Don't know what changes have been made to underslung gear since my time, but Whirlwind underslung detail at Tern Hill mid 60s - hooked up and checked, sent off on circuit. Visual check on return approach - no load. Pilot totally unaware and reluctant to land believing load still attached. Eventually convinced him and checked hook - support bar open ... but over-centre lock still closed!! Cancelled the detail and consulted armourers - "Can't happen , impossible" They insisted that it MUST have been the ground contact on touch-down which closed the latch. I KNEW this was not so ! Some 18 months or so later, it happened again. This time I kept the aircraft hovering and had it hover-taxy back to the pan and had the armourers look at it before touch-down. Amazed and puzzled - in theory there is a geometric lock which couldn't allow it to happen. It DID.
Never found the answer.