
Cycles Since Overhaul (CSO): the cycles accumulated since overhaul.
These parameters give Reliability Engineers the context required to evaluate component performance against its actual operational exposure rather than relying on removal counts alone.
What MTBUR Tells You About Component Reliability
Another important measure is Mean Time Between Unit Removal (MTBUR), which represents the average operating time, expressed in hours or cycles, between the installation and subsequent removal of a generic part.
Aircraft components may have an expected or documented MTBUR that provides a reference for anticipated in-service performance. The longer a component can remain installed without requiring removal, the lower the potential maintenance burden associated with that component and the smaller its contribution to aircraft downtime.
Airlines can therefore monitor the actual MTBUR achieved by components in service and compare this with documented or expected performance. A lower-than-expected MTBUR may indicate that further investigation is required into areas such as component design, material performance, maintenance practices or the operational environment.
The value of MTBUR increases when it is analysed across multiple dimensions. Comparing performance by aircraft registration, part number, serial number, installation position or configuration can help determine whether a deviation is isolated to an individual unit or forms part of a broader reliability trend.
What Data Is Needed to Analyse Unscheduled Component Removals?
Reliable analysis depends on having a complete history around each installation and removal event. Reliability Engineers need to understand which part and serial number was installed, on which aircraft and position, when the installation and removal took place, and how many hours or cycles accumulated during that period.
The analysis may also require the reason for removal, whether the event was scheduled or unscheduled, the relevant aircraft utilisation, component life parameters and associated maintenance history. Where components have been overhauled or repaired, the corresponding TSO and CSO information may also be relevant.
This information makes it possible to move beyond a simple list of removed components and establish the actual time between installations and removals. Combined with average or projected aircraft utilisation, historical time-between-installation data can also help engineering teams understand the likely in-service duration of a particular part number.
The quality of the underlying information matters. Incomplete installation dates, inconsistent aircraft utilisation or missing component position information can directly affect the reliability indicators engineers are attempting to interpret.
How to Analyse Unscheduled Component Removals
A practical component reliability analysis typically begins by identifying components with the highest frequency of unscheduled removals or behaviour that deviates from comparable parts elsewhere in the fleet.
The next step is to place those events against component and aircraft utilisation. Raw removal counts can be misleading when aircraft operate at different utilisation levels, so measures such as MTBUR provide a more meaningful basis for comparison.
Engineers can then investigate the data by part number, individual serial number, aircraft registration, installation position or configuration. This allows them to determine whether the observed behaviour is associated with a generic component type, a particular serialized unit, one aircraft or a specific installation environment.
Actual component MTBUR can also be compared with documented or expected MTBUR to identify deviations that may justify further engineering investigation.
The purpose of this analysis is not simply to calculate another reliability indicator. It is to identify where component performance differs from expectations and provide engineers with enough operational context to determine what should be investigated next.
How Should Component Removals and Tracked Hours and Cycles Be Maintained?
Keeping track of serialized parts installed on an aircraft forms an important part of maintaining the aircraft's configuration status. Modern M&E and MRO systems typically support the tracking of serialized components installed on individual aircraft, while operators must also maintain defined processes for recording component removals and installations.
A component removal or installation is itself a maintenance action and therefore needs to remain connected to the corresponding maintenance record and Aircraft Technical Logbook information.
For reliability analysis, maintaining this history consistently is particularly important. If engineers cannot confidently identify when a serialized component was installed, when it was removed and how much utilisation it accumulated between those events, MTBUR and related component performance analysis become significantly more difficult to interpret.
Why Component Assemblies Matter in Reliability Analysis
Individual components should not always be analysed in isolation. Many parts installed on an aircraft belong to a larger component assembly. A brake, for example, forms part of a wheel assembly, while the wheel assembly itself sits within the broader landing gear system.
Maintaining the relationship between parts, serial numbers and their parent assemblies can support more targeted technical investigations. A reliability issue may not be associated with a generic part number across the entire fleet. It may instead appear only when a particular combination of components, serial numbers or assembly configurations is present.
Preserving the aircraft's component hierarchy therefore provides additional context when engineers investigate abnormal removal behaviour.
Why Installation Position Can Reveal Reliability Trends
The position in which a component is installed can provide another important analytical dimension.
If identical components operate in multiple positions on an aircraft, comparing reliability performance by position code may reveal trends that would remain hidden in an overall part-number analysis. A component that repeatedly performs differently in one installation position may point towards a position-dependent operating condition, configuration issue or another factor requiring investigation.
Tracking position information alongside component history allows Reliability Engineers to move from identifying that a reliability issue exists to narrowing down where that issue may originate.
When Component Reliability Analysis Becomes an Information Challenge
The technical calculations behind component reliability are only as reliable as the operational information supporting them.
Installation and removal histories may exist in the M&E system, aircraft utilisation may be maintained elsewhere, component repairs may involve external organisations, and relevant defect or maintenance history may need to be connected to the component record before an engineer can fully understand the event.
As this information moves across systems and engineering processes, small inconsistencies can create significant additional validation work. A missing installation date, unclear removal classification or incomplete position history can affect the interpretation of MTBUR and other component reliability indicators.
This is why meaningful component reliability analysis depends not only on collecting events and statistics, but also on maintaining the continuity, consistency and traceability of the operational information behind them.
From Component Reliability Metrics to Engineering Action
Reliability analysis creates value when the information leads to engineering action.
A declining MTBUR, repeated unscheduled removals or abnormal behaviour associated with a particular aircraft, serial number or installation position should become the starting point for deeper investigation. Reliability Engineers can then review the relevant maintenance history, defect information, repair records, configuration and operating context to understand what may be driving the observed performance.
Depending on the findings, the response may involve closer monitoring, further technical investigation, changes to maintenance practices or another appropriate engineering action.
The objective is to move beyond reporting what happened and establish enough context to understand why it may be happening and where engineering attention is required.
Supporting Reliability Analysis with EXSYN
EXSYN's Reliability Analysis App is designed to help Reliability Engineers move from raw operational and maintenance data to actionable fleet and component reliability insight.
The application supports reliability teams in identifying trends, investigating recurring issues and analysing performance across fleets, systems and components. By connecting reliability analysis with the organisation's existing M&E environment, engineering teams can spend less time manually preparing and reconciling information and more time investigating the reliability patterns that require attention.
For component reliability, the objective is to make deviations easier to identify, provide engineers with the context required for root-cause investigation and support faster, data-based maintenance decisions.
Frequently Asked Questions About Unscheduled Component Removals and Aircraft Reliability
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An unscheduled component removal occurs when a component is removed from an aircraft before its planned maintenance or replacement interval, typically because of a defect, failure or performance concern. Monitoring these removals helps Reliability Engineers identify components that may be underperforming and investigate recurring reliability issues across the fleet.
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MTBUR stands for Mean Time Between Unscheduled Removals. It measures the average operating time between unplanned removals of a component and is commonly used to evaluate component reliability. A declining or unusually low MTBUR can indicate that a component requires further investigation, particularly when compared across aircraft, positions or operating conditions.
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Airlines track component reliability by combining component installation and removal history with aircraft utilisation, maintenance records and operational data. Reliability Engineers can then analyse measures such as unscheduled removal frequency and MTBUR, compare performance across aircraft or installation positions, and identify recurring patterns that may require engineering action.