Crack growth in aerostructures

Poor aircraft analysis

“Don’t fly in 20 years,” my college teammate leaned over and whispered.

It was April 2010; I was on my final leg of undergraduate education. This spring semester, we were watching half of our aeronautical engineering peers give final presentations for their Capstone Designs. The presentations ranged from excellent, to awful, to alarming.

To be fair, the school produced an impressive amount of excellent engineers. But those are not the people who stick out in memory. Perhaps that’s one of my personality flaws.

Regardless, that off-hand remark is proving to be prophetic.

While everyone is handwringing about the events in Washington, DC, a real, insidious, and largely unknown threat is lurking in the bones of every aircraft.

Time-based failure, specifically crack growth in metals (fatigue), appears to be an afterthought in many analyses I’ve reviewed in recent memory. This failure mechanism is responsible for the Comet crashes in the 1950s, F-111 in 1969, Aloha Airlines in 1988, Chalks Airlines in 2005, F-15 in 2008, Southwest Airlines in 2011, and a firefighting C-130 in 2012. In the Chalks and C-130 cases, the failures were directly a result of poor repair design.

These are just the cases which have gained some publicity. We actually find cracks on aircrafts all the time. With proper fleet management, we’re able to deal with these before they become catastrophic. However, recent interactions have me wondering if not everyone is as concerned as they should be about this failure mechanism.

Companies keep asking the question, “What do Millennials want?” Apple was able to figure this out with the iPod. Most people didn’t know they wanted an iPod until they acquired one.

I submit to you that what Millennials want is not coffee shops, not nap rooms, and not hammocks. They want safe spaces…to fly! They want aircrafts to stay airborne. They want (whether they know it or not) engineering rigor behind every piece of aircraft structure.

Fellow engineers, we are the champions of airworthiness, and we need to take our task seriously!

Recent experiences have raised red flags. I am concerned with the analyses I see coming out of other offices and other companies. I have a common thought: “I really hope they don’t deal with commercial aircraft this way.” For some reason in military aviation, the thought that “the pilot has a parachute” is enough to let some engineers get cavalier with their analyses.

A parachute may save the pilot, but it won’t save my house as the abandoned plane barrels into the ground!

In all seriousness, the “lazy way out” has become a common denominator. If we don’t address this problem now, we will either be praying for luck or become statistics.

People don’t like to be corrected. But you can’t ignore problems like poor analyses, lack of understanding the problem, and incorrect free-body diagrams.

Yet when you address these, the engineer or the lead gets defensive! Rather than listen to the substance of the concern, pride becomes the foundation of the analysis. Fellow engineers: be the mentor and provide correction to your subordinates!

I too was once a cocky, inexperienced engineer who thought he knew everything. Thankfully, I had a mentor who wasn’t concerned with friendship. He was able to be painfully blunt and correct my attitude before I became dangerous. Leads, supervisors, and senior engineers: you must do this with your new engineers.

Without this mentorship or training, you end up with flat-out-wrong Finite Element Analyses, misapplication of newer technologies, and analyzing the wrong components! I just reviewed a document where an engineer “refined” the mesh of a Finite Element Model, and the stress went down 40 percent. Where did all that load go? That’s not the way it works. Perhaps the model was wrong to start. The engineer’s lead or supervisor should have reviewed and pointed this out before providing the so-called analysis.

Fellow engineers: trust in new technologies is not your deliverance. Proper testing and characterization is required. FEA and 3D printing will not fix analytical holes in your assumptions and analysis!

All this begs the question: why is there such little focus on aerostructures? It appears as if all you have to do is say the word “laser,” or “synergy,” for that matter, and you have all levels of management and customers saying “take my money!” Maybe we should coin the term Laser-Focus-Analysis-Aerostructures.

Could it be that the next aircraft crisis is right around the bend?

I look at our local universities. One is dropping many classes in fatigue crack growth. Another focuses heavily on aerodynamics and very little on structure. I wish my own schooling had included more concerns with aerostructures.

Our next wave of engineers is going to be intimately familiar with composites, 3D printing, the various computer-aided design tools, and the latest-and-greatest technologies. Like anything else, all these need some form of validation, testing, and understanding.

Yesterday I read an article about Elon Musk’s plan for space tourism. That’s fantastic! Does his engineering team have a group of engineers intimately familiar with fatigue?

It took us over 30 years to finally get the metals right. The composites field is getting to that point after nearly 30 years. We still learn something new every day. The task is far from complete.

Some will accuse me of “standing in the way of progress.” If progress is gaining speed as you fall out of the sky due to fatigue, I proudly stake my claim as that barrier.

As the “graybeards” retire, the knowledge tends to go with them. I fear that in 10 years, we will have a group of un-mentored, and more dangerously, unaware engineers who are too proud to accept errors spotted in their analysis.

I hope we can pull our collective head out of the clouds and start addressing these deficiencies before they address us.