Sometimes you find gold in unexpected places. In this case, it’s from a video about the CH-53 helicopter. The CH-53 Sea Stallion first entered service in 1966. It’s a huge helicopter, capable of lifting immense amounts of weight. All that weight hangs from 6 (or 7 on some models) rotor blades.  The problem was detecting problems with the blades before they grew into a catastrophic failure.

Much like a fixed-wing aircraft wing, the CH-53 rotor blades are designed with a spar as the main structural member.  On early designs, the spar was extruded aluminum. The CH-53D and other variants moved to cold-formed titanium.

All of these blades shared the same problem – detecting tiny cracks in the metal before they grow into blade failure. Detecting cracks turned out to be rather easy. The blades are sealed and pressurized with nitrogen gas. Cracks allow the gas to leak out. A barber pole pressure indicator on the blades allows mechanics to tell at a glance if everything is ok.

That’s all fine and good on the ground, but if a blade begins cracking in the air, the pilots want to know about it immediately. What was needed was something akin to the tire pressure monitoring system on modern cars. A pressure sensor that would turn on a light in the cockpit.  Modern automotive TPMS systems use wireless sensors inside the tire. In the early 60’s though, electronic components were not reliable enough to survive rotating on a helicopter blade. Wiring the sensors through the spinning rotor head using slip rings would be incredibly complex. Expecting wireless electronic components to survive life in a rotorhead would be even more problematic.

The solution turned out to be simple: Throw a little ionizing radiation into the mix. Called IBIS, short for Inflight Blade Monitoring System keeps the CH-53 safe. The magic happens in the same pressure indicator we mentioned earlier. Rather than just a shift from a green to a barber pole indication, the IBIS module also exposes a tiny amount of radioactive Strontium-90.  The Strontium is a beta emitter, meaning its radiation can be detected by a Geiger counter inside the helicopter.  It’s also relatively safe for humans as long as they don’t eat or breathe it in. No batteries, no electronics on the rotor blade. It’s a simple mechanical solution to a difficult problem.

The solution is so elegant it’s still being used today. The newest versions of the CH-53 of fiber optics to detect faults in the newest all-composite blades. The older variants? They’re still going with the nuclear option.