GoKawiilCH-53 Sea Stallion's blade crack detector uses radioactive gas indicator
The CH-53 Sea Stallion helicopter's rotor blades contain a spar filled with pressurized nitrogen gas, which leaks out if a crack forms, causing a visible barber-pole indicator to shift as a ground-check warning. Because engineers in the 1960s couldn't rely on wireless electronics surviving inside a spinning rotor head, they developed IBIS, an Inflight Blade Monitoring System that uses a small radioactive source within the same pressure indicator to alert pilots mid-flight if a blade is failing.