Considerations Related to Aircraft MSG 3 Inspections – Accidental Damage (AD) Environmental Damage (ED) and Fatigue Damage (FD)

Introduced by Sofema Online (SOL) www.sofemaonline.com

Background to MSG3 Significant Structural Inspection (SSI) Program

Using MSG 3 Maintenance Philosophy we have 3 drivers for Structural Deterioration.  

       · Accidental Damage (AD)
       · Environmental Damage (ED)
       · Fatigue Damage (FD)

Requirements for detecting Accidental Damage (AD), Environmental Deterioration (ED), Fatigue Damage (FD), and procedures for preventing and/or controlling corrosion form the basis for the MRB structural maintenance.

Note 1 - All FD inspection requirements may not be available when the aircraft enters service. (In such cases the manufacturer shall propose, prior to the entry of the aircraft into service, an appropriate time frame for completing the FD inspection requirements.

Note 2 - If the need arises, procedures should be developed for any new material (e.g., new composite material) whose damage characteristics do not follow those previously understood procedures.

Non-metallic Structure

Also susceptible to damage and/or deterioration (e.g., Disbonding and Delamination). 

Note 3 - Such structure that is classified as an SSI will require inspections to ensure adequate strength throughout its operational life.

Further Considerations related to Non-Metallic Structure

       · Susceptibility to Long-term Deterioration (Should be assessed with regard to the operating environment.
       · Likely candidates for inspection include areas such as:

o   Major attachments

o   Joints with metallic parts

o   Areas of high-stress levels

Note about Composite Materials – Accidental Damage

The consequence of composite damage may not be readily apparent and may include internal damage, e.g., disbonding or delamination.

Structural Significant Inspection (SSI) Methodology

MSG 3 Aircraft Structural Analysis Procedures identify tasks for are designed to relate the scheduled maintenance tasks to the consequences of structural damage remaining undetected.

Each structural item is assessed in terms of its:

        · Significance to continuing airworthiness,
        · Susceptibility to any form of damage, and
        · The degree of difficulty involved in detecting such damage.

Structural Significant Inspection (SSI) Zonal Inspections

Some parts of the inspection requirements for SSIs and most of the items categorized as Other Structure can be provided by the zonal inspections.

Note - Tasks and intervals included in the zonal inspections should be based on operator and manufacturer experience with a similar structures.

        · For structure containing new materials and/or construction concepts, tasks and intervals may be established based on an assessment of the manufacturer's recommendations.
        · Thresholds are normally established as part of the damage tolerance certification requirements. These are subject to change as service experience, additional testing, or analysis work is obtained.

Development of Structural Scheduled Maintenance Tasks - Inspection Thresholds

The inspection threshold for each SSI task is a function of the source of damage.

Accidental Damage (AD)

The first inspection (threshold) Normally corresponds to a period equal to the defined repeat inspection interval, from the time of first entry into service.

Accidental Damage – Causes

         · Ground and cargo handling equipment,
         · foreign objects,
         · erosion from rain & hail,
         · lightning,
         · runway debris,
         · spillage,
         · freezing, thawing, etc..,
         · Human error during aircraft manufacture, operation or maintenance that are not included in other damage sources.

Environmental Deterioration (ED)

The initial inspection thresholds for all levels of inspection are based on:

         · Existing relevant service experience,
         · Manufacturers recommendations, and/or
         · A conservative age exploration process.

 Structural deterioration as a result of a chemical interaction with its climate or environment.

Assessments are required to cover:

         · Corrosion,
         · Stress corrosion,
         · Deterioration of non-metallic materials.

Note: Corrosion may or may not be time/usage dependent.

For example,

         · Deterioration resulting from a breakdown in surface protection is more probable as the calendar age increases;
         · Corrosion due to galley spillage is a randomly occurring discrete event.

Fatigue Damage (FD)

FD Detection - Inspections related to FD detection in metals are applicable after a threshold, which is established during the aircraft type certification process.

At the time, the fatigue-related inspections are implemented, sampling can be used, where it is applicable and effective.

Inspections directly related to fatigue damage detection will occur after a threshold(s) to be:

            · Established by the manufacturer and
            · Approved by the appropriate regulatory authority.

Which is characterized by:

            · The initiation of a crack or cracks due to cyclic loading and subsequent propagation.
            · A cumulative process with respect to aircraft usage –

Note Concerning - Widespread Fatigue Damage (WFD)

The likelihood of the occurrence of fatigue damage in an aircraft’s structure increases with aircraft usage.

The design process generally establishes a design service goal (DSG) in terms of flight cycles/hours for the airframe.

Any cracking that occurs on an aircraft operated up to the DSG:

             · Will occur in isolation (i.e., local cracking),
             · Originate from a single source, such as a random manufacturing flaw (e.g., a mis-drilled fastener hole) or a localized design detail.

Next Steps

Sofema Aviation Services (SAS) www.sassofia.com & Sofema Online (SOL) www.sofemaonline.com provide regulatory & vocational training offering more than 550 Classroom Training Courses and More than 200 Online Courses. For additional information please email team@sassofia.com