A few airships have been metal-clad, with rigid and nonrigid examples made. Each kind used a thin gastight metal envelope, rather than the usual rubber-coated fabric envelope. Only four metal-clad ships are known to have been built, and only two actually flew: Schwarz's first aluminum rigid airship of 1893 collapsed, while his second flew; the nonrigid ZMC-2 built for the U.S. Navy flew from 1929 to 1941 when it was scrapped as too small for operational use on anti-submarine patrols; while the 1929 nonrigid Slate Aircraft Corporation "City of Glendale" collapsed on its first flight attempt.