The article is very vague on how the radioactivity allows detection of leaking gas.

The video at the bottom explains from 7:50 in. Gas pressure pressing on a spring holds a radiation shield in place. If the gas leaks the shield moves and the radiation can be detected.

  MH-53E maintainer here. The system is IBIS (Inflight Blade Inspection System) and the little pods are BIM (blade inspection monitors). The box is pretty obvious sitting just behind the main rotor on the deck above the center engine. I found a random picture on the web:

  The IBIS devices provide in-flight warning of blade failure on various models of helicopters. One device is located on the root of each blade attached to the rotor. The IBIS detects a decrease in pressure in the blades of a CH-53 model helicopter. Any cracks in the blades will result in a decrease in air pressure in the blade. Such cracks could result in structural failure in the blade, thereby resulting in a potentially dangerous operational situation for the helicopter, such as a hard landing and/or injury to the craft and/or personnel up to, and including, death. Each device contains approximately 500 microcuries (18.5 MBq) of strontium-90 (Sr-90) in the form of a rolled metal foil, encased in a small stainless steel protective cylinder about the size of the press button on a ballpoint pen. The external radiation level of the IBIS is 0.8 mR/hr at 3 inches in the normal (shielded) mode, and 75 mR/hr at 12 inches in the failure (extended) mode.

I don’t have a good intuition for the aerodynamic environment around a helicopter blade in flight. It feels like it ought to be very blustery. Presumably there’s a huge amount of pressure on the underside and a drop in pressure on the top side, maybe even below atmospheric pressure?

The blades are a foot wide and 30’ long, so five of them carrying 25,000lbs is about 1psi of pressure. 3x that for a laden Stallion. I guess I expected it to be a lot more because, from the perspective of the rotor blade, it is in a very windy regime.

The other major open question for me is with the cracks. The system implies there is a non instantaneous progression from unbroken, to having a small crack on the surface, to having a crack large enough for nitrogen to escape, to having a crack so large that the rotor blade fails. I’d love to know what sort of timeframes sit between each of these phases.

I can just imagine the conversation when they came up with that:

Boss: we need to transmit 1 bit of information indicating pressure loss from a spinning rotor blade to the main body of the helicopter. The blade spins too fast for electronics to work properly.

Engineer: What if we make the blades radioactive when they depressurize, then point a Geiger counter at them to detect if that happens.

Boss: ... where did you say you got your degree from?

Often in software development I encounter code that seems overly complex. It's not always obvious at first glance whether that complexity is because it's solving a really difficult problem, or because the developers made it more complex than it needed to be.

Having no domain knowledge of the problems of detecting cracks in blades, this sounds like either a genius solution to a super hard problem, or unnecessary complexity.

This definitely come from the same category as F-16 potentially releasing hydrazine when it crashes, as it is used to power its auxiliary power unit in an emergency:

https://en.wikipedia.org/wiki/Hydrazine#Gas_producers_and_pr...

https://pmc.ncbi.nlm.nih.gov/articles/PMC12542269/

https://aviation.stackexchange.com/questions/23417/why-is-hy...

First you usually go "WTF, why would they do that ?" but on a second look it usually makes perfect sense & is quite an elegant, if unorthodox engineering solution.

Heh...I actually knew about this.

A friend's cousin was a CH-53 pilot out of MCAS Miramar. One day, he was telling us, someone didn't properly secure one of the (chairs? racks? don't know the rotorcraft term...)in the cargo hold. The doors were open, and they were flying above the desert at a few thousand feet when...one of the aluminum & webbing chairs got sucked out the window and hit a rotor.

The number & intensity of the warning lights in the cockpit, and the accompanying alarms when that happens is pretty spectacular. The pilot knows right away.

What if the helicopter would have to operate in a contaminated area? Wouldn't it cause false positives?

> The CH-53 Sea Stallion first entered service in 1966

1966 is Cold War era, so, I think, an idea that a military helicopter would have to fly in an area that has been recently nuked wasn't totally inconceivable...

When I read the title I thought it would be about depleted uranium, which other aircraft have used as weight: https://en.wikipedia.org/wiki/Depleted_uranium#Trim_weights_...

See too the Porsche 917[0].

[0] https://en.wikipedia.org/wiki/Porsche_917?#Construction

> 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.

Huh? So it's not really in use today as much as hasn't been retrofit out. Those are very different. This also doesn't read well. Maybe rewritten without proofreading

Some really hyperbolic comments on there:

JohnU says: "It’s all fun and games until helicopter crashes and what should be a minor incident turns into a nuclear emergency where you have to call very specialized decontamination units to remove radioactive material from the site."

Alan Reid says: "unless they shoot ’em down on your own territory

then you don’t have a helicopter but you have mini-hiroshima to clean up."

How much Strontium-90 do they think these things are carrying?

Do they have smoke detectors at home (I hope so...)?

Terrible explanation

Assuming only one rotor cracks, then the radiation is from one rotor IBIS and the detector electronics could detect radiation levels that ripple synchronously with the rotor speed.

The Strontium-90 is a beta radiation emitter.

  The external radiation level of the IBIS [is] 75 mR/hr at 12 inches in the failure (extended) mode.

Strontium-90 is a beta emitter, apparently, so you could just measure Alpha, Beta, and Gamma and detect false positives due to elevated background radiation that way. If you're in the air you're probably much closer to your Beta source as well so you could detect it above background due to distance. Gamma is essentially light so it falls off with the square of distance. But Alpha and Beta are probably much worse than Gamma in that respect, so I'm guessing this is fine most of the time and then they rule out false positives by looking at the total background in each type of radiation.

I imagine that if you're flying through an active nuclear war, false positives from the blade crack detector are pretty far down your list of things to worry about.

Exposure to a nuclear blast doesn't necessarily make things radioactive. Hiroshima and Nagasaki are not radioactive wastelands. Even in Chernobyl, the other reactors continued to operate for a time after the accident, and nature is thriving.

But, the radiation detector is on the upper surface of the rear fuselage. Hypothetical Beta particles from the ground will strike the underside of the aircraft and not the detector.