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We're showing 25.571 compliance via similarity to the 737 DT analysis. The new wing skin uses Al 7475-T7351 instead of 2024-T3. What MMPDS data do I need and which ACs cover the methodology delta?

C

For the material substitution from 2024-T3 to 7475-T7351, you need:

MMPDS Data Required

MMPDS-17 Table 9.3.6.2 for 7475-T7351 plate da/dN curves, and Table 9.3.6.4 for fracture toughness KIC values at the relevant thickness.

Applicable Advisory Circulars

AC 25.571-1D §6.2 covers the similarity methodology. For the material change specifically, AC 20-107B §7 addresses composite-to-metal substitution substantiation.

MMPDS-17AC 25.571-1DAC 20-107BFAR 25.571ASTM E647

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FAR 25.571

Damage Tolerance & Fatigue

AC 25.571-1D

Advisory Circular

CS 25.571

EASA Equivalent

AC 20-107B

Composite Structures

ASTM E647

Crack Growth Testing

MMPDS-17

Material Properties

SC 25-529

Special Condition

Cross-references resolved automatically · 6 connected regulations

Part 25 Compliance Matrix
Subpart C — Structure
Complete (102)In Review (68)Draft (677)
Reg.TitleStatus
25.301Loads
Linked to AC 25.301-1, AC 23.629-1B. Awaiting flight loads survey data.
Draft
25.303Factor of Safety
Standard 1.5x ultimate factor. No special conditions required.
Draft
25.305Strength & Deformation
Linked to AC 25.305-1. Static test plan under ODA review.
In Review
25.307Proof of Structure
Full-scale static test approach. Linked to 25.305 test program.
Draft
25.571Damage Tolerance
Linked to AC 25.571-1D, ASTM E647, and 3 special conditions. Similarity to 737 DT analysis approved.
Complete
25.603Material Suitability
MMPDS-17 material data package in progress. 7475-T7351 qualification pending.
Draft
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Similarity Argument — FAR 25.571
Citations verified

Section 4.2 — Fatigue Substantiation

The fatigue and damage tolerance evaluation of the wing lower skin panels follows the methodology established in AC 25.571-1D §6.2. The crack growth analysis utilizes NASGRO 4.02 with material data sourced from MMPDS-17 Table 9.3.6.2.

The inspection program is based on the damage tolerance assessment per FAR 25.571(a)(3), with threshold and repeat intervals derived from full-scale fatigue test evidence documented in TR-2024-0571-DT. The applicant demonstrates that the principal structural elements meet ASTM E647 for crack propagation rate characterization.

Citations verified

AC 25.571-1DMMPDS-17FAR 25.571(a)(3)TR-2024-0571-DTASTM E647

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