Non-Destructively Checking Airplanes for Cracks

By now, everyone has seen the story about the Southwest airliner that took off from Phoenix and had a hole open up in the fuselage skin. I thought it would be interesting to take a look at one of the ways Southwest is testing their planes.

My academic background is microbiology and electrical engineering but I do have a little bit of experience (it's been a while) in material science and non-destructive testing. I spent one summer in graduate school checking ferrite plastic materials, made from a mix of plastic and iron for microscopic cracks using something called an Eddy-Current Tester.

Most of the testing we did that summer was done on little donut shaped toroids we cast ourselves using a mixture of bakelight and iron powder. After casting, these magnetic/plastic toroids were wrapped with wire and used in sensor circuits - things like ground fault interuptors that sense a rapid change in household electrical current and open the circuit before someone gets electrocuted.

This work was done in Dr Jim Masi's materials lab at Western New England College (WNEC). When Jim retired from WNEC he came over to Springfield Technical Community College for three years and was the first ICT Center Director (our Center was called the Northeast Center for Telecommunications Technologies back then) which is the position that I now hold. When Jim retired I inherited his old desk and guess what he left for me inside - some of those old toroids! These are the first things I see when I open my desk and they always make me think of Jim. Here's a picture of a couple.

So, what do little donut shaped pieces of plastic with iron mixed in have to do with testing 737 jetliners? It turns out - quite a bit. The trick with those toroids was to find a way to test them for cracks without actually breaking them - something called non-destructive testing. There are  different ways this kind of testing is typically done, one uses holography and lasers and works really well for small objects but does not work so great with large things like 737's.The other method is something called eddy-current testing and this is what Southwest is using to test their planes.

Here's how Eddy-current testing works:

• It uses electromagnetic induction to check for microscopic cracks in conductive materials. Conductive is a key work here - the material has to be able to conduct electricity. Eddy-current testing does not work on things like wood or plain-old plastic. 

• The testing process on something as large as an airplane involves using a circular coil (a toroid!) passed across the surface being tested. The toroid is made of magnetic material and wrapped with wire.

• Alternating current is then passed through the coil wire, generating an alternating magnetic field around the coil.

• The coil's alternating magnet field inductively interacts with the conductive airplane metal and generates eddy currents.

• A second search or detector coil is then used to detect eddy currents coming from the material.

If there are no cracks or other flaws in the material, eddy current patterns are smooth and continuous. Any variations/disruptions (amplitude and phase) in wave patterns are easily picked up by the search coil and can indicate flaws in the material being tested. I like to think of a smoothy flowing stream with a large rock in the middle and how the flow pattern changes as the water goes around the rock. Think of a crack as that rock, disrupting the flow pattern of the surrounding magnetic field.

There are some limitations with Eddy Current testing - the one that concerns me the most when it comes to airplane testing is limited depth of penetration but I'm thinking airplane skins are fairly thin so maybe that really does not worry me. There are also problems detecting cracks that are parallel to the detecting coil which can be usually be worked around by rotating the detector so that one does not worry me much either.

When done correctly, eddy-current testing is a simple and highly effective method to find small microscopic cracks that can't be seen - perhaps hidden behind paint on objects like airplane skins. Eddy-current testers are inexpensive, portable, provide immediate feedback, and do not need to contact the item being tested. It works.

On Sunday, Southwest COO Mike Van de Ven issued a statement saying:

Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft.

I would hope the FAA, manufacturers and airlines are using eddy-current testing as part of routine airplane maintenance.