Complex sentence: It was concluded that the main factor for the accident was an error in reading the correct heading from the flight plan by the commander, compounded by the co-pilot copying the setting from the commander's panel instead of checking the flight plan.
Split into simpler sentences: It was concluded that the main factor for the accident was an error in reading the correct heading from the flight plan by the commander. This mistake was compounded by the co-pilot copying the setting from the commander's panel instead of cross-checking the flight plan.