7.4.3.3 Add HFEC (4) Inspection of the Chord The completed DTR Check Form for EX-3A(III) is shown on Page 7.0.37. An HFEC inspection of the chord (Direction 4), with a repeat interval of 8,000 flight cycles (4C), gives a ΔDTR = 12 and a Total DTR = 12+0.3 = 12.3. Since this exceeds the Required DTR of 6, a phased inspection program can be used.
To achieve the Required DTR, the HFEC inspections must provide a ΔDTR = 6 - 0.3 = 5.7. Entering the ΔDTR scale at 5.7 and reading across to the HFEC (4) curve, the value of N = 15,000 flights.
The operator elects to use the 4C-check interval for the HFEC inspections. Therefore, the percentage
of phased aircraft to be inspected is given by 8,000/15,000 x 100 = 53.
The HFEC inspection of the chord cannot detect a crack in the fail-safe angle. Therefore, HFEC is not
applicable to EX-3B.
7.0 DOCUMENT USAGE D6-82669 Original
As shown previously in Section 7.4.3.2, a detailed inspection of the fail-safe angles every 4C provides
more DTR than required. Therefore, a phased inspection program can be used.
Consider the 53 percent of phased aircraft developed for EX-3A.
The completed DTR Check Form for EX-3B(III) is shown on Page 7.0.38.
The effective inspection interval N = 100 X 8,000/53 = 15,000 flight cycles.
Entering the N at 15,000 and reading the DET (2) curve, the 'DTR = 6.0. This meets the Required DTR
exactly and is therefore acceptable.
The program objective can be achieved for this SSI with the following inspections:
EXTERNAL -General Visual of the SSI and adjacent structure every 2,000 flight cycles (C-check).
- HFEC inspection of the upper chord every 8,000 flight cycles (4C) with a phased inspection program of 53 percent of the eligible fleet during each interval.
INTERNAL - Detailed inspection of the SSI every 8,000 flight cycles (4C) with a phased inspection program of 53 percent of the eligible fleet during each interval.
GENERAL VISUAL - Inspection of structure adjacent to the SSI at each scheduled inspection as noted in Section 5.1.11.
The primary objective is to inspect ALL eligible airplanes within a repeat interval corresponding to N, which for this example is 15,000 flight cycles. The primary purpose of specifying Ro is to assure a reasonably uniform rate of inspection during the interval N. The value of Ro can be considered approximate. For this example the operator has 10 eligible airplanes and Ro = 53 percent. The program objective can be achieved by inspecting any six eligible airplanes during the first 8,000 flight cycles (4C), followed by the remaining four plus a second inspection on any one of the first six, during the second 8,000 flight cycle (4C) interval.
This gives an average of 55 percent, and all of the eligible airplanes are inspected within 15,000 flight cycles.
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