By the Pact of Madrid in 1953, visa requirements were eliminated for US visitors to Spain. This stimulated the commencement of transatlantic flights between Spain and United States the following year. In addition, the amendments made in Montreal to the Convention on International Civil Aviation on June 14, 1954 were liberal to Spain, allowing impetus for mass tourism using charter planes. Iberia responded to the increased traffic by acquiring a fleet of Lockheed Super Constellation long range aircraft and these served the routes to North and South America from 1954. The pressurised twin-engined Convair 440 was introduced to the company's European routes from 1957.