When did the last aeroplane fly beneath the Bridge?
Until the 1930's daredevil pilots occasionally flew beneath the Bridge in bi-planes, after which the faster planes made it too dangerous. There are stories that Spitfires and others were flown underneath the Bridge during WW2 but such claims have not been substantiated. The last known fixed-wing flight beneath the Bridge was in 1957 when Flying Officer Crossley of 501 Squadron, R.A.F. flew a Vampire Jet at 450 mph from east to west under the Bridge (in spite of a ban against such escapades and against all safety regulations!). He crashed into the cliffs on the Leigh Woods (south) side and was killed instantly.
Today it is illegal to fly any aircraft within 500ft. of any highway or structure. This is a general rule not a specific bridge byelaw. The exceptions would be when coming in to land, or when the pilot has the permission of the CAA, although the police and other emergency services have such permission for search and rescue. A police helicopter flew beneath the Bridge in 1997 whilst conducting a search.