Check his name is the linked RAF casualty list
http://www.flightglobal.com/pdfarchive/view/1943/1943%20-%200728.html?search=Sweeney
On my smart phone so cant check it confortably.
I think you will find his name among the Died on Active Service names, hence not related to an aircraft accident. This status would point towards a death from illness or a traffic accident etc.
It can be airmen killed by propellers on airfields. Check the link first.
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