The cause of the infamous Hindenburg crash has baffled experts for over 80 years, with theories about the airship’s fire ranging from deliberate sabotage to a spark generated by the stormy conditions in which it landed. But little-known amateur footage of the crash has resurfaced, showing the airship’s final seconds from a fresh angle and in unrivaled clarity. Taking clues from the footage and other sources, NOVA leads a fresh investigation at a leading scientific lab with eye-opening experiments that point to a final solution of the mystery. (Premiered May 19, 2021)
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Hindenburg: The New Evidence
PBS Airdate: May 19, 2021
NARRATOR: The airship Hindenburg: in 1937, the fastest way to cross the Atlantic.
RICK ZITAROSA (Historian & Curator Director, Navy Lakehurst Historical Society): There was no other aircraft, at the time, that could do this type of distance.
GUILLAUME DE SYON (Professor of History, Albright College): For the few who could afford an airship trip, this was actually quite an opportunity. The image is one of prestige.
NARRATOR: Prestige also for the country that built it.
DAN GROSSMAN (Aviation Historian and Author): 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…
BARBARA WAIBEL (Director of Zeppelin Archives): Whoa! Yeah, I’ve never seen this material.
NARRATOR: …and a new investigation.
LIEUTENANT COLONEL JASON O. HARRIS (Unite States Air Force Reserve Pilot, Aviation Consultant): That’s the upwind pattern that it was flying, as it’s coming overhead the hangar, before it did its initial turn.
PAT DOYEN (Archival Technician, Colorlab): And you believe that the film was shot with this exact camera?
DAN GROSSMAN: Yes, I do.
NARRATOR: Can we still find answers?
DR. KONSTANTINOS GIAPIS (Professor of Chemical Engineering, California Institute of Technology): Initially, I thought it was going to be relatively simple.
JASON HARRIS: Whoa!
KONSTANTINOS GIAPIS: But, very quickly, I realized that there were a lot of unanswered questions.
JASON HARRIS: What is going on right now?
If one thing had been different on that day, we wouldn’t have the same outcome.
NARRATOR: Hindenburg: The New Evidence, right now, on NOVA.
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 (Radio clip): It’s burst into flame! And it’s crashing, crashing, terrible! Oh, my. Get out of the way, please! It’s burning. It’s bursting into flames. It’s falling onto the mooring masts. 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?
DAN GROSSMAN: 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.
JASON HARRIS: 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.
JASON HARRIS: 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.
DAN GROSSMAN: 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.
DAN GROSSMAN: 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 reexamining 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.
DAN GROSSMAN: The ship left Germany on May 3rd, intending to arrive at Lakehurst early in the morning, about 6:00 a.m., on May 6th.
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.
DAN GROSSMAN: They were hoping to, 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.
DAN GROSSMAN: 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.
DAN GROSSMAN: Everyone knew that hydrogen burned and it burned furiously. But the Germans had this 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.
RICK 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.
DAN GROSSMAN: 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.
RICK ZITAROSA: 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.
DAN GROSSMAN: 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.
DAN 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 propelled 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, 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.
RICK ZITAROSA: 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.
DAN 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.
RICK 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 ship makes propaganda flights for Germany’s Nazi government at Nuremburg 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.
DAN GROSSMAN: 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.
RICK 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.
RICK ZITAROSA: The first problem is bad weather all the way across, which delays them.
DAN GROSSMAN: And so the ship was about 12 hours behind schedule.
RICK ZITAROSA: They arrive over Manhattan that afternoon, and they head directly to Lakehurst. Lakehurst cannot receive them; the weather conditions are unsettled.
DAN GROSSMAN: There were thunderstorms.
JASON 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.
RICK ZITAROSA: As 7:00 is approaching, Commander Rosendahl signals that “conditions now suitable for landing, recommend landing now.”
NARRATOR: The ship begins its final approach.
RICK ZITAROSA: The Hindenburg makes a wide circle of the field and approaches from the north.
HERBERT MORRISON (Radio clip): 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.
HERBERT MORRISON (Radio clip): Thousands of people have come out to witness the landing of this great airship.
RICK ZITAROSA: The barometer is dropping; the wind is shifting.
DAN 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.
DAN GROSSMAN: Roughly four minutes after dropping these landing lines, fire erupted.
NARRATOR: In less than a minute, there’s nothing left but smoking wreckage. Of 97 passengers and crew, 35 are dead, plus one ground crewman.
BARBARA WAIBEL: Maybe, up until then, one couldn’t really imagine that such a horrible disaster could actually happen. One felt quite safe and that one could navigate hydrogen-fueled airships quite well. I would say it was a rude awakening.
NARRATOR: What happened? Even before any investigation starts, Hindenburg’s commander, Max Pruss, one of 62 survivors, says what many are thinking.
ALEXANDER PRUSS (Grandson of Hindenburg Commander, Max Pruss): My grandfather was in charge as a captain on the Hindenburg flight, he made no secret of his opinion that it was sabotage. That someone must have placed a bomb somewhere.
NARRATOR: Pruss’ boss, Ernst Lehmann, agrees it had to be sabotage.
DAN GROSSMAN: If you’re one of the German officers who made the decisions that lead to that public destruction of Nazi power, you’re going to be very careful about what you say.
You must know a source, may have it in his book.
GUILLAUME DE SYON: Lehman 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.
DAN GROSSMAN: 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.
GUILLAUME DE SYON: The reality is that all evidence suggests it could not have been sabotage.
NARRATOR: If not sabotage, then what?
DAN 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.
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, or, enormous, like lightning.
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 4, 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.
HERBERT MORRISON (Radio clip): 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.
DAN GROSSMAN: All 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.
DAN GROSSMAN: I was here at Lakehurst for the 75th anniversary. 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…
BOB SCHENCK (Nephew of Harold Schenck): This is right where we met.
DAN GROSSMAN: …in 2012…
BOB SCHENCK: Yeah.
DAN GROSSMAN: …where you showed me this film on your laptop.
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.
BOB SCHENCK: Yup. Yup.
DAN GROSSMAN: Because it was like, “This is special!”
BOB SCHENCK: Yeah.
My dad had bought this nifty Kodak camera, wind-up movie camera, 8 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.
DAN GROSSMAN: And as soon as I start 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.
BOB 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. You have something for me to look at, right?
DAN GROSSMAN: I am excited for you to look at it. So, here is the film we’ve been talking about.
PAT DOYEN: Wow.
DAN GROSSMAN: And I also brought you the camera that it was filmed on.
PAT DOYEN: Oh, wow!
NARRATOR: Film archivist Pat Doyen is an expert in preserving and restoring rare vintage film.
PAT DOYEN: Good provenance here. And you believe that the film was shot with this exact camera?
DAN GROSSMAN: Yes, I do.
PAT DOYEN: 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. There’s an address, there’s a stamp from the time. So, this is all really good information.
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.
DAN GROSSMAN: Oh, okay.
PAT DOYEN: That was written on the box, and you can see it’s also on this leader.
DAN GROSSMAN: And who wrote that? Would Kodak have written that?
PAT DOYEN: That, yes, that would have been for processing.
DAN GROSSMAN: Okay.
PAT DOYEN: So, right now, I’m going to look for what they call a date code. So, Kodak put 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.
DAN GROSSMAN: So, how do you know what a triangle and a square means?
PAT DOYEN: So, there’s a reference to check that out. And we can see this film was manufactured between July to December, 1936.
DAN GROSSMAN: Ah.
NARRATOR: 1936, the year before the accident.
PAT DOYEN: When someone would buy a film for 1937.
DAN GROSSMAN: Great.
PAT DOYEN: We can see the aperture plate: that 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.
PAT DOYEN: This one here, which matches our film, has the square in between the two perforations.
DAN GROSSMAN: Is, that is exactly what we’re seeing, right here?
PAT DOYEN: Mmm hmm.
DAN GROSSMAN: Oh, yeah, of course. It looks just like your book.
PAT DOYEN: It tells us that it was shot with this model of camera, the Cine Kodak Eight 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.
PAT DOYEN: It also tells me that it was camera original.
NARRATOR: “Camera original”: this film was exposed in a camera, it’s not a copy.
PAT DOYEN: If it was a print, you wouldn’t see the circles or the squares, because the printer blocks that off.
DAN GROSSMAN: So, what’s your verdict on the film?
PAT DOYEN: 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…
DAN GROSSMAN: Wow.
PAT DOYEN: …that it was shot at that time.
DAN GROSSMAN: 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.
DAN GROSSMAN: 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.
BOB SCHENCK: And as it exploded, he had the camera at his side, and it was a wind up camera, so he had the presence of mind to switch the switch on and pick it up at that moment.
DAN GROSSMAN: Thanks to that aperture plate, you actually see the nose and the tail at the same time.
PAT DOYEN: Is that unusual?
DAN GROSSMAN: Yes, it is.
NARRATOR: The spring runs down. After rewinding, he rolls again, getting the aftermath.
DAN 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.
DAN 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 1.
JASON HARRIS: 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.
JASON HARRIS: When an aircraft is out of trim, it’s not in balance. And when you look at how massive this aircraft was, and to 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.
RICK ZITAROSA: Depending on how heavy you wanted to make the ship, you held a 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.
RICK 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?
DAN GROSSMAN: It seems most likely that it was tail heavy because there was a preexisting hydrogen leak.
NARRATOR: They now have a choice: proceed with the landing or stop and diagnose the problem.
RICK 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.
JASON HARRIS: 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?
DAN GROSSMAN: 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.
RICK ZITAROSA: Lehmann was very, very conscious of the fact that they were twelve 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.
JASON HARRIS: 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.”
DAN GROSSMAN: There was no cockpit voice recorder from Hindenburg. We don’t know what they said to each other.
NARRATOR: All we know is what they did.
BOB SCHENCK: The ship explodes.
DAN GROSSMAN: And the ship started to burn, and look how quickly it crashes. And he gets the early stage of the fire, 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.
JASON HARRIS: That’s totally different than anything I’ve ever seen, from all the other footage I’ve seen.
DAN GROSSMAN: Right. Because the person with the 8-millimeter camera was in a different location.
NARRATOR: Where exactly was Harold Schenck?
DAN GROSSMAN: Most of the press photographers and all of the newsreel film photographers were over in that direction, where the mooring mast was. It looks from Schenck’s photographs that he was located around Hangar 1.
JASON HARRIS: So, he basically is seeing the aircraft go from right to left as it continues to go down to the…
DAN GROSSMAN: 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 over there, looking at the bow of the aircraft, as it was flying towards them.
JASON HARRIS: Right.
NARRATOR: But for all it reveals, Harold Schenck’s film does not show what ignited the hydrogen, the spark that doomed Hindenburg.
KONSTANTINOS 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.
KONSTANTINOS 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.
JASON HARRIS: When you look at this, it’s almost unbelievable to think that anyone, actually, was able to walk away from this.
KONSTANTINOS GIAPIS: Yes, 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.
NARRATOR: But the key question remains.
KONSTANTINOS GIAPIS: 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.
KONSTANTINOS GIAPIS: 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 the spark happen close to where the hydrogen was leaking?
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.
DAN GROSSMAN: The Zeppelin, and the industry that it set off, was a really important part of the town’s history.
JASON HARRIS: This is amazing, just walking in and seeing that airship!
DAN GROSSMAN: Isn’t it incredible?
JASON HARRIS: Oh, yeah.
NARRATOR: Dan has been coming here for years; this is Jason’s first visit.
JASON 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.
BARBARA WAIBEL: If you’re interested in airship travel and airship construction, the Zeppelin archive is the most important archive worldwide. It’s the biggest archive on this topic.
DAN 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 a hundred of them.
NARRATOR: Nevertheless, Hindenburg is not the first zeppelin to burn.
DAN GROSSMAN: 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.
RICK 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.
SIMONE LIPSKI (Public Relations, Zeppelin Museum): They would have preferred to use helium, but helium was only available in the U.S.A. The U.S.A. refused to deliver helium to Germany, so they had to continue working with hydrogen.
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 to Senior Archivist Jürgen Bleibler.
BARBARA WAIBEL: Whoa, yeah. I’ve never seen this material. You can see it so clearly, how the way of the fire is.
JÜRGEN BLEIBLER (Senior Archivist, Zeppelin Archive): This moment, escaping of the passengers is unbelievable.
DAN GROSSMAN: Yeah. Isn’t it? They had so little time.
BARBARA WAIBEL: I’ve never seen it from this point of view, so it’s really a new material for me.
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?
DAN GROSSMAN: One of the things we’d like to do is test the electrical conductivity of the trail rope, the landing rope, the landeseile. And we’d like to get a sample of that rope and see what it looks like.
BARBARA WAIBEL: Here you go. I believe this is a piece of the rope.
DAN GROSSMAN: Excellent.
JASON HARRIS: And is this one of the actual ropes?
BARBARA WAIBEL: It was not part of the accident, but it’s the same type of rope, used for replacement.
DAN 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.
Fourteen centimeters, excellent. Fourteen centimeters, and it’s manila hemp rope.
NARRATOR: Back at Caltech, Professor Giapis has immersed himself in Hindenburg, focusing on how the leaking hydrogen may have been ignited.
KONSTANTINOS 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.
RICK ZITAROSA: 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.
JASON HARRIS: So, what you were looking for was how did the, the spark in that particular place, connect with the hydrogen in that explicit moment in time?
KONSTANTINOS GIAPIS: 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.
DAN GROSSMAN: Port and starboard trail lines hit the ground.
NARRATOR: But nothing happens. From the moment the ropes touch the ground, it takes four minutes for the fire to start. If the ropes created a path for electricity to flow, then why the delay?
JASON HARRIS: What was important about the rope and the four minutes?
KONSTANTINOS GIAPIS: 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.
KONSTANTINOS GIAPIS: 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?
JASON HARRIS: Where’d you acquire this rope from, and how is it similar to what they had 80-plus years ago?
KONSTANTINOS GIAPIS: So, we had to search 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 twelve-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 trying to see if any current flows through it when you apply a voltage across 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.
KONSTANTINOS GIAPIS: What you see, here, is something that I think is pretty remarkable. We see a current flowing through the rope when we apply almost three kilovolts. To my immense surprise, the 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.
KONSTANTINOS GIAPIS: Now, we want to find out what happens when I make this wet.
So, we have the same voltage you 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.
JASON HARRIS: 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.
KONSTANTINOS GIAPIS: So, it becomes very conductive.
NARRATOR: Over 10-times more current flows when the rope is even slightly wet.
KONSTANTINOS GIAPIS: Now that it’s wet, look at what happens as we come down. You see that the voltage now, two inches below, is about the same. As they come down that high 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?
RICK ZITAROSA: 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.
RICK ZITAROSA: The airship is isolated from the ground. The mooring ropes are dropped. They become conductors.
NARRATOR: But there’s a problem.
KONSTANTINOS 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 the explosion should have happened the moment that 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.”
DAN GROSSMAN: 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.
BARBARA WAIBEL: You can see this is the cotton layer, without an additional coat. Then the one with iron oxide to make it more dense, and then the aluminum powder mixed in as well. And the whole thing was pasted over with a final layer to achieve the smoothest surface possible.
NARRATOR: But it’s the electrical properties of Hindenburg’s skin that concern Professor Giapis.
DAN GROSSMAN: Barbara, one of the things we care about is whether there was an electrical connection between the fabric and the metal.
BARBARA WAIBEL: As you can see here, there is no connection between the fabric and the metal.
You can see clearly how the two panels of fabric are connected through the lugs, with rami twine, onto the frame. Of course, they tried to pull it as tight as possible, but there’s a space between the metal and the fabric. They also placed little wooden pegs in between.
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.
JASON HARRIS: It seems to be that this wooden dowel was actually put there to separate the skin, as a protection-slash-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.
KONSTANTINOS 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.
KONSTANTINOS 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.
JASON HARRIS: So, what are we replicating here in this experiment?
KONSTANTINOS GIAPIS: I’m trying to simulate what was happening in the top of the airship, as this was standing about 100 meters away from Earth, off 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 of these two panels that we see 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 the airship.
JASON HARRIS: The airship is grounded. It has the ability 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 dry?
KONSTANTINOS GIAPIS: Correct.
NARRATOR: The electrodes apply a charge, like that which would have built up on the skin of Hindenburg.
KONSTANTINOS GIAPIS: I charged up the electrodes, connected the frame to the ground, and I would observe no spark. My dope was very dialectic, 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.
KONSTANTINOS GIAPIS: Now, I want to find out what happens if we actually, you know, do this in the rain. There was rain falling. The ship had also just crossed the ocean. And there were salt particles on its surface. Now rain and salt make a conductive mixture.
Right. So, let’s see. Let’s wait a little bit.
JASON HARRIS: Whoa.
What is going on right now?
Oh, wow, that was significant!
KONSTANTINOS GIAPIS: That’s the spark that matters. 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?
KONSTANTINOS GIAPIS: Rain makes the top of the skin conductive and allows, eventually, for charges to move.
NARRATOR: Making the skin 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 moment the ropes hit the ground?
KONSTANTINOS GIAPIS: 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.
KONSTANTINOS GIAPIS: 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 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.
KONSTANTINOS GIAPIS: 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.
KONSTANTINOS 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.
JASON HARRIS: Yeah.
KONSTANTINOS GIAPIS: I have one volt up here.
JASON HARRIS: What is that telling us, at this point in time?
KONSTANTINOS GIAPIS: 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?
JASON HARRIS: So, like they’re going back and forth again.
KONSTANTINOS GIAPIS: That was it!
JASON HARRIS: Tell us, what did we just experience right there?
KONSTANTINOS GIAPIS: 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 the rain did contribute to the disaster. Wetting the skin made it easier for the charge to move where frame members were located.
KONSTANTINOS GIAPIS: That’s the spark. That’s how you get the spark to occur under the skin.
JASON HARRIS: So, it happens underneath, only after all of these series of events have taken place?
KONSTANTINOS GIAPIS: Yes.
JASON HARRIS: The rope hits the ground…
KONSTANTINOS GIAPIS: Yes.
JASON HARRIS: …the rope then gets wet. There is a charge on the top of the surface of the airship, and there’s rain on top of the airship. So, all of those things have to happen. And we, pretty much, just walked through that one without the other means nothing.
KONSTANTINOS GIAPIS: Yes.
JASON HARRIS: But once you put the rain in there, that’s where we get the magic of the spark.
KONSTANTINOS GIAPIS: The magic ingredient. Yes.
JASON HARRIS: Wow.
NARRATOR: But another mystery remains. Why did the spark happen where it did?
KONSTANTINOS GIAPIS: What were the chances in this, this enormous ship, that this spark, this 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.
KONSTANTINOS GIAPIS: 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.
KONSTANTINOS GIAPIS: 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.
HERBERT MORRISON (Radio clip): 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.
RICK 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 to the skin-to-frame attachment, but it didn’t matter.
DAN GROSSMAN: After the Hindenburg disaster, no rigid airship 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.
JASON HARRIS: 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.
KONSTANTINOS 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.
DAN GROSSMAN: 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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A NOVA Production by Morningstar Entertainment for GBH Boston.
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This program was produced by WGBH, which is solely responsible for its content. Some funders of NOVA also fund basic science research. Experts featured in this film may have received support from funders of this program.
Original funding for this program was provided by Draper, Analog Devices, Mint Mobile, the David H. Koch Fund for Science, the NOVA Science Trust and the Corporation for Public Broadcasting.
Image credit: (The German passenger airship Hindenburg seconds after catching fire, May 6, 1937.)
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- Guillaume De Syon, Pat Doyen, Konstantinos Giapis, Dan Grossman, Jason Harris, Simone Lipski, Alexander Pruss, Bob Schenck, Barbara Waibel, Rick Zitarosa