MSG-3 Analysis (Part IV): Protecting Aircraft from Lightning and Radiated Fields (L/HIRF)

The primary objective of L/HIRF maintenance under MSG-3 analysis is to minimize the risk of a failure cause—such as a lightning strike—impacting aircraft airworthiness. This approach also considers the occurrence of a common failure cause (e.g., Environmental or Accidental Damage – ED/AD) across redundant channels of L/HIRF protection that could compromise safety.

Key Components of L/HIRF Protection

L/HIRF protection relies on both internal and external protection components:

• Internal Protection Components: For Line Replaceable Units (LRUs) equipped with internal protection components, the aircraft manufacturer ensures that the LRU manufacturer validates the effectiveness of the protection. This may be confirmed through LRUCMM procedures or other regulatory-accepted data. In this scenario, MSG-3 analysis is not required.

• External Protection Components: External L/HIRF protection components must undergo analysis if their failure could affect safety. The Type Certificate Holder (TCH) may combine Zonal and L/HIRF assessments when appropriate. All findings should be reflected in the PPH (Periodic Maintenance Program Handbook).

MSG-3 Analysis Procedure for L/HIRF Protection

The majority of L/HIRF protection maintenance is typically covered by Zonal Inspections. However, when Zonal Inspections fail to detect degradation of L/HIRF protection components, additional tasks are necessary.

Simplified MSG-3 L/HIRF Analysis. Source: Aircraft Maintenance Programs (Lapesa Barrera, 2022).

Step 1: Identifying Significant L/HIRF Items (LHSI)

The first step in the MSG-3 analysis is to identify L/HIRF Significant Items (LHSI). These items perform critical functions essential to ensuring L/HIRF protection for key systems and structures. Dedicated L/HIRF maintenance is only required for components susceptible to Environmental or Accidental Damage (ED/AD).

Step 2: Assessing Degradation

For each location where LHSIs are installed, ED/AD threats are evaluated:

• If degradation is detectable through Zonal Inspections, no further dedicated L/HIRF maintenance is required.

• If degradation isn’t covered by Zonal Inspections:

  • If degradation is detectable without disassembly, the appropriate task and interval are selected.

  • If disassembly is needed, the potential effects of disassembly must be weighed against the likelihood of degradation.

Based on this assessment, the required tasks are determined, and if disassembly would cause further damage or deterioration, redesign may be necessary.

Step 3: L/HIRF Protection Assurance Plan (PAP)

The Working Group reviews the L/HIRF Protection Assurance Plan (PAP) developed by the Type Certificate Holder (TCH). This includes sample size, test points, and other details to determine if the existing plan covers the task requirements. If additional stand-alone tasks are necessary, these are identified and incorporated into the maintenance schedule.

Conclusion

By following MSG-3 analysis for L/HIRF protection, aircraft manufacturers can ensure that lightning strikes and radiated fields do not compromise airworthiness. The inclusion of L/HIRF tasks under ATA 20 Standards Practices—Airframe provides an organized approach to maintaining aircraft safety, minimizing risks from L/HIRF threats, and ensuring effective airframe protection.

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