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7.4.3.4  Use X-Ray (7) Instead of HFEC Inspection of Chord The completed DTR Check Forms are shown on Pages 7.0.39 (EX-3A(IV)) and 7.0.40 (EX-3B(IV)). The DTR Check Forms are completed with the same procedure used for Section 7.4.3.3 (HFEC). Unlike the HFEC, the X-ray procedure provides opportunities for detecting the chord crack while it is
hidden beneath the skin.

This improves the 'DTR values for a given value of N allowing the phased inspection size to be reduced
to 31 percent.
Similarly, the X-ray procedure provides additional opportunities for detecting a crack in a fail-safe angle.

Therefore, a 31 percent phased inspection program provides a total DTR of 12.6 which is well in excess of the required DTR of 6.
7.4.3.5  Use of Both HFEC (4) and X-ray (7) Inspection Methods The completed DTR Check Forms are shown on Pages 7.0.41 (EX-3A(V)) and 7.0.42 (EX-3B(V)). For EX-3A, the HFEC, plus X-ray, inspections must provide a 'DTR = 6 - 0.3 = 5.7. Taking cuts across the HFEC (4) and X-ray (7) curves, at N = 39,000, the 'DTR values are 1.9 and 3.8,
respectively.
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The corresponding phased inspection Ro = 100 x 8,000/39,000 = 21 percent.

For EX-3B, a phased inspection of 21 percent provides a Total DTR of 8.1 which exceeds the
Required DTR of 6 and is therefore acceptable.
Summary
A summary of the five procedures considered for EX-3 is provided on Page 7.0.43.

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SUMMARY: The following table summarizes the effect of exercising Options a) through e) on the DTR achieved for Example 3.
REQUIRED DTR = 6  ITEM EX-3A  ITEM EX-3B 
Δ DTR FROM SCHEDULED MAINTENANCE ON 100% FLEET  Δ DTR FROM SUPPLE-MENTAL INSPECTIONS  TOTAL DTR  Δ DTR FROM SCHEDULED MAINTENANCE ON 100% FLEET  Δ DTR FROM SUPPLE-MENTAL INSPECTIONS  TOTAL DTR
C  4C  C  4C 
I  EXISTING MAINTENANCE  0.3  - - 0.3  - 5.2  - 5.2 
II  IMPROVE INSPECTION METHOD (GENERAL VISUAL TO DETAILED)  1.1  0.1  - 1.2  - 12.0  - 12.0 
III  ADD HFEC INSPECTION ON A 53% PHASED INSPECTION AT 4C-CHECK  0.3  - 5.7  6.0  - - 6.0  6.0 
　
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