hydrogen Montgolfier balloon incident

Following the first flight of the Montgolfier brothers, the first severe accident with hydrogen on a Montgolfier-balloon occurred in 1785, when trying to cross the Channel from France to England. Two Frenchman were flying a balloon filled with hydrogen, which was also carrying a small hot-air balloon to control the altitude. This eventually ignited the hydrogen gas demonstrating its flammability. Both pilots were killed in this accident.

The INITIATING CAUSE was the accidental ignition of the hydrogen used in the balloon.

The ROOT CAUSE was a conceptual design failure, which did not took into account he presence of strong ignition source in the neighbourhood of the hydrogen onboard.

Montgolfier

The event was witnessed about 30 seconds after the explosion by two employees 1000 feet away (30 m).
Authorities were notified and calls were placed to other personnel about 30 minutes after the explosion.
Teams of firefighters and emergency medical personnel were sent to the area to verify that no one was injured and to extinguish small residual fires.

The first balloons for transport were using hydrogen as buoyancy gas because it was easy to be produced via electro-chemical process. However, the presence of hydrogen and of a flame onboard was definitively not a good idea from safety point of view. Helium has similar buoyancy characteristics, but it was much more difficult and expensive to produce via separation methods. The modern transport by balloon is using helium, because not reactive.