Complex sentence: The airfield consisted of free draining grass landing runways however when allocated for USAAF use in 1942, hardened runways were scheduled, but work did not commence until the spring of 1943.
Split into simpler sentences: The airfield consisted of free draining grass landing runways however when allocated for USAAF use in 1942. Once turned over to the Americans hardened runways were scheduled, but work did not commence until the spring of 1943.