Escape systems are alien territory unless you have worked round them !!
In the Canberra (metal nav hatch) a/c - the nav hatch could be blown either by operating one of the navs bang seat handles OR by using the jettison switch,in the link I sent you - one can clearly see that the nav hatch has been jettisoned before a wheels up landing was attempted.
We had the same case at Cottesmore,the crew could not get the wheels down and therefore blew the hatch before the wheels up landing
rgds LR