A helicopter safety system for in-flight monitoring of rotor blade integrity, introduced at Forum 32 (Washington, DC, 1976). Called the IBIS, the system utilized a differential pressure indicator on the rotor blade spar. When activated by loss of blade gas pressure, or vacuum resulting from a cracked blade, a small radioactive source was moved from a shielded to an unshielded position. A radiation detector signal was then fed to a processor that activated a light on the pilot's instrument panel. From Vertiflite, January/February 1977 (page 17).