CONCERN has been raised that the closure of United States military airfields in the North Pacific will affect the operations of airlines flying between Asia and North America. Apart from a safety risk from the reduced number of diversion airports, twin-engined aircraft are not allowed under international civil aviation regulations to be more than 180 minutes from an airfield. Airlines using such aircraft may need to take longer, more northerly routes over the Pacific Ocean that could add millions of dollars to fuel costs and revenue losses from reduced cargo or passenger loads, industry experts said. According to Flight International magazine, the US Department of Defence's base closure programme will close or downgrade the Aleutian Island bases of Adak and Shemya. Also affected are Midway, from which approach aids have been withdrawn, and Wake and Johnston Islands, which face downgrades. The magazine said a joint Federal Aviation Administration and Air Transport Association committee in Washington was considering raising the international limit for flights to 210 minutes from 180. The certification for extended twin-engine operations states that a twin-jet can safely operate for a specific amount of time over water on one engine. As a new aircraft comes into service, it must go through a lengthy programme to receive the certification, which first allows it 90 minutes between airports, then 120 minutes, followed by 138 minutes, and finally 180 minutes. Most aircraft flying Pacific routes are four-engined Boeing B747s and Airbus Industrie A340s, or three-engined McDonnell Douglas MD-11s, that do not require ETOPS. But at least two airlines - Asiana of South Korea and United of the US - fly long-range Boeing B767 aircraft that have two engines. The issue has greater implications for future services, an industry expert said, as Boeing was considering extending the range of its new B777 twin-jet to 18 hours. Dubbed the B777-100X, Boeing is trying to market it as an aircraft best suited for services over the Pacific. The company has sold more B777s in Asia-Pacific than in any other region. 'The list of requirements for flying a long-range twin is very high,' an expert said. 'The airfields need to have adequate facilities and that doesn't only mean having a long strip of concrete. You need people on the ground. [Closures mean] you won't be able to operate long-range twins over those routes without huge diversions - under existing rules. 'There's also the safety aspect. Whether it's a four-engined aircraft or a twin, you need diversion airfields in case there's some emergency like a fire in the cargo hold or whatever. 'With a four-engined aeroplane it's important, but with a twin it's absolutely critical.' Cathay Pacific Airways' director of flight operations, Captain Gerry Clemmow, said the closures were not a great concern because the airline used only B747s on services to North America. But, he said, 'the more [airports] the better'. It may also have an impact in future, as Cathay was considering buying a B777-100X for non-stop services between Hong Kong and New York. 'You may have to fly further north, and sometimes that would mean more fuel,' Capt Clemmow said. 'For us, it's good for the flexibility of operations [to have more airfields], although it's not totally necessary because we don't use twins.' An Airbus official said its view was that 'four-engined aircraft are always the best for ultra long haul services'. Airbus is considering extending the range of its four-engined A340, which already has the longest range of any passenger aircraft, to 18 hour non-stop.