Review by Sofema Aviation Services (SAS)
Introduction
Many Industry developments related to the structural integrity of aircraft were introduced following the incident concerning Aloha Airlines Flight 243 – April 28, 1988.
Corrosion Prevention and Control Programs (CPCPs) were developed in the early 1990s by type certificate holders (TCHs) with the assistance of aircraft operators and regulatory authorities and were issued as FAA Airworthiness Directives (ADs).
In 1993 MSG-3 was amended to include assessment of damage to aircraft structures caused by corrosion. Since then, CPCPs have been incorporated into maintenance programs using MSG-3 guidance.
Corrosion Prevention and Control Programs (CPCP)
A Corrosion Prevention and Control Program should be established to maintain the aircraft’s resistance to corrosion as a result of systematic (e.g. age-related) deterioration through chemical and/or environmental interaction. This Program applies to damage tolerant and safe-life structures.
The program is expected to allow control of the corrosion on the aircraft to Corrosion Level 1 or better. The CPCP should be based on the Environmental Deterioration (ED) analysis, assuming an aircraft operated in a typical environment.
If corrosion is found to exceed Level 1 at any inspection time, the corrosion control program for the affected area must be reviewed by the operator with the objective to ensure Corrosion Level 1 or better.
Note on Corrosion Levels
Level 1 Corrosion is the damage occurring between successive inspections that is local. and can be reworked/blended-out within allowable limits as defined by the manufacturer in a structural repair manual (SRM), service bulletin, etc.
Level 2 Corrosion is damage occurring between successive inspections that require rework or blend-out that then exceeds the manufacturer’s allowable limits, requiring a repair or complete/partial replacement of a principal structural element.
Level 3 Corrosion is damage found during the first or subsequent inspection(s) which is determined by the operator to be a potential airworthiness concern requiring expeditious action.
General Comments
A CPCP requires a method to notify the Relevant Competent Authority as well as the aircraft’s manufacturer regarding findings as well as any data associated with such damage.
The definitions for different corrosion levels have changed over time, causing issues for air carriers with mixed fleets so to check carefully the applicable documentation relevant to your aircraft type.
Across Industry there are some differences in reporting requirements for the three corrosion levels.
A potential issue exists related to “successive blending” (i.e., blending performed in a series versus multiple blends at different times) of corroded areas.
For example, if a Level 1 area is blended a second time, it becomes Level 2.
However, successive blends after the second blend (up to five in a row) may remain defined as Level 1.
Question – How does your organisation track successive level 1?
Are you tracking blending very well or not at all? This information may have to be built into maintenance program revisions. Operators CAMO’s may be required to show they are recording these blending repairs especially if they wish to take advantage of successive blending allowances.
Concerning Damage Tolerance (DT)
With Level 1 repairs, the amount of material loss during the blending process does not affect the strength requirement; therefore, this would not require DT considerations.
However, a Level 2 corrosion repair that would necessitate a major repair to restore the structural integrity and function of the structure may require a DT inspection or task. – How is this managed?
Next Steps
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Corrosion Prevention Corrosion Protection, SAS training, SOL Training, Corrosion Prevention Control Program
