Complex sentence: In September 1935 he was posted to the Fleet Air Arm where he spent a year in the Mediterranean flying from the aircraft carrier HMS "Courageous" which had been converted from a battlecruiser to a carrier as a result of the Washington Naval Treaty.
Split into simpler sentences: In September 1935 he was sent to the Fleet Air Arm where he flew from the aircraft carrier HMS "Courageous" in the Mediterranean. The ship had been converted from a battlecruiser to a carrier as a result of the Washington Naval Treaty.