Complex sentence: In a test flight for endurance, the aircraft remained airborne and fully operational for 35.4 hours, however the aircraft was designed to remain airborne for a full week in the event of an emergency.
Split into simpler sentences: In a test flight for endurance, the aircraft remained airborne and fully operational for 35.4 hours. Although untested, the aircraft was designed to remain airborne for a full week in the event of an emergency.