an aircraft will still be able to reach a diversion airport using the remaining operational engine, albeit at a reduced speed or altitude. The standard typically applies to long-distance routes over water or remote areas, which were historically restricted to three- and four-engine aircraft.

Twin-engine passenger airliners are typically restricted to areas within a one-hour flying time of a suitable airport on one operational engine. ETOPS-90 certification extends this to a 90-minute flying time,