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One comment the FAA inspector made to me was that the last IA inspecting the center section said it looked pristine inside. So the corroded rivets were hidden- and ultrasonic inspection will not tell you whether your rivets are corroded either. You have to determine that for yourself by drilling it out. For anyone who doesnt know ultrasonic testing, it is just like using sonar in a submarine to find a ship and determine it's distance away. It is a pulse-echo technique. I set up the tester using a known good standard which is a piece of aluminum of the same length of a good rivet- lets just say 1 inch for reference. There is a tiny transducer which sends ultrasonic sound waves into the calibratoin standard along it's length. WHen the sound waves come to the end of the standard (where the aluminum block meets the air) the sound waves will bounce back and come back to the transducer. The display is very primitive, it is just a green horizontal line across the screen- but there will be a vertical peak in the line where the transducer touches the aluminum block, and another peak at the other end which represents where the sound waves are bouncing back from the end of the block. It looks much like a heart rate monitor in a hospital. So- I then put the transducer onto a rivet (with some goop called "couplant") and I watch for the exact same 2 vertical spikes in the display, at the same place on my display screen. This tells me that the rivet is the same length as the test block; which being interpreted means that there is no break in the rivet along it's length. If the rivet was sheared half way in the middle, then there would be a metal-to-air gap in the rivet, and the soundwaves would bounce back and return in a shorter time. The second vertical spike would now be about in the middle of my display and I would then know that about roughly half way through the rivet there was some type of break in continuity.
That's all the tester tells you- it's up to you to then start removing rivets to see what's exactly going on-
Jan Zanutto
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