The Hidden Risk: Untracked "REMOVABLE STRUCTURAL COMPONENT (RSC)" in Aircraft

Straight to the point:

Many structural components from the aircraft during its service may be replaced with new or used components such as, but not limited to, door assemblies, wing slats and wing flaps, flap-track beam, winglets, engine mounts. These structural components are called “Removable Structural Component (RSC)”.

Why should we care?

Well, the aircraft utilization is monitored in Flight Hours (FH) and Flight Cycles (FC). If an RSC from one aircraft is removed and re-installed on different aircraft, the utilization (FH/FC) profile of the replaced RSC would vary from the actual utilization of aircraft, making it difficult to determine the component’s age (or total FH / FC).

The tracking of RSC’s age is extremely important as many Airworthiness Limitations (ALS or ALI) and Airworthiness Directives (ADs) are sometimes "component-specific". Furthermore, RSCs may contain Fatigue-Critical Structure or may be subject to Fatigue requirements during repairs and modifications to RSCs.

Example: if a “slat” is removed from a new A320 (MSN: ABC) aircraft, having ZERO FH / FC, and is installed on another aircraft A320 (MSN: XYZ) which has 40,000 FH / 20,000 FC. There will be difference between the aircraft and installed slat’s age (hence, actual life of components may not be tracked correctly).

What should be done?

Aircraft Operators / Continuing Airworthiness Management Organization (CAMO) should develop and implement a system for tracking and control of RSC either at:

1. aircraft level (when RSC follows the life of the aircraft); OR
2. at component level (when an RSC follows a different life than the aircraft it was initially installed on)

The implementation of this system will help ensure compliance with the Maintenance Program requirements specific to repairs and modifications installed on an affected RSC.

What if the actual age of RSC could not be determined?

As per EASA AMC 20-20B and FAA AC 120-93:

If the actual age of RSC cannot be determined, the component’s age may be conservatively assigned using one of the following fleet leader concepts, depending upon the origin of the component:

1. If component times are not available, but records indicate that no part changes have occurred, aircraft flight cycles or flight hours can be used.
2. If no records are available, and the parts could have been switched from one or more older aircraft under the same maintenance programme, it should be assumed that the time on any component is equal to the oldest aircraft in the programme. If this is unknown, the time should be assumed to be equal to the same model aircraft that is the oldest or has the most flight cycles or flight hours in the world fleet.
3. A manufacturing date marked on a component may also be used to help establish the component’s age in flight cycles or flight hours. This can be done by using the above reasoning and comparing it to aircraft in the affected fleet with the same or older manufacturing date.

If none of these options can be used to determine or assign a component age or total number of flight cycles or flight hours, a conservative implementation schedule can be established for the initial inspection, if required by the Damage-Tolerance data.

Role of Regulatory Authorities and Aircraft OEMs:

EASA and FAA have published means of compliance and guidelines related to the tracking and controlling of RSCs (EASA AMC 20-20B & FAA AC 120-93). Similarly, aircraft manufacturers like Airbus and Boeing have also identified the baseline RSC list for tracking and control. Moreover, the “Airworthiness Assurance Working Group (AAWG)” together with the “Airline for America (A4A)” issued “ATA Spec 120: Removable Structural Components Industry Guidelines” for operators.

Conclusion:

To conclude, aircraft structural components like doors, wing slats, and engine mounts, known as Removable Structural Components (RSCs), may be replaced during an aircraft's service life. Tracking and controlling the RSCs' age is crucial for airworthiness and safety, as many Airworthiness Limitations and ADs are component-specific. If an RSC's actual age cannot be determined, conservative estimates can be made using fleet leader concepts as defined in EASA AMC 20-20B and FAA AC 120-93. Regulatory authorities and aircraft manufacturers have issued guidelines and baseline RSC lists for tracking and control. Operators and Continuing Airworthiness Management Organizations (CAMO) must develop systems to track and control RSCs to ensure compliance with maintenance program requirements.

Further reading:

1. A4A Spec 120 “Removable Structural Components Industry Guidelines”.
2. Airbus Magazine Flight Airworthiness Support Technology (FAST) # 60

References:

1. FAA AC 120-93.
2. EASA AMC 20-20B.
3. Airbus Magazine Flight Airworthiness Support Technology (FAST) # 60