The de Havilland DH.66 Hercules was a British 1920s seven-passenger, trimotor airliner built by de Havilland Aircraft Company. With the Hercules, Imperial Airways took over responsibility for the airmail service from the Royal Air Force, which had been operating the obsolete Airco DH.10 Amiens. The Hercules effectively provided long-distance service to far-flung regions for Imperial Airways. Although slow, they pointed the way for future airliners. Design and development The Hercules was designed for Imperial Airways when it took over the Cairo–Baghdad air route from the Royal Air Force. The Hercules was a three-engine two-bay biplane with room for seven passengers and the ability to carry mail. Using three engines reduced the risk of forced landings over remote desert areas. To reduce the risk of deterioration in tropical areas the fuselage was a tube steel frame, with the cabin and rear baggage compartment of plywood mounted inside it. The two pilots were in an open cockpit above the nose while the cabin had room for a wireless operator and seven passengers. With a contract for five years to run the Cairo to Baghdad air mail service, and a requirement to start a service between Cairo and Karachi, Imperial Airways ordered five aircraft. In June 1926, while the prototype was still being built, the type name Hercules was chosen in a competition in the Meccano Magazine. The prototype first flew on 30 September 1926 at Stag Lane Aerodrome. Four were built in 1929 for West Australian Airways with modifications to meet Australian requirements including an enclosed cockpit and seating for 14 passengers while retained space for mail and baggage. Two additional aircraft were built for Imperial Airways in 1929 with the enclosed cockpit, which were retro-fitted to the earlier aircraft. Operational history Imperial Airways Following a period of crew training the prototype left the United Kingdom for Cairo on 18 December 1926 to be based at Heliopolis. The second aircraft left Croydon for Cairo on 27 December 1926, carrying the Secretary of State for Air Samuel Hoare. It flew on to India and arrived in Delhi on 8 January 1927. The prototype, later to be named the City of Cairo by King Fuad, operated the first commercial service between Basra and Cairo on 7 January 1927. The service to Karachi did not start until two years later after permission was granted from the Persian Government. With the start of the service between