Complex sentence: On 30 April 1922 he was posted as a supernumerary because of illness and by 16 September 1922 he had recuperated enough to be posted back to flight status.
Split into simpler sentences: On 30 April 1922, he was posted as a supernumerary because of illness. However, by 16 September of that year, he had recuperated enough to be posted back to flight status.