♪ ♪ ♪ ♪ NARRATOR: The airship Hindenburg.

In 1937, the fastest way to cross the Atlantic.

There was no other aircraft at the time that could do this type of distance.

For the few who could afford an airship trip, the image is one of prestige.

NARRATOR: Prestige also for the country that built it.

The German government used Hindenburg as a propaganda symbol.

NARRATOR: A revolutionary vision of connecting the world with a fleet of airships.

RICK ZITAROSA: They were looking to have 40 to 50 airships linking the cities of the globe by 1945.

NARRATOR: Then, disaster.

36 lives lost in a horrifying instant, an entire industry essentially destroyed, the precise cause never conclusively proven.

Now, after more than 80 years, new evidence.

Whoa, yeah.

No, I've never seen this material.

NARRATOR: And a new investigation.

That's the upwind pattern that it was flying as it's coming overhead the hangar, before it did its initial turn.

And you believe that the film was shot with this exact camera?

Yes, I do.

NARRATOR: Can we still find answers?

KONSTANTINOS GIAPIS: Initially, I thought it was going to be relatively simple.

(loud sparking) Whoa!

But very quickly, I realized that there were a lot of unanswered questions.

What is going on right now?

JASON HARRIS: If one thing had been different on that day, we wouldn't have had the same outcome.

NARRATOR: "Hindenburg: The New Evidence," right now, on "NOVA."

♪ ♪ ♪ ♪ NARRATOR: In a nondescript building in a Washington, D.C., suburb, an investigation begins.

The focus is a cold case over 80 years old that was never definitively solved.

An aviation accident.

The legendary crash of the airship Hindenburg.

HERBERT MORRISON (archival): It's burst into flames-- and it's crashing.

It's crashing, terrible.

Oh, my, get out of the way, please, it's burning, bursting into flames, and, and it's falling on the mooring mast... oh, the humanity!

♪ ♪ NARRATOR: Hindenburg.

Like Titanic, synonymous with disaster.

The images seen countless times by millions.

Despite two investigations of the accident, the precise cause-- the exact chain of events-- remains a mystery.

But now, after more than 80 years, there is new evidence.

This film.

Never seen by investigators in 1937, it has remained essentially unknown.

Now, decades after the tragedy, can this film shed new light on one of the most notorious aviation accidents in history?

Might it hold a clue to the cause of the disaster?

This right here where we're standing right now is the actual crash site, so you can sort of see where we are in relation to it.

Got it.

NARRATOR: These men hope so.

The discovery of this long-lost film has prompted them to begin a new investigation of Hindenburg.

♪ ♪ Lieutenant Colonel Jason O. Harris, an Air Force Academy graduate, flew multiple combat tours and has training in accident investigation.

Today, he's a commercial airline pilot.

When we look at aircraft accidents, whether it's an airship or an airplane or even a helicopter, you want to establish a chain of events.

When we evaluate it, we get to see exactly where things begin to break down.

And they were going to fly basically toward the hangar in this direction... NARRATOR: Harris's colleague in this investigation is aviation historian Dan Grossman.

A bestselling author and world-renowned authority on airships, Grossman has extensive knowledge of Hindenburg and the 1937 investigations.

No one's ever taken a fresh look at the expert conclusions, either based on testing or based on the experience of these experts, and it's time to do that.

NARRATOR: So, inspired by a newly found reel of film, Dan Grossman and Jason Harris are re-examining the case of the Hindenburg.

They'll work with specialists who have expert knowledge about vintage motion picture film, travel to Germany to examine evidence where the airship was built, and observe specially designed engineering tests to see if anything new can be learned about Hindenburg.

♪ ♪ It's May 3, 1937, when the airship takes off with 36 passengers and 61 crew members.

GROSSMAN: The ship left Germany on May 3, intending to arrive at Lakehurst early in the morning, about 6:00 a.m., on May 6.

NARRATOR: Lakehurst, New Jersey, is a U.S.

Naval Air Station and hub with connections to New York.

At Lakehurst, Hindenburg will be serviced for the return to Europe.

GROSSMAN: They were hoping to have a day to turn the ship around, refuel, replenish.

And they were planning on leaving that evening with a full load of passengers back to Germany.

NARRATOR: Settling in for a comfortable and scenic two-and-a-half-day trip, the 97 people onboard are probably feeling quite safe.

In over 25 years of service, no Zeppelin passenger airship has ever had a fatal accident.

And yet right over their heads lurks potential danger.

What lifts Hindenburg into the air is hydrogen gas.

Seven million cubic feet of it are stored in 16 gas cells, giant bags that fill the ship from end to end.

Hydrogen is the lightest element on the periodic table.

Because it's lighter than air, it's buoyant.

It will go up if surrounded by air.

NARRATOR: But mixed with air, it's also extremely flammable, a bomb waiting to explode.

GROSSMAN: Everyone knew that hydrogen burned, and it burned furiously.

But the Germans had the feeling, this overconfidence, that after 37 years of working with hydrogen, "We got this, we know how to deal with hydrogen safely."

♪ ♪ NARRATOR: It's 1909 when the German Zeppelin company starts the world's first passenger airline.

Two decades later, just after Charles Lindbergh crosses the Atlantic, their airship Graf Zeppelin makes an international publicity flight.

ZITAROSA: In 1929, the Hindenburg's predecessor, the Graf Zeppelin, flew from Germany here to Lakehurst with paying passengers, and then did a circumnavigation of the globe.

Their idea, their vision, was that they were going to have a fleet of these ships crossing weekly in the same way that there was a fleet of ocean liners that crossed weekly.

NARRATOR: Over the next several years, the Graf Zeppelin carries thousands of passengers without a single mishap, and proves the concept.

The next step: expand to the U.S.

They had already established service with the Graf Zeppelin to South America.

It was a tremendous public relations and investment opportunity for German airship interests.

NARRATOR: All they need now is more and bigger ships.

The Hindenburg will be the first of the new model.

♪ ♪ It's over three times longer than a 747, constructed around a lightweight aluminum frame.

Hindenburg basically was a metal framework that was kind of an engineering miracle in that it had to be very big, it had to be very strong, and it had to be very, very light.

NARRATOR: Outside the frame, a painted fabric skin.

GROSSMAN: The fabric covering was there to give it an aerodynamic shape and to protect the gas cells that were inside the covering.

NARRATOR: Two diesel engines on each side propel the ship through the air.

A rudder steers it left and right; elevators up and down.

The crew controls the ship from a small car mounted to the underside.

Above the control car, inside the skin and beneath the gas cells, are two decks.

The lower holds a few passenger cabins, kitchen, and crew's quarters.

The upper, 25 double-berth cabins, a lounge, writing room, dining room, and promenades.

It was definitely a, a rich person's luxury way of travel, sailing above the great sights of ocean and Earth with glasses of wine in their hand, eating gourmet meals, looking down.

NARRATOR: And it's fast.

GROSSMAN: You could cross the Atlantic in two-and-a-half days on Hindenburg.

It took you five to six days on an ocean liner.

ZITAROSA: The Hindenburg was the Concorde of its day.

It was a premium-priced service particularly popular with American businessmen who were always in a hurry.

NARRATOR: Starting in 1936, the Hindenburg makes propaganda flights for Germany's Nazi government at Nuremberg rallies and the Berlin Olympics.

That year, the ship crosses the Atlantic 34 times.

Hindenburg has carried over a thousand passengers without a single mishap.

The 1936 service was a testing period to see if this thing could be made to work.

And it worked very successfully.

NARRATOR: For the 1937 season, there is one overriding priority.

ZITAROSA: The key in the mind of the Germans was to now tighten up the schedule and make for more prompt arrivals and departures.

NARRATOR: But on the very first flight, the schedule slips.

The first problem is bad weather all the way across, which delays them.

GROSSMAN: And so the ship was about 12 hours behind schedule.

ZITAROSA: They arrive over Manhattan that afternoon and they head directly to Lakehurst.

Lakehurst cannot receive them, and the weather conditions are unsettled.

There were thunderstorms.

(thunder claps) HARRIS: You're trying to get this airship on the ground, you're now under a lot more stress than you ordinarily would be.

NARRATOR: Hindenburg circles over New Jersey in a holding pattern, waiting for Charles Rosendahl, commander at Lakehurst, to approve landing.

As 7:00 is approaching, Commander Rosendahl signals that conditions now suitable for landing, recommend landing now.

NARRATOR: The ship begins its final approach.

ZITAROSA: The Hindenburg makes a wide circle of the field and approaches from the north.

MORRISON (archival): Well, here it comes, ladies and gentlemen, we're out now, outside of the hangar... NARRATOR: Reporter Herbert Morrison is recording a description of Hindenburg's arrival for later broadcast on radio.

MORRISON: Thousands of people have come out to witness the landing of this great airship.

The barometer is dropping, the wind is shifting.

(wind blowing) GROSSMAN: It made a turn to realign so its nose was pointing into the wind.

They dropped two lines, called trail lines.

NARRATOR: The lines let the ground crew pull the ship into position and secure it.

In subsequent investigations, these ropes will come under intense scrutiny.

Roughly four minutes after dropping these landing lines, fire erupted.

(flames billowing, panicked voices) ♪ ♪ (screams) ♪ ♪ NARRATOR: In less than a minute, there's nothing left but smoking wreckage.

Of the 97 passengers and crew, 35 are dead, plus one ground crewman.

(speaking German): NARRATOR: What happened?

Even before any investigation starts, Hindenburg's commander, Max Pruss, says what many are thinking.

My grandfather was in charge as a captain on the Hindenburg flight, and he made no secret of his opinion that it was sabotage.

That someone must have placed a bomb somewhere.

NARRATOR: Pruss's boss, Ernst Lehmann, agrees.

It had to be sabotage.

If you're one of the German officers who made the decisions that lead to that public destruction of this symbol of Nazi power, you're going to be very careful about what you say.

Lehmann had been mortally injured.

He died within 24 hours.

He reportedly said on his deathbed that he thought it must have been sabotage, that it could not have been something else.

There were a lot of people, even in 1937, who didn't like the Hitler government.

It was perfectly natural for people to ask, "Did somebody bomb this airship?"

♪ ♪ NARRATOR: Two investigations begin, one German, one American.

But no one finds any evidence of foul play.

The reality is that all evidence suggests it could not have been sabotage.

NARRATOR: If not sabotage, then what?

GROSSMAN: They looked at a lot of things.

Diesel engine exhaust, or a propeller breaking and entering the airframe, or someone from the ground shooting at the airship, but both agreed that it was leaking hydrogen ignited by some electrostatic discharge.

(thunder rumbling) NARRATOR: Electrostatic discharge-- a sudden flow of electricity between two electrically charged objects.

In other words, a spark.

It can be tiny-- like the spark you feel when you walk across a carpet and touch something.

(static) Or enormous, like lightning.

(thunder rumbling) Although investigators eventually conclude that leaking hydrogen was ignited by a spark, they never precisely demonstrate the cause of the spark.

But the source of the hydrogen is obvious: a leak, somewhere in one of the gas cells.

A surviving crewman reported that he saw an orange glow in gas cell four, near the tail.

Observers on the ground also saw the first flames near the tail.

With so much of the physical evidence destroyed, investigators have to rely on these eyewitness accounts.

But there is one other type of evidence.

About a dozen press and newsreel photographers were covering the landing.

MORRISON (archival): The landing crew of the airbase here is superbly trained to handle these massive ships of the sky.

Safety comes first, as it always should.

NARRATOR: But for investigators, every image of the accident caught on film has the same limitation: they're all shot from the same angle.

All of the newsreel photographers were gathered in a small area close to the mooring mast where the ship was expected to land.

NARRATOR: Not only are the films shot from the same place and angle, they all start at essentially the same time: after the fire is well underway.

There's no film capturing the moment of ignition.

So for over 80 years, the origin of the spark that doomed Hindenburg has remained elusive, what exactly caused it and where in the ship it occurred lost to history.

But now, a new piece of the Hindenburg puzzle has surfaced.

Ironically, it was available from the beginning, but no one had been interested at the time.

GROSSMAN: I was here at Lakehurst for the 75th anniversary, and we had a memorial service, and a guy comes up to me and says, "I've got some film on the Hindenburg disaster.

"You probably don't really care, "but this was taken by my uncle, and if you want to see it, I'll show it to you."

So this is right where we met in...

This is right where we met.

In 2012.

Yeah.

Where you showed me this film on your laptop.

Yeah.

And if you remember, I was so excited, I took my cellphone and I took some photos-- I asked your permission-- and I took photos of the film on your laptop.

Yup, yup.

Because it was, like, this is special!

Yeah.

My dad had bought this nifty Kodak camera, a wind-up movie camera, eight-millimeter.

And he couldn't come because he worked.

So he asked my uncle and my mom if they would take some shots and see the Hindenburg land.

And as soon as I started looking at it, I realize it looked really different, and it looked really interesting.

NARRATOR: And yet, Harold Schenck's film-- which starts earlier and is shot from a different angle than all the other photographers-- is never seen by investigators.

SCHENCK: It was, at the time, publicly put out that he had it.

Nobody ever asked for it.

There was plenty of footage taken by the newsreels.

And nobody really cared, I guess, about angles.

NARRATOR: But perhaps this new angle will make a difference.

After 80-plus years, might this footage show something new?

And what could a closer inspection of the film reveal?

To learn more about the film's history, Dan Grossman brings it to Colorlab, a world-class facility that restores historic film for the Library of Congress, National Archives, and others.

PAT DOYEN: I'm excited that you have something for me to look at, right?

I am excited for you to look at it.

So, here is the film we've been talking about.

Wow.

And I also brought you the camera that it was filmed on.

Oh, wow!

NARRATOR: Film archivist Pat Doyen is an expert in preserving and restoring rare vintage film.

DOYEN: Good provenance here.

And you believe that the film was shot with this exact camera?

Yes, I do!

I can see that this is the kind of box that this film would have been packaged in.

I can see that you had it processed by Kodak, uh, there's an address, there's a stamp from the time.

So this is all really good information.

(reel creaking) And when we look at it over the light table, there's a few things we can tell.

Now, there's a number here, 36814.

Oh, okay.

That was written on the box, and you can see it's also on this leader.

And who wrote that?

Would Kodak have written that?

That-- yes.

That would have been for processing.

Okay.

So, right now, I'm going to look for what they call a date code.

So Kodak put, um, some symbols on the film to tell us when it was manufactured.

So I'm looking at the date code, and I see a triangle square.

So how do you know what a triangle and a square means?

So there's a reference to check that out.

And we can see...

This film was manufactured between July to December 1936.

Ah.

NARRATOR: 1936, the year before the accident.

When someone would buy a film for 1937.

Great.

We can see the aperture plate, the little cutout on the left side.

NARRATOR: The camera's aperture plate defines the frame of the picture where the image extends in between the sprocket holes.

DOYEN: This one here, which matches our film, has the square in between the two perforations.

Is that, is it exactly what we're seeing right here?

Mm-hmm.

Oh, yeah, of course, it looks just like your book.

It tells us that it was shot with this model of camera, the Cine Kodak 8, model 20.

NARRATOR: A year before the disaster, in an eerily prophetic ad featuring the Hindenburg, Kodak suggested using their cameras to film "moments that make history."

It also tells me that it was camera-original.

NARRATOR: Camera-original.

This film was exposed in a camera-- it's not a copy.

If it was a print, you wouldn't see the circles or the squares, because the printer blocks that off.

GROSSMAN: So what's your verdict on the film?

So...

It's a little shrunken and it's got some aging here, it's got a little silver mirroring, which tells me that it's an old film.

This doesn't happen right away, overnight.

It takes years and years, sometimes decades, so all of this taken together, I can't say with a hundred percent certainty, but everything points to this film being an authentic film.

Wow.

DOYEN: That it was shot at that time.

This is a good day.

♪ ♪ NARRATOR: After digitally scanning the film, Dan and Pat take a look on a large screen.

This is the first time this footage has been widely seen.

(film reel whirring) Wow!

Look at how much detail we get from this scan.

NARRATOR: The roll of film will last only two minutes.

To conserve it, Harold Schenck shoots brief moments: the ground crew assembling, the giant ship passing over the hangar.

The landing lines are the last thing Harold Schenck records before disaster strikes.

And as it exploded, he had the camera at his side, and it was a wind-up camera, so he, he had the presence of mind to switch the switch on and pick it up at that moment.

♪ ♪ Thanks to that aperture plate, you actually see the nose and the tail at the same time.

Is that unusual?

Yes, it is.

NARRATOR: The spring runs down.

After rewinding, he rolls again, getting the aftermath.

GROSSMAN: You can see details of the girder structure.

Where the gas cells were would be a lot of information for us about how this flame progressed.

This is really great, thank you for doing this for us.

NARRATOR: Confident of the film's provenance, Dan now shares the new digital transfer with Jason.

GROSSMAN: You can see the mooring mast.

There's the ship, it's flying over the building we're in right now, that's hangar one.

That's the upwind pattern that it was flying as it's coming overhead the hangar before it did its initial turn.

NARRATOR: The sequence of events during Hindenburg's landing approach has clues about what went wrong.

Surviving crew members indicated that they were having trouble trimming the ship-- keeping it level.

The tail was heavy.

When an aircraft is out of trim, it's not in balance.

And when you look at how massive this aircraft was, and then try to control it, and it's out of trim, it is not going to do what you're asking.

NARRATOR: To correct the problem, they valve off gas from the bow, making it heavier.

Depending on how heavy you wanted to make the ship, you held the gas valve open for 15 seconds, 30 seconds.

NARRATOR: They release gas multiple times.

They're still tail-heavy.

Then they drop weight, water ballast, from the tail to make it lighter.

ZITAROSA: They've already dropped about 1,300 pounds of water ballast.

Now they've moved six men into the nose.

That's another 1,200 pounds.

The ship is still tail-heavy.

NARRATOR: Why might Hindenburg be tail-heavy?

It seems most likely that it was tail-heavy because there was a pre-existing hydrogen leak.

♪ ♪ NARRATOR: They now have a choice: proceed with the landing or stop and diagnose the problem.

ZITAROSA: It would have been relatively simple to send a few riggers back to look into the condition of whether the rear gas cells were all intact.

If they needed to wait longer, they could have just hung out and waited longer.

It's not an issue of running out of fuel.

It could stay up there for an indeterminate amount of time because it's an airship.

NARRATOR: But who will make the decision?

The official commander of the flight was Max Pruss, but the director of flight operations, or the chief pilot, Ernst Lehmann, was also onboard.

So Pruss was operating under the eye of his boss.

Lehmann was very, very conscious of the fact that they were 12 hours behind schedule, and they had a full load of passengers that had to get onboard and get back to Europe, and this was now his ball game.

How do you tell your boss, "Hey, boss, we're late, and I actually want to make us more late"?

"I know we're supposed to land, but I don't think it's safe."

GROSSMAN: There's no cockpit voice recorder from Hindenburg.

We don't know what they said to each other.

NARRATOR: All we know is what they did.

There's the line going down, you see it hit the ground.

And the ship started to burn, and look how quickly this crashes, right?

In the time we have just talked about this for the past few seconds, that is all the time these people had to escape.

Wow.

That's totally different than anything I've ever seen from all the other footage I've seen.

Right.

Because the person with the eight-millimeter camera was in a different location.

HARRIS: So, so from what you've... NARRATOR: Where exactly was Harold Schenck?

GROSSMAN: Most of the press photographers and all of the newsreel film photographers were over in that direction where the mooring mast was.

♪ ♪ GROSSMAN: It looks from Schenck's photographs that he was located around hangar one.

HARRIS: So he basically is seeing the aircraft go from right to left as it continues to go down to this landing site.

Exactly.

And so, because he was all the way over there, he got a beautiful broadside view of Hindenburg.

As opposed to the newsreel photographers, who were looking at the bow of the aircraft as it was flying toward them.

HARRIS: Right.

♪ ♪ NARRATOR: But for all it reveals, Harold Schenck's film does not show what ignited the hydrogen, the spark that doomed Hindenburg.

GIAPIS: How did the spark actually find its way to the location in this enormous airship, where actually hydrogen was coming out, mixing with air?

NARRATOR: To try and learn more about that spark, Jason and Dan have turned to Konstantinos Giapis, professor of chemical engineering at Caltech.

GIAPIS: You see almost a mushroom cloud right here.

And this is hydrogen being released massively from the central airbags.

That hydrogen wants to rise up because it's a very light gas, and as it rises, it takes a lot of heat with it.

When you look at this, it's almost uncanny to think that anyone actually was able to walk away from this.

Well, you know, if you happen to be underneath the fire, you, you don't suffer as terrible consequences, and I believe this is the reason why so many people survived, actually.

NARRATOR: But the key question remains.

I see a few things, but I don't see the origin of the fire, I don't see how the fire started.

NARRATOR: So Professor Giapis will design experiments to learn more about how the fire started.

GIAPIS: The experiments should include addressing the origin of the spark, addressing the importance of the rope falling and becoming conductive, and addressing the issue of, how did a spark happen close to where the hydrogen was leaking?

(jet engine roaring) NARRATOR: To get more information to help Professor Giapis design historically relevant experiments, Jason and Dan travel to Friedrichshafen, Germany, home of the Zeppelin company and Zeppelin Museum.

Hindenburg made its first test flights over this lake.

The Zeppelin and the industry that it set off was a really important part of the town's history.

HARRIS: This is amazing.

Just walking in and seeing that airship hanging...

Isn't it incredible?

Yeah.

NARRATOR: Dan has been coming here for years; this is Jason's first visit.

HARRIS: I didn't know very much about lighter-than-air aircraft.

I've read a few things, but my background is all fixed-wing aircraft.

And so I was looking to fully understand how the airships worked, and even some of the different concepts in terms of how the airship was constructed.

WAIBEL (in German): GROSSMAN: And you know, this World War I exhibit really gives you an understanding of just how experienced the Germans were with Zeppelins.

And it actually explains a lot about their confidence and overconfidence operating Hindenburg, because they had flown these hydrogen-filled Zeppelins for 37 years.

They'd flown over 100 of them.

NARRATOR: Nevertheless, Hindenburg is not the first Zeppelin to burn.

There were a lot of hydrogen airships that burned, even outside of combat, as a result of operating accidents.

NARRATOR: In fact, the Zeppelin company was hoping to abandon hydrogen, because of the danger.

ZITAROSA: The Hindenburg had originally been designed with the intention of using helium gas.

However, helium was a strictly American resource in those days.

Most of the world's helium supply existed within a 250-mile radius of Amarillo, Texas.

NARRATOR: In 1927, Congress passes the Helium Control Act, which forbids selling helium to any foreign nation.

If Hindenburg's designers want to use helium, they'll need Congressional approval.

(in German): NARRATOR: Of all the resources in this museum, Dan and Jason are most interested in the historical archive.

But first, they show Harold Schenck's film to Zeppelin Archive director Barbara Waibel and Zeppelin department head Jürgen Bleibler.

Whoa, yeah, I've never seen this material.

You can see it so clearly, how, how the way of the fire is.

BLEIBLER: This moment... WAIBEL: Mm-hmm.

BLEIBLER: Escaping for, of the passengers is unbelievable.

GROSSMAN: Yeah, isn't it?

They had so little time.

WAIBEL: Mm-hmm.

I've never seen it from this point of view.

So it's really new material for me, yeah.

NARRATOR: The fire started roughly four minutes after the landing ropes hit the ground.

So Professor Giapis is interested in that rope.

Could it conduct electricity, which might contribute to a spark?

One of the things we'd like to do is test the electrical conductivity of the trail rope, the landing rope, the Landestau.

Mm-hmm.

We'd like to get a sample of that rope and see what it looks like.

WAIBEL (in German): Excellent.

HARRIS: And is this one of the actual ropes?

(in German): Ah.

GROSSMAN: So let's go ahead and see how, how big the rope is, what, what its circumference is, so that we can either acquire or recreate something that matches.

14 centimeters.

Mm-hmm.

Mm-hmm.

Excellent.

And this is... (speaking German) 14 centimeters, and it's manila hemp rope, right?

Yeah.

Yup.

NARRATOR: Back at Caltech, Professor Giapis has immersed himself in Hindenburg, focusing on how the leaking hydrogen may have been ignited.

GIAPIS: I read the reports of various committees.

They both agreed that there was a hydrogen leak.

But there were certain things that didn't make sense.

How did the spark happen, where it happened, and the time sequence, the timeline of how it happened.

NARRATOR: The German committee believed the landing ropes allowed a spark to happen, because they gave electricity a path from the ship to the ground.

In a house, electricity flows from one side of an outlet through whatever is plugged into it, and back to the other side of the outlet.

But it only flows when it has a path.

Take away the path, the flow stops.

Why this matters to the Hindenburg is because the airship is carrying electricity on its skin.

Any craft moving through the air will accumulate a charge.

NARRATOR: As long as Hindenburg's electrical charge has no path, it can't flow.

To find out if landing rope could create an electrical path to the ground, Professor Giapis will test a sample to see if it conducts.

Jason is back at Caltech to observe.

So, what you were looking for was, how did the spark in that particular place connect with the hydrogen in that explicit moment in time?

Yes.

The committees talked about the skin charging up.

And the question is, what happens to that charge?

I can try to find out where the charge goes and whether, in doing so, it can create a spark.

NARRATOR: Hindenburg approaches, carrying a powerful electrical charge on its skin.

But the charge has no path to go anywhere.

Port and starboard trail lines hit the ground.

NARRATOR: But nothing happens.

From the moment the ropes touch the ground, it takes about four minutes for the fire to start.

If the ropes created a path for electricity to flow, then why the delay?

What was important about the rope and the four minutes?

So, the idea from both investigative committees was that the rope was not conductive to begin with.

It took four minutes or so for it to get wet to create the spark.

NARRATOR: During the final landing approach, a light rain is falling.

The theory is that as the rope got wet, it became more conductive.

So I want to probe that.

I want to find out if the rope was initially conductive at all, and how quickly did it become conductive when it became wet?

Where'd you acquire this rope from and how is it similar to what they had 80-plus years ago?

So, we had to search, uh, quite a bit, actually, to find this rope.

However, we found one that is made of the same material, which is manila hemp.

And this one is an eight-braid rope, whereas the original one was a 12-braid rope.

But it's approximately the same diameter.

And it has a lot of surface area, which is important for our experiment.

My first experiment was, try to see if any current flows through it when you apply a voltage across the, the two ends of the rope.

I will increase the voltage that I apply at the top.

NARRATOR: Professor Giapis applies almost 3,000 volts to the top end of the rope.

GIAPIS: What you see here is something that I think is pretty remarkable.

We see a current flowing through the rope when we apply...

It's almost three kilovolts.

GIAPIS: To my immense surprise, dry rope had some conductivity.

Now, when I talk about conductivity, what we're talking about is the ability to ground the airframe.

NARRATOR: So even dry rope provides an electrical path from the ship to the ground, which, theoretically, could trigger a spark.

But the test isn't over.

Now we want to find out what happens when I make this wet.

So we have the same voltage we have dialed before, about three kilovolts, and I will make this wet.

So I'm using deionized water to try to simulate the absorption of water by the rope.

Pay attention to this.

So it's increasing with every bit of wetness.

With every bit of water, you add to it, it's increasing.

And so you figure, for four minutes, it was constantly having this done with four minutes of rain and moisture.

So it becomes very conductive.

NARRATOR: Over ten times more current flows when the rope is even slightly wet.

Now that it's wet, let's look at what happens as we come down.

You see that the voltage now, two inches below, is about the same.

As I come down, that higher voltage is communicated.

This thing is fully conductive.

NARRATOR: So wet or dry, the landing rope does conduct electricity.

But how would that cause a spark?

The Zeppelin is flying.

She's got an electrical charge that she has picked up.

NARRATOR: But the charge on Hindenburg's skin can't go anywhere-- yet.

ZITAROSA: The airship is isolated from the ground.

The mooring ropes are dropped.

They become conductors.

NARRATOR: But there's a problem.

GIAPIS: My very first experiment showed that the rope had some conductivity, and for the kinds of voltages that I think were possible on the airship, that conductivity meant that... (snaps) The explosion should have happened the moment the rope hit the ground.

NARRATOR: So once the ropes hit the ground, what explains the four-minute delay before the explosion?

Dan and Jason found a clue in Germany.

In the Zeppelin Museum, they got details of Hindenburg's skin and the paint that covered it, called "dope."

Let's talk about the dope, the Cellon that went onto the fabric.

NARRATOR: The Cellon dope paint is what gave Hindenburg its metallic sheen.

WAIBEL (in German): NARRATOR: But it's the electrical properties of Hindenburg's skin that concern Professor Giapis.

GROSSMAN: Barbara, one of the things we care about is whether there was an electrical connection between the, the fabric and the metal.

WAIBEL (in German): Right.

WAIBEL (in German): Right.

WAIBEL (in German): NARRATOR: The wooden pegs, and the space between skin and metal frame, would theoretically prevent a charge on the skin from reaching the frame.

It's crucial information for Professor Giapis.

It seems to be that this wooden dowel was actually put there to separate the skin as a protection/safety mechanism in the building of the airship.

NARRATOR: This design means there's no electrical connection between skin and frame.

As the ship comes in to land, the skin is electrically charged.

When the ropes drop, the frame is electrically connected to the ground.

So there's now a powerful charge right next to a grounded frame, with a small air gap in between.

It's like a person who crossed a carpet almost but not quite touching the light switch.

A spark waiting to happen.

GIAPIS: So there is electrical communication between the frame and the ground.

So now we need to find out what was happening between the skin and the airframe.

NARRATOR: Professor Giapis wants to better understand how a charge that's built up on the skin could discharge in a spark that jumps to the frame, and why it took roughly four minutes to happen.

GIAPIS: The second test that I developed, tried to understand this charging-discharging issue.

So I developed a scaffold similar to the frame of the original airship.

NARRATOR: He'll use a reproduction of a section of Hindenburg's skin covered with dope, stretched over but not touching an aluminum frame.

HARRIS: So, what are we replicating here in this experiment?

I'm trying to simulate what was happening in the top of the airship.

As this was standing about 100 meters away from Earth, the top of it, at least, collecting rain and collecting also charge from the ambient environment.

I need to figure out a way to bring uniform charge to these two panels that we're seeing here.

And I have done this by creating these electrodes, and I will charge those so that I can apply a voltage that I think was existing at that time on, on the airship.

The airship is grounded.

It has the ability to, to conduct, but the surface is actually just dry.

So, you're simulating what it looks like, or what happens when the surface itself is just dry.

Correct.

NARRATOR: The electrodes apply a charge, like that which would have built up on the skin of Hindenburg.

I charged up the electrodes, connected the frame to the ground, and I would observe no spark.

My dope was very "dielectric," as we say in the jargon.

The charge was not going anywhere.

NARRATOR: With the skin dry, the charge does not jump to the frame.

But these laboratory conditions do not fully replicate the situation at Lakehurst.

Now I want to find out what happens if we actually, you know, do this in the rain.

There was rain falling.

(thunder rumbling) The ship had also just crossed the ocean, and there were salt particles on its surface.

Now, rain and salt make a conductive mixture.

All right.

So, let's see.

(spritzing) Let's wait a little bit.

(sparking loudly) Whoa!

(sparks) What is going on right now?

(sparks) Oh, wow, that was... That's it.

Yes, that was significant!

That's the spark that matters.

GIAPIS: Charging the top surfaces, adding the rain to the mix, you've got the spark across the skin.

NARRATOR: But why?

What changes when the skin is wet?

Rain makes the top of the skin conductive and allows eventually for charges to move.

NARRATOR: Making the skin more conductive lets the charge move across it more easily, until it reaches a spot over a frame member, where it can jump across the gap.

But there's still the question of the four-minute delay.

Why didn't the spark happen the instant the ropes hit the ground?

So, then it occurred to me that the moment the airframe grounds, you form a capacitor capable of storing more charge than what initially existed on the surface of the airship.

And that means that it will take time to charge up.

A capacitor is a very simple device that allows you to store energy.

NARRATOR: A capacitor typically contains two conductive plates separated by a non-conducting insulator.

Charge builds up on the plates, positive and negative, until it's strong enough to jump across the gap.

On the Hindenburg, the skin represents the top surface and the grounded frame represents the bottom surface of the capacitor.

NARRATOR: Positive charge from the air collects on the skin.

Negative charge from the ground collects through the ropes onto the frame.

With every passing second, the electric field between skin and frame increases, until finally it's strong enough to jump across the gap, making a spark.

To see how long it would take to fully charge Hindenburg's skin, Professor Giapis calculates how much charge the ship can hold based on its surface area and compares that with the rate of atmospheric electricity flowing in the stormy conditions that day.

So then I wrote down the numbers of how long it would take for it to charge, and I ended up with four minutes.

And then it all clicked, because nobody has been able to explain the four minutes it took for it to explode.

NARRATOR: Rope hits the ground, turning Hindenburg into a giant capacitor.

Charge is building up.

It will take about four minutes to fully charge the ship.

Rain is accumulating on the skin, making it easier for the charge to move to locations of underlying frame members.

♪ ♪ For his final test, Professor Giapis repeats the experiment, adding the rope.

GIAPIS: The rope to the ground as if it's just thrown down.

And then we're going to make the rope wet in the correct sequence.

So we're going to try to find out what happens when all of this is together.

I have zero volts down here.

HARRIS: Yeah.

I have one volt up here.

What is that telling us at this point in time?

It's telling us that it's a perfect conductor.

The frame is connected to the ground very efficiently.

So that allows for a maximum charge to accumulate up there.

So, I will go now and try to recreate the spark.

Ready?

(spritzing) (spark buzzing) HARRIS: Oh!

Whoa!

GIAPIS: That was it!

Tell us, what did we just experience right there?

There is a capacitor forming between the skin and the frame.

The capacitor is fully charged.

But the charge cannot move through the rope to the ground.

Despite the fact that the rope is wet, fully wet.

However, when I drop a little bit of rain on top, magic happens.

NARRATOR: Professor Giapis has shown that rain did contribute to the disaster.

Wetting the skin made it easier... Whoa!

NARRATOR: ...for the charge to flow to where frame members were located.

That's the spark.

That's how you get the spark to occur under the skin.

So, it happens underneath, only after all of these series of events have taken place.

Yes.

The rope hits the ground.

Yes.

The rope then gets wet.

There's a charge on the top of the surface of the airship, and there's rain on top of the airship.

Correct.

So, all of those things have to happen, and we pretty much just walked through...

Yes.

...that one without the other means nothing.

Yes.

But once you put the rain in there, that's where we get the magic of the spark.

The magic ingredient, yes.

Wow.

NARRATOR: But another mystery remains.

Why did the spark happen where it did?

What were the chances in this enormous ship that the spark, the tiny spark, happened right there where the hydrogen was leaking or in the vicinity of where it was mixing with air?

How was it possible to get the spark right there, where, you know, things were happening?

NARRATOR: Professor Giapis believes Hindenburg's frame, horizontal girders, and vertical rings in effect formed individual panels.

I realized that each panel, each crossing of these girders, is a separate capacitor.

NARRATOR: There didn't have to be one spark in just the right place.

Why?

Because there were multiple sparks!

One of them was bound to happen near it, because it was happening everywhere!

♪ ♪ NARRATOR: Ironically, the design keeping skin and frame electrically separate, possibly intended as a safety feature, actually made this spark possible.

MORRISON (archival, crying): "I, I can't talk, "ladies and gentlemen.

"Honest, it's just laying there, a mass of smoking wreckage.

"I'm going to have to stop for a minute "because I've lost my voice; this is the worst thing I've ever witnessed."

NARRATOR: Ultimately, although a spark almost certainly caused the fire, it was something else that caused the tragedy.

ZITAROSA: The story of the Hindenburg is a story very familiar, even today, of human error compounded by some very unfortunate circumstances.

The Hindenburg had been put by her command into a great deal of jeopardy.

NARRATOR: After the accident, the Zeppelin company made some design changes in the skin-to-frame attachment, but it didn't matter.

After the Hindenburg disaster, no rigid airship ever carried a paying passenger again.

By the time Hindenburg actually left its hangar, there were airplanes that could do things better.

NARRATOR: Although Harold Schenck's film did not show how the hydrogen ignited, it did inspire a new examination of Hindenburg, new experiments, and new results.

Wait a little bit... (sparking) Whoa!

So the science gave us an answer to a previously unsolved question that was 80-plus-years-old that we thought we'd never be able to answer.

GIAPIS: There is an opportunity here to use science to answer an unsolved mystery.

We come up with a new theory, we break it apart into pieces, we go to the lab, and we try to validate every one of these pieces.

NARRATOR: Yet no matter how many questions we answer about the details of what happened, it's the image of Hindenburg that never loses its grip on our imagination.

Today, we're used to seeing horrible stuff on television.

People in 1937 were not used to seeing a disaster with their own eyes.

And to see this airship filled with people burn and be destroyed in a matter of seconds was really shocking and dramatic.

I think the fact that this disaster was caught on film is why we still think of it today.

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