Of the 125,000 aircrew who served in Bomber Command 47,268 were killed in action. A further 8,195 were killed in accidents, 9,838 became prisoners of war, and 8,403 were wounded (figures from The Bomber Command War Diaries). In the very broadest terms, then, 4 in 10 aircrew died during the war, the chances of surviving unscathed were about 50-50.

A standard Bomber Command operational tour was 30 successful sorties, the chances of completing it would vary depending when your great uncle started flying; up to March 1944 RAF loss rates averaged 4.4%, which I believe gave crew about a 1 in 4 chance of completing a tour. From April to August 1944, as the Luftwaffe was heavily hit and focus switched to targets in support of the Normandy invasion, the loss rate fell to 2%, and further still to 1% from September 1944 to the end of the war, so crew starting later had a significantly better chance of surviving.