Complex sentence: The next available break in the weather was to be the 21 September and the aircraft was fueled during the previous night from 50 barrels of gasoline, when the aircraft was weighed it was found to be overweight.
Split into simpler sentences: The next available break in the weather was to be the 21 September and the aircraft was fueled during the previous night from 50 barrels of gasoline. When the aircraft was weighed it was found to be overweight.