Complex sentence: Once Parkinson had notified everyone on the LZ of their planned route of departure his team mounted their gear and slogged through the mud to the western side of the mountain, where they came to the crashed helicopter, lying on its side on a steep embankment.
Split into simpler sentences: Once Parkinson had notified everyone on the LZ about their planned route of departure his team moved out through the mud to the western side of the mountain. There they found the crashed helicopter, lying on its side on a steep embankment.