May 09, 2025

Steven Bentley

Sofema Aviation Services (SAS) considers the specific requirements of Part 91 Subpart L

Introduction

Subpart L of 14 CFR Part 91 focuses on maintaining the airworthiness and structural integrity of ageing aircraft. Below is a breakdown of the specific requirements:

  • 91.1505 – Repairs Assessment for Pressurised Fuselages

This regulation mandates repair assessments for specific pressurised aircraft models to ensure fuselage structural integrity over extended service periods.

Applicability:

  • The rule applies to aircraft models such as Airbus A300 (excluding -600 series), British Aerospace BAC 1-11, Boeing 707, 720, 727, 737, 747, McDonnell Douglas DC-8, DC-9/MD-80, DC-10, Fokker F28, and Lockheed L-1011.
  • Operators must comply beyond the specified flight cycle implementation time or by May 25, 2001, whichever comes later.

Requirements:

  1. Repair Assessment Guidelines (RAGs)
    • Operators must incorporate approved repair assessment guidelines within their inspection programs.
    • These guidelines apply to the fuselage pressure boundary, including:
      • Fuselage skin
      • Door skin
      • Bulkhead webs
    • The responsible Aircraft Certification Service office must approve the RAGs.
  2. Flight Cycle Implementation Time for Each Model:
    • Airbus A300 B2: 36,000 flights
    • Airbus A300 B4-100 (B4-2C): 30,000 flights (above window line), 36,000 flights (below window line)
    • Airbus A300 B4-200: 25,500 flights (above window line), 34,000 flights (below window line)
    • Boeing 707: 15,000 flights
    • Boeing 720: 23,000 flights
    • Boeing 727: 45,000 flights
    • Boeing 737: 60,000 flights
    • Boeing 747: 15,000 flights
    • McDonnell Douglas DC-8: 30,000 flights
    • McDonnell Douglas DC-9/MD-80: 60,000 flights
    • McDonnell Douglas DC-10: 30,000 flights
    • Lockheed L-1011: 27,000 flights
    • Fokker F-28 (Marks 1000, 2000, 3000, 4000): 60,000 flights.
  • 91.1507 – Fuel Tank System Inspection Program

This regulation establishes a fuel tank safety inspection program, ensuring compliance with SFAR 88 and FAA requirements to mitigate potential fuel tank hazards.

Applicability:

  • Applies to transport category, turbine-powered aeroplanes with a type certificate issued after January 1, 1958, that have:
    • A maximum passenger capacity of 30 or more
    • A maximum payload capacity of 7,500 lbs or more.

Requirements:

  1. Auxiliary Fuel Tanks Installed Under Field Approvals
    • Operators must submit proposed maintenance instructions for auxiliary fuel tanks installed before June 16, 2008, to the Aircraft Certification Service Office.
    • These instructions must comply with SFAR 88 requirements.
  2. Mandatory Inspection Program Revisions
    • After December 16, 2008, no operator may fly a qualifying aircraft unless its inspection program includes:
      • Applicable inspections
      • Procedures
      • Limitations for the fuel tank system.
  3. Compliance with Fuel Tank System ICA
    • Operators must revise inspection programs based on fuel tank system Instructions for Continued Airworthiness (ICA).
    • ICA revisions must align with:
      • SFAR 88 requirements
      • § 25.1529 and Appendix H of Part 25 (as of June 6, 2001)
    • ICA must cover all fuel tanks, including:
      • Supplemental type certificate-installed tanks
      • Other approved design modifications.
  4. Post-Alteration Inspection Program Updates
    • After December 16, 2008, any aircraft undergoing fuel tank alterations must update its inspection program before returning to service.
    • The updates must integrate ICA-based inspections and procedures.
  5. Submission to the Flight Standards Office
    • Any fuel tank system revisions (initial or subsequent) must be submitted to the Flight Standards Office for review and approval.

Summary Subpart L of 14 CFR Part 91 enforces critical airworthiness standards to maintain the safety and longevity of ageing aircraft. The key takeaways include:

  • § 91.1505: Ensures structural integrity of pressurised fuselages by implementing mandatory repair assessments.
  • § 91.1507: Requires operators to enhance fuel tank system safety by revising inspection programs to comply with SFAR 88.

By integrating these measures into maintenance programs, operators prevent potential safety hazards, ensure regulatory compliance, and extend aircraft service life.

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Continued Airworthiness, SAS blogs, Aging Aircraft, ICA, Safety Hazards, safety improvements, Subpart L, Part 91 Subpart L, Repairs Assessment for Pressurised Fuselages, Repair Assessment Guidelines (RAGs), Aircraft Certification Service, Airbus A300 B2, maximum passenger capacity, Auxiliary Fuel Tanks, extend aircraft service life