Fire of the Zeppelin LZ 129 „Hindenburg“

As the airship approached the docking mast, it performed a sharp turn to face into the wind, which was shifting.
The engines were run at full speed astern for about a minute to slow the airship. Dust came from the lines as they dropped to the ground, but the lines wetted in the light rain. Only the port line had been attached to a ground winch used for drawing the airship down to ground level for disembarking passengers and cargo. The starboard line was being handled by ground crew at the time of the fire. The nose cable had been lowered about 50 ft. but was not connected to the docking mast when the fire began. The airship never got closer than 700 ft. (horizontal) to the docking mast.

Ground crew saw the outer cover at the tail of the airship fluttering and the skin of the airship seemed to be rippling. Since the propeller slipstream was far below that area, the ship had little headway, and the wind was light, the only reasonable explanation for the cover flutter is that hydrogen was leaking from a gas cell and causing the cover to move. Just after the port line had grown taut, a small tongue of flame emerged from where the skin had been fluttering. Ground crew then noted a red glow. Seconds later, burning hydrogen burst from the top of the airship.
Photographs and witness testimony allowed investigators to conclude that the fire did start at the top of the craft. The stern of the craft was engulfed in flame and began dropping. As the airship bow began pointing skyward, hydrogen flames shot up through the bow like a blowtorch. The entire craft was afire and the frame collapsed. About 32 seconds had elapsed from the time the ground crew noted the red glow until the ship lay smouldering on the ground. Secondary fires, mainly of diesel fuel used for the propeller engines, burned for another 3 hours. As the Zeppelin continued to settle by the stern, the slope continued increasing, allowing faster and faster flame spread.

Leakage and ignition of hydrogen appears the most plausible IINITIATING CAUSE, from indirect evidence gathered by witnesses.
However, there are other theories regarding the main fire.

It is not clear what caused the leak, but the occurrence of a thunderstorm during the docking operation hints at this as damage/ignition cause.

A possible ROOT CAUSE could be the intrinsic fragility of the whole design, and the may be identified in the unavailability in Germany of helium and its replacement with hydrogen.

gas on zeppelin airship

Heavy storm was ongoing


Normal docking procedure was carried out. {Dichristina, M., What really Downed the Hindenburg, Popular Science (1997) 70-76; Tittel L., 1936-1939 LZ 129 "Hindenburg", Schriften zur Geschichte der Zeppelin Luftschifffahrt No. 5, Zeppelin-Museum Friedrichshafen, Germany, 4th edition (1997; Bain A., Schmidtchen U., Ein Mythos verglueht, Warum und wie die "Hindenburg" verbrannte.}

DESCRIPTION OF THE VEHICLE
There were 14 independent gas carrier cells. The wall of that contained the hydrogen consisted of 2 layers of cotton with a gas-tight film (gelatine) in between. This wall had a permeation of 1 l/(m2 day). The storage quantity was 200,000 Nm3 in 16 gas cells. The actual pressure was atmospheric.

It is worth noting that the Hindenburg accident was just one of a number of hydrogen airships that met with an accident involving the hydrogen lift gas. Of 129 hydrogen airships in the early 1900's, over one-third met with that type of accident

Investigation to identify the causes of the accident were conducted for many years.

The American investigators concluded that the first open flame was on the top of the ship forward of the entering edge of the vertical fin over gas cells 4 and 5. The investigators believe that there was no detonation explosion after the fire began, just a very rapid burning of the hydrogen as it escaped from the gas cells. That that it was not an overpressure event, just a fire, is confirmed by the post-accident photographs which show that the docking mast was intact and the few injuries to the nearby ground crew and spectators.
A plausible cause for the leakage was the fracture of a shear wire in the airship hull, a wire might have snapped during the last sharp turn to line the airship nose up with the mast and face into the wind. Possible gas combustion ignitors were discussed, including the pressure sensor, outgoing radio transmissions, mechanical friction heat from the airship structure, chemical reactions, electrical energy and drive engine exhaust. Electrostatic energy seemed the most promising cause. A theory was advanced that a brush discharge from the airship fabric to the grounded part of the airship occurred because of the voltage potential gradients that existed at the field after thunderstorm passage. A brush discharge was proven to be able to ignite hydrogen in tests, and this discharge could not be seen in daylight. The American investigation concluded that a leak in the vicinity of gas cell 4 or 5 formed a combustible mixture and it was probable that a brush discharge ignited the mixture.

The German investigation of the accident concluded that, while definitive causes could not be found, the most probable cause was that a leak developed in gas cell 4 or 5 possibly caused by a failed support wire inside the airship. The leak caused a flammable hydrogen-air mixture to form in the upper part of the ship's stern, where ignited either by (a) a brush discharge after the ship was electrically grounded or (b) the cover did not ground as quickly, allowing a voltage potential difference to develop and create a spark between the fabric and the aluminium alloy frame. The German investigators, including Dr. Eckener, favoured scenario (b). He stated that a leak of hydrogen on the order of 40 to 50 m2/s could cause the sort fabric flutter described by the witnesses. He also stated that this leakage rate would not immediately be noticed on instruments in the control car.