Complex sentence: On 16 September 1918 he was piloting one of six De Havilland DH4s on their way to bomb Mannheim when his aircraft was hit by anti-aircraft fire and he was forced to drop out of formation.
Split into simpler sentences: On 16 September 1918 he was piloting one of six De Havilland DH4s that were to bomb Mannheim. Johns' aircraft was hit by anti-aircraft fire and was forced to drop out of formation.