Douglas Aircraft modified the design into a civil transport longer than the DC-4 . The civil DC-6 first flew on 29 June 1946 , being retained by Douglas for testing . The first airline deliveries were to American Airlines and United Airlines on 24 November 1946 . A series of inflight fires ( including the fatal crash of United Airlines Flight 608 ) grounded the DC-6 fleet in 1947 . The cause was found to be a fuel vent next to the cabin cooling turbine intake ; all DC-6s were modified and the fleet was flying again after four months on the ground
United Airlines Flight 608
United Airlines Flight 608
Douglas DC-6