Complex sentence: F/Os H. Connelly and J. Hunt had gone up together to practice circuits and landings; as they made a circuit, preparatory to coming in, the Mosquito suddenly stalled and crashed from 500 feet.
Split into simpler sentences: F/Os H. Connelly and J. Hunt had gone up together to practice circuits and landings. As they made a circuit, preparatory to landing, the Mosquito suddenly stalled and crashed from 500 feet.