Twin-engined aircraft are subject to extended-range twin-engine operations (ETOPS) regulations, requiring them to operate routes that stay within a given maximum single-engine flying time from a usable diversion airfield. An ETOPS-equipped Boeing 777 is allowed to fly up to 3h from a diversion airfield, plus 15% under certain circumstances. Three- and four-engined aircraft are under no such restrictions.

Boeing is thought to be pushing for either no restrictions or at least 4h ETOPS for appropriately equipped 777s, which would mean that virtually no commercially viable routes would be closed to the type. Airbus says there are still some situations in which a twin cannot provide "equivalent safety" with a four-engined aircraft. For example, it argues, between the North Pole and northern Siberia a twin-engined aircraft would have to divert immediately if one engine had to be shut down; a four-engined aircraft could continue if that were the safer option.

Source: Flight International