on 13 july 1910, a partly cloudy day, the "erbslöh" crashed near pattscheid (today a quarter of leverkusen). the cause that was accepted by the subsequent investigation committee was that a sudden exposure of the envelope to the sun after the ship had been flying under cloud cover and cool morning fog had heated the lifting gas, causing it to expand. the relief valves could not vent the developing excess pressure quickly enough. this allowed hydrogen to leak from the envelope and come into contact with sparks from the propulsion system, causing the gas to explode.