The Laughably Simple Flaw:

It had square windows.

This is one of those things that is easy to miss (the designers missed it, for instance) but easy to understand once it's explained.

Here's a Kit Kat style candy bar. Where would you say this thing is most likely to break when pressure is applied?

seriouseats
Along the willpower line, probably.

Right there at those sharp notches, obviously. That's why they're there, and it's why no one builds important structures out of Kit Kats.

Well, a square window is made up of four 90-degree notches cut out of your wall, creating four of these weak points. You don't need a diagram -- if you have brick or stucco on your house, go outside and look. You'll find cracks there, protruding right from one of those sharp corners:

Getty
To fix, place head in bucket of sand and hum loudly.

In engineering, that sharp corner (or groove in the Kit Kat) is called a "stress concentration," a spot where the shape of the object makes it more likely to break under stress.

You don't want the red bit.

So if you're an airplane maker, how the hell do you fix that?

Well, have you ever noticed how on every plane you've ever been on, the windows you look out of have rounded corners? Those curves are pretty much the only thing keeping the plane from tearing itself apart in midair like in that scene from Fight Club. It distributes the stress to all of the various points along the rounded curve, rather than on that one sharp corner, which otherwise would (as they found out) tend to pull apart and form a crack over time.

Trust us, this was not easy to figure out. Experts had no idea why the planes weren't holding together until they tested the structure by simulating the repeated pressurization of the cabin. Sure enough, the fuselage eventually burst like a bootleg condom, and the break started with cracks right at those window corners.

Getty / plane-crazy
Top: Safe and enjoyable ride. Bottom: Explosive Caesarean section.

Representatives from competing companies Boeing and Douglas both said that their engineers hadn't thought of it either, and that if the Comet hadn't been first, it would have been one of theirs that crashed. Planes have had windows with rounded corners ever since.

It's a floating runway. How else would you design it?

Well, that design was kind of a suicide factory. As you can see, planes waiting to take off sit at the other end of the runway you're trying to land on. If you don't get stopped in time, you're going to create one hell of a fireball. And getting stopped in time was no small thing -- catching the arresting wire (the thing that stopped the plane) was a tricky business. Eventually carriers went with the cartoon-logic solution and installed barrier nets to stop planes if they missed all the wires. However, it wasn't all that uncommon for aircraft to bounce over the barrier.

militaryvideocom
Like skipping stones over a pond, if the pond had sharks and you had no arms.

So what was the brilliant innovation that allowed them to make landings that much safer?

They angled the landing strip about nine degrees.

Wikipedia
And as the Navy learned from trailer parks, double-wide is always better.

Don't laugh -- it took years to come up with it. While some of the greatest technological advances in history, including space flight and splitting the goddamn atom, came from developments during World War II, we didn't think of angling the flight deck until 1952. Prior to that, every landing was a potential rear-end collision.

By angling the deck, a plane that missed the wires could go to full throttle, take off again and come around for another pass. Planes waiting to take off are near the bow, out of harm's way.

See? Absolutely no planes in the way.

Angling the deck also allowed for the tactical advantage of being able to launch and recover aircraft simultaneously, whereas in WWII, launching had to be postponed while landings were occurring, and vice versa. Who knows how many lives could have been saved if someone had thought of doing this about 10 years sooner.

When designing their newest hotel to be built in downtown Kansas City, the fine people at Hyatt Regency wanted all the bells and whistles in it. The architectural firm in charge of the building design came up with a series of aerial walkways suspended from the ceiling so that guests could people-watch from a heightened vantage point. All in all, it was a pretty nifty feature. Until it suddenly collapsed and killed more than a hundred people.

Wikipedia
"We can claim these were either terrible walkways or aggressively efficient elevators."

The Laughably Simple Flaw:

One long rod was replaced with two short ones.

If there's one principle consistent across all human nature, it's that we will always prefer the path of least resistance (i.e., "if you can get away with a half-assed job, do it"). The original plan was for two walkways that were directly on top of one another to both be supported by one very long rod that would anchor into the ceiling. Like so:

This is a highly technical diagram.

Looks pretty simple, right? It all hangs off one long rod, which makes it strong, but also makes it a pain in the ass to assemble -- the rod has to extend through both walkways and then alllll the way up into the ceiling. Just in general, big pieces are hard to work with -- what's easier, to carry a whole assembled desk into your house, or a series of small pieces? The rod also has to be threaded all the way along its length so you could screw that nut up to that top platform spot.

Got to be an easier way, right? So, the steel company in charge of making the rods made a design change by replacing the single rod with two shorter ones, shown below.

The guy with the hat is Rodney, a multimillion-dollar investor. The other is Nutter.

Easier to work with, easier to install, works exactly the same. Right?

That little change killed 114 people, injured 216 more and cost $140 million in lawsuits.

Look at the first image again.

Nutter has an idea for an FTL drive, but also a tool that injects bacon with peanut butter.

One rod, two nuts. Each nut only has to carry the weight of its own platform. Which is good, because each nut (and the welded beam it's screwed to) is only rated to carry the weight of one platform.

Now look at the second image. See the nut we've labeled "OH s**t"?

The twain shall never meet, and civilization is hollower for it.

That one single nut now has to carry the weight of BOTH platforms, and all the doomed tourists standing on them. Look obvious? Congratulations, because none of the professionals at either company caught it.

And so, one night during a dance competition, the stressed "OH s**t" nut cleaved clean through the beam and the walkways collapsed.

Wikipedia
Considering this was 1981 and something called a tea dance, we're willing to rule it a suicide on the part of the building.

During the ensuing lawsuits, it came out that neither the steel company nor the engineering firm in charge of construction had even bothered to do a back-of-the-envelope calculation that would have shown them this glaring flaw.

Until 1942, when a fire killed 492 people.

But the thing is, most of those people died not due to fire, but due to door hinges.

The Laughably Simple Flaw:

A busboy had been groping around for an electrical socket, so he lit a match to see what he was doing. It accidentally lit some of the gaudy tropical decorations, which were ridiculously flammable, and quickly engulfed the club in smoke and flames. The fire burned so quickly that afterward some of the bodies were found sitting with their drinks still firmly in hand.

Wikipedia
"I didn't bow to Prohibition, and I damn well won't bow to this fire."

Of the many safety violations, including overcapacity and decorating the nightclub with dry pine needles, there was one fatal flaw that you wouldn't even think of: Namely, that the exit doors all swung inward.

The main entrance was a revolving door that quickly became jammed with people trying to get out, so they flocked to other entrances and were pressed against the doors so hard that they couldn't open. The fire department estimated that if the doors had swung out, over 300 people could have survived.

Wikipedia
"Form an orderly, single-file line and you'll all be allowed to die at your own pace."

Unfortunately, this was not the first time (nor the last) that having inward-opening doors killed people. The Iroquois Theater, Lakeview Grammar School, Triangle Shirt Waist Factory, Beverly Hills Supper Club and Dupont Plaza Hotel are all examples of mass fire-related deaths because the doors swung the wrong damn way.

If you think you're paranoid for checking which way the nearest exit door swings, don't worry -- we're looking, too.

Among engineering and physics students, it is literally a textbook example of what not to do and how, if you manage to screw up something big enough, no one will ever forget it. So what was wrong with it?

The Laughably Simple Flaw:

The bridge didn't have holes in it.

Ever notice how, well, flimsy large bridges look? You can practically see through the things:

jonpetitt / Getty
Just one more reason catapults are a superior transportation option.

You might have thought it was because the builders are just really cheap and want to save steel, but there's actually a reason for it: to let the wind through.

No matter how sturdily built a structure is, it will still move with the wind; the Burj Khalifa (aka that building Tom Cruise dangles off of in the new Mission: Impossible movie) can sway up to six feet on windy days. You have to account for it.

The Tacoma Narrows Bridge didn't bother with that s**t. Ain't no wind holes here:

Wikipedia
Well, except for that little one there.

Right from the start, people knew something was up. Whenever the wind would start blowing, the air would catch the I-beams and shake the bridge like a paint can. The bridge deck would rise and fall several feet, and crossing it was like driving on a waterbed. It twisted and moved so much that the locals nicknamed it "Galloping Gertie."

Making things worse, the wind often happened to be blowing at just the right speed to get the bridge to vibrate at its natural frequency: a particularly dangerous situation. Realizing that bridges aren't supposed to have seizures, the state hired an engineering professor to try to fix the design mistakes. Among the solutions he proposed was the idea of just drilling some holes in the I-beams so the wind could pass through it without the bridge getting all pissy. An easy fix; we bet they felt silly for not thinking of it earlier.

But, before that or any of the other ideas could be enacted, the bridge collapsed.

Wikipedia
"You know what's really safe? No bridge at all!"

They would rebuild it a decade later. And see if you can spot the simple difference in design:

Wikipedia
It's still flipping the bird at nature. Behind its back.

Human stupidity notwithstanding, most of the criticism focuses on the atrocious lack of safety features, but there is one obscure flaw that was intentionally designed.

The Laughably Simple Flaw:

The center propeller didn't work in reverse.

The Titanic had three steam-driven propellers, with the outer two driven by piston engines and the center screw driven by a steam turbine. Steam turbines have the advantage of generally being smaller and more efficient than their piston counterparts, but have the drawback of being one-way; that is, the steam can only flow forward and the shaft can only turn in one direction.

Wikipedia
It's like trying to force a screw to go in counterclockwise. It's not going to happen.

So when First Officer Murdoch slammed the big girl into full reverse in order to avoid the iceberg, the outer two screws started turning the other way, while the center one just stopped (correctly portrayed in the movie). It sort of makes sense; if you're trying to go backward, you don't want one of your propellers still pushing you forward.

However, the center screw was directly in front of the rudder, and shutting it down meant less water was washing over the rudder, which crippled the ship's handling.

Wikipedia (fr)
Coque indeed.

Had the center prop been designed in such a way that it kept turning in the event of a reversal (or if they hadn't reversed at all), it's pretty likely the ship would have missed the iceberg completely, saving the lives of 1,514 people and eight dogs.

When he's not pointing out other people's failures, Chris writes for his website and tweets.

For more minuscule yet catastrophic mistakes, check out 5 Tiny Computer Glitches That Caused Huge Disasters and The 7 Most Disastrous Typos Of All Time.

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