In 1928 Boeing, Pratt & Whitney and Vought formed United Aircraft & Transport Corp., Frederick Brant Rentschler of P&W being president, office at Hartford, CT, where P&W and Vought had large and expanding plants. In 1929 added Hamilton Metalplane (soon becoming Hamilton Standard, leader in propellers), Sikorsky, Stearman, Standard Steel Propeller and 3 airlines which, with Boeing Air Transport, were merged 1930 into giant United Airlines. In 1934 Air Mail Act prohibited any one management from both manufacturing and operating aircraft; United Airlines was hived off and UATC became United Aircraft, comprising Pratt & Whitney, Vought, Sikorsky and Hamilton Standard, while Boeing went free again with Stearman as subsidiary. In June 1935 UAC split into United Aircraft Manufacturing Corp. and United Aircraft Export Corp., latter being international sales/marketing organization. In Second World War 11 companies built UAC products without royalty. On 1 May 1975 UAC changed name to United Technologies as result of progressive acquisition of additional divisions: Chemical Systems (rockets, previously UTC), Essex (automotive), Norden (mainly electronic) and Otis Elevator.