I’m sure a lot of you have been wondering why I haven’t written about the Southwest 737 that had a big hole open up inflight last week. It was big news for sure, but I just didn’t know enough beyond the basic information to make a post worthwhile. Southwest is still not saying much at all, but I was able to piece some things together from other sources to get a better idea of what’s actually going on. It all starts with a lap joint.
What Happened
Last week, a Southwest 737-300 airplane was flying from Phoenix to Sacramento when a hole opened up in the roof. The airplane lost pressurization (of course) but the pilots were able to land the airplane in Yuma and everyone was fine. You may have heard people talk about how the airplane plunged after the hole appeared, but that was on purpose.
When an airplane loses pressurization at altitude, you can’t breathe. And that’s a problem. There are oxygen masks but those don’t have an endless supply of oxygen in them. So anytime this happens, the pilots are trained to go into a steep descent until they get to around the 10,000 foot mark where the air is breathable. It may seem like you’re plunging, but it’s all part of the plan.
Once on the ground, it was easy to see that this was no small hole. It happened in the crown of the airplane along a lap joint. That’s a horizontal line where two pieces of the skin come together and are fastened to each other. As you can imagine, this area is cause for concern regarding fatigue because joints are the weakest points in a structure.
Tear Straps
The part that’s really concerning here isn’t the tear itself so much as it is the size of the tear. See, on all these airplanes, they install what are called tear straps. The aircraft of particular concern are the 737-300, -400, and -500s, collectively called 737 Classics. Next Generation (or “NG”) airplanes make up the bulk of the 737s you’re likely to fly, including all of the ones that American and Delta operate. Those had a different design and are newer so they aren’t impacted by this. I’m sure, however, that the FAA and Boeing will be watching this closely.
But back to the Classic airplanes. On the older models, these tear straps were placed every 10 inches horizontally along that lap joint. In 1993, a change was made that resulted in the straps being needed only every 20 inches. These tear straps are meant to stop any crack from spreading further. In other words, even if a hole opened up, it should never go further than 10 or 20 inches depending upon the airplane because the tear strap will stop it.
As you may have seen, this went for feet, not inches, and that means that the tear straps were breached. That is not good. So, Boeing, the FAA, and the airlines are diving in to try to figure out what exactly happened here. But for now, they are simply mandating inspections for cracks so that this never even becomes an issue. Why weren’t these being inspected before? That’s a different story.
Shouldn’t These Have Been Inspected?
For the older 737 Classic models that were built before 1993, there were directives issued that required inspections (using technology, not visual) for aircraft with more than 45,000 cycles. That was eventually lowered to only 35,000 cycles. One cycle is one takeoff and landing. This metric is used because that’s a good measure of how much stress is put on the airframe going through the pressurization process.
At US Airways media day yesterday, that airline confirmed that all of its 737s fall into this category, and they’ve been doing the inspections since the FAA mandated them early last decade. More than half of Southwest’s 737-300s fall into this category, so presumably the airline has been conducting these checks as required. But it didn’t use this as a standard 737-300 maintenance procedure for the full fleet. On those airplanes built after the 1993 manufacturing change, none of these non-visual inspections were done, because they didn’t have to be done.
With hindsight, that’s too bad, because had Southwest inspected all of its 737-300s, it would have found the cracks. The airplane that opened up a hole had more than 39,000 cycles. But the newer manufacturing process wasn’t expected to have problems this early on in the life of the airplane. In fact, Boeing thought that 60,000 cycles would have been a good conservative number for an inspection. Now, the FAA has mandated checks on these newer airplanes starting at 30,000 cycles.
So as you can see, there’s a lot up in the air. Nobody knows why cracks are showing up on these airplanes so early in life, but stepped up inspections will make sure that they are safe to fly regardless. Now the investigation can focus on why this is happening.
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