Eddy current and false positive & other burning questions

[koopas]

Hi all,

First of all, happy holidays everyone. I haven't posted here in a while...and neither have a lot of people from what I've noticed.

A few general questions that have been bugging me:

1. A fuselage skin gouge is blended out. Inspection conducts a High Frequency Eddy Current check from the exterior and sees a spike on the machine. They conclude there's a crack indication. The inside is opened-up, and no crack indication shows up from the "backside". I suggest dye-pen to confirm, but am quickly admonished that the blending may have caused the surface cracks to get filled, thus rendering dye-pen inaccurate in this case.

What's going on here? Is it a false positive for cracks? Do you keep blending away until you no longer see a spike (knowing that very soon, you'll fall outside SRM limits for material removed)? What do YOU do? I am a little weak on NDT.


2. Do ALL your repairs always meet 100% of the lost Ftu? Even temporary repairs on secondary metallic structure?

3. Related to question 2. Is it a violation of the FAR's to design a repair that does not meet 100% of the lost Ftu, knowing that the repair is issued as time-limited with repetitive inspections for cracks?

4. Not to start the major/minor discussion again, but is a repair to primary structure (i.e. pressurized fuselage) automatically classified as major?

Thanks
Alex

 

[aviat]

I can’t say much about your Eddie Current inspection. As long as the machine is properly calibrated for the material etc. there is not much I can add. As regards your LPI it is true that metal will be smeared when working the area, but AC 43.13 and ASTM –E-1417 both call for a chemical etching to remove and clean this prior to applying penetrate. A 10X magnifying glass and a light can also be used to check for cracks.

Assuming you are referring to damage tolerant repairs in your other questions the answers are in the most part answered in AC 25.1529-1. Type certificate holders (TCH) have to provide instruction in order for operators to maintain their product in an airworthy condition. They are called Instruction for Continued Airworthiness (ICA’s). These instructions are contained in manuals (SRM, IPC, MM, OH etc.), SB (some are mandatory by AD’s), airworthiness limitations, SID’s (mandatory by AD), CCPP, and other approved or specified data. For A/C that were designed to damage tolerant per 25.571 all your PSE’s will be in the SRM and it will tell you if your repair is major or not, IAW AC 25.1529-1 (5)(e). For A/C not designed to DT, it does not mean that it doesn’t require repair to same standard. The instructions may be in another manual, such as SID’s, and some research will be required to determine it.

A list of items that may be PSE’s are contained in AC 25.571-1C (page 10) for info. only. AC 25.1529-1 gives a flow chart in figure 1 that may be helpful for you.

I have used the FAA as examples but the rules are similar for other authorities, but if you have any questions you should get your regulating body involved in the process, as well as the OEM.
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Good luck!