Complex sentence: Hewitt could see that the control rod to the elevator trim tab on Learmonth's Beaufort had separated from the tab and was hanging down, allowing the tab and elevator to oscillate and drive the violent shaking of the whole aircraft.
Split into simpler sentences: Hewitt could see the control rod to the elevator trim tab on Learmonth's Beaufort, hanging down. It had separated from the tab, allowing the tab and elevator to oscillate and drive the violent shaking of the whole aircraft.