When I joined Sleeve Wing and the merry band I was slightly mystified by the need to take the Pneumatics General CAA exam...all was explained by Pete Hill. And a remarkable explanation it was. IF you happened to have a total electrical failure there was a big red guarded button on the OHP. What it did was to activate a pneumatic ram which forced the main cb closed and prevented it from opening......short circuit or not! Anyone remember the starting technique of pulling CB133A and turning all the air relights on? I was threatened by the late Norman Brewitt with loss of vital organs for simply knowing of the technique. The fun we had.......