Complex sentence: The crew member, occasionally two navigation crew members for some flights, were responsible for the flight's navigation, including its dead reckoning and celestial navigation, especially when flown over oceans or other large bodies of water where radio navigation aids were not originally available.
Split into simpler sentences: The crew member, occasionally two navigation crew members for some flights, was responsible for the trip navigation, including its dead reckoning and celestial navigation. This was especially essential when trips were flown over oceans or other large bodies of water, where radio navigation aids were not originally available.