Concorde had considerable difficulties that led to its dismal sales performance. Costs had spiralled during development to more than six times the original projections, arriving at a unit cost of £23 million in 1977 (equivalent to £ million in ). Its sonic boom made travelling supersonically over land impossible without causing complaints from citizens. World events had also dampened Concorde sales prospects, the 1973–74 stock market crash and the 1973 oil crisis had made many airlines cautious about aircraft with high fuel consumption rates; and new wide-body aircraft, such as the Boeing 747, had recently made subsonic aircraft significantly more efficient and presented a low-risk option for airlines. While carrying a full load, Concorde achieved 15.8 passenger miles per gallon of fuel, while the Boeing 707 reached 33.3 pm/g, the Boeing 747 46.4 pm/g, and the McDonnell Douglas DC-10 53.6 pm/g. An emerging trend in the industry in favour of cheaper airline tickets had also caused airlines such as Qantas to question Concorde's market suitability.