Aircraft Composite Structure: Fabrication, Inspection, Damage Assessment and Repair (SAE AIR4938 Part 2)

Objective:
Given discussion, lecture, and feedback, the student will identify team member skills, roles, and responsibilities as related to composite repair processes. The student will state the differences between structural classifications and repair categories.

Topics:
A. Roles and Responsibilities
    i. Engineer (degreed)
    ii. Inspector
    iii. Aircraft Maintenance Technician
B. Structure Classifications
    i. Primary
    ii. Principal Structural Elements (PSE)
    iii. Fatigue Critical Baseline Structure (FCBS)
    iv. Secondary
    v. Interior
C. Repair Categories
    i. Category A – Permanent
    ii. Category B – Interim
    iii. Category C – Time Limited

Objective:
Given discussion, lecture, and feedback, the student will be able to identify and verify that the repair material is the correct material called in the repair document.

Topics:
A. Repair Material Requirements
    i. Aerospace Material Specifications
    ii. Material Certification Requirements
    iii. Receiving Inspection
    iv. Product Identification

Topics:
A. Thermoplastics
B. Thermosets
C. Mixing and Mix Ratios
    i. Weight (accuracy of scales)
    ii. Volume
    iii. Mixing Process
    iv. Improper Mix Ratios
D. Curing of Resins
    i. A, B, and C Stages
    ii. Flow versus Gelation
    iii. Glass Transition
    iv. Viscosity
    v. Curing Reactions
    vi. Effect of Mis-Cures (under-cured and over-cured)

Objective:

Topics:
A. Fumes, Vapors, and Dust
B. Skin Contact
C. Safety Data Sheet (SDS)
D. Personal Protective Equipment (PPE)
E. Exothermic Reactions
F. Waste Disposal
G. Flammability

Objective:

Topics:
A. Film
B. Liquid
C. Paste
D. Supported/Unsupported
E. Surface Preparation
F. Bond Line Control
    i. Micro Beads
    ii. Carriers
        a. Scrim Cloth
        b. Knit
        c. Veil
G. Failure Modes
    i. Adhesion
    ii. Cohesion
H. Foams
    i. Core Splice

Objective:

Topics:
A. Typical Sandwich Design
B. Solid Laminate and Monolithic Design
C. Advantages/Disadvantages of Solid Laminates versus Sandwich Structures
D. Joint Types
    i. Bolted
    ii. Bonded
E. Impact Resistance
F. Balance and Symmetry
G. Electromagnetic Effects (EME)
H. Conductive Layers
I. Sealants
J. Primers
K. Finishes
L. Acoustic Properties
M. Sonic Fatigue
N. Elevated Surface Temperature Requirements
O. Erosion Protection
P. Galvanic Compatibility
Q. Transmissivity

Objective:

Topics:
A. Heat Transfer Basics
B. Equipment Usage
    i. Autoclave
    ii. Oven
    iii. Hot Bonders
    iv. Heat Blankets
    v. Hot Air Blower
    vi. Infrared
    vii. Heat Lamps
C. Thermocouple Placement
D. Insulation and Airflow Control

Objective:

Topics:
A. ATA Specification 100
B. Structural Repair Manuals
    i. Allowable Damage Limits (ADL)
    ii. Repair Limits
C. Component Maintenance Manual
D. Engineering Orders
E. Maintenance Organization Specific Documents
F. SAE AMS-CACRC ARPs
G. Regulatory Documents
    i. Airworthiness Directives
    ii. Service Bulletins
    iii. Advisory Circulars
    iv. Rule Making Task
H. Material and Processes Specifications
    i. OEM
    ii. Vendor
    iii. Internal

Objective:

Topics:
A. Repair Design
    i. Prepreg
    ii. Wet Lay-Up
    iii. Support Tooling Requirements
    iv. Doubler Overlay
    v. Pre-Cured Patch
    vi. Bolted
    vii. Nonstructural (cosmetic)
B. Taper Sanding and Step Sanding
C. Part Drying Requirements
    i. Physical Water
    ii. Absorbed Moisture
D. Ply Stack-Up
    i. Small Ply Down
    ii. Large Ply Down
E. Repair Area Limitations

Objective:

Topics:
A. Types of Damage
B. Pre- and Post-Repair Inspection
C. Visual
D. Tap Testing
E. Low-skill Inspection Instrument
F. Ultrasonic (Pulse-Echo, TTU, Bondtesters)
G. Thermography
H. X-Ray
I. FT-IR (Fourier Transform Infrared for heat damage)
J. Comparison of NDI Techniques for Metal versus Composite Structure

Objective:

Topics:
A. Part Identification
B. Material Identification
C. Ply Identification
D. Damage Mapping (Sizing)
E. SRM Repair Zones

Two of the repair exercises will use 180 °C (350 °F) nominal cure temperature prepreg materials. For some of the repairs, foam-cored parts are an alternative to honeycomb parts. At least one of the repairs must be a fiberglass part. 50% of the repairs will use a hot bonder as the control method. One of the repairs requires restoration of the electromagnetic effects (EME) surface layer. One of the repairs is to be a part with a significant curvature.

I. Damage Assessment

II. Repair to honeycomb sandwich part, skin, and core damage, using 121 °C (250 °F) prepreg materials. This repair is to use a two-stage repair method and double-sided heat for the first stage. This will be the heat blanket and hot bonder cure method.

    i. Assessment:
        a. Core removed without damage to far side inner skin
        b. Taper sanding meets repair document requirements
        c. Correct material and orientation of original plies identified
        d. Repair plies are positioned and oriented correctly
        e. Cure cycles meet repair document requirements
        f. Repair meets post-repair inspection requirements

IV. Repair to honeycomb sandwich part, with both skins and core damage, using wet layup or prepreg materials. This repair can use either a single-sided or double-sided heating method. The repair is to be performed from one side access. The repair can be either a one or two-stage repair method. The cure cycle heat application can be any of the listed methods in this document.

V. Repair to the edge band of the honeycomb sandwich part, the damage is to extend into the core and have ply drop-offs. This repair can use either single-sided or double-sided heating methods. The repair can use either a one or two-stage repair method. The cure cycle heat application can be any of the listed methods in this document.

    i. Assessment:
        a. Core removed without causing additional damage to far side inner skin
        b. Taper sanding meets repair document requirements
        c. Correct material and orientation of original plies identified
        d. Repair plies are positioned and oriented correctly
        e. Cure cycle(s) meets repair document requirements
        f. Repair meets post-repair inspection requirements

    i. Assessment:
        a. Taper sanding meets repair document requirements
        b. Correct material and orientation of original plies identified
        c. Repair plies are positioned and oriented correctly
        d. Cure cycle meets repair document requirements
        e. Repair meets post-repair inspection requirements