Prior to the present fibre composite blades, the military Puma had hollow alloy blade spars which were pressurised with nitrogen (Wessex similar?). In the event of a blade crack, the spar depressurised and set off the BIM caption. We had a lot of false captions. One day, my instructor decided to fly 10 minutes back to base with a BIM caption on as he thought it was probably spurious. In fact it wasn't and the spar had only one third of intact metal remaining across the crack. He took a few days off after discovering what he had just escaped.... <img border="0" title="" alt="[Eek!]" src="eek.gif" />