The Helicopter with Radioactive Blades | Hacker News Reader

The Helicopter with Radioactive Blades

hackaday.com

203 pointszdw62 comments

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.

The titanium spar is filled with gas. If there was a crack in the spar the gas would escape, therefore tripping the pressure sensitive radiation shield. Releasing radiation that can be detected inside the helicopter.

Wireless systems aren’t used for opsec reasons.

Wireless arent used because they need power/wires and are less reliable than IBIS. Civilian versions of sikorsky helos use the same system.

carrying a radiation source doesn't sound very opsec either to be honest...

Why? It's not like it's gamma radiation.

Since the radiation source is only exposed if it's indicating a potential main rotor crack, and it's not a very strong radiation source in the first place (so it's not like it allows long distance tracking), it seems like a minor risk -- if the radioactive capsule is exposed, the pilot is going to be returning to safe place to land.

Beta particles typically only travel a few meters in air before they lose most of their energy.

Wait until you find out about smoke detectors. Or microwave ovens. Or light bulbs. Or anything with nonzero temperature.

There's a good comment:

  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:

https://assets.verticalmag.com/wp-content/uploads/2017/01/MH...

And:

  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.

From https://public-inspection.federalregister.gov/2019-08531.pdf...

What does “inches” mean here

Presumably the measure of distance from the source to receive dosages under those conditions.

It's a medieval measurement, still in popular use. Nowadays it means approximately 2.54 centimeters. Historically, it's the height of the ash pile a "witch" would leave behind, and the word has morphed only slightly from "witches" into "inches."

Actually nowadays it means exactly 2.54 cm as that’s how it is defined. Which also means all distance measured in US Customary units is ultimately metric too

Inches from the source. Inverse square law applies

Fast electrons through air mostly have elastic scattering making their long distance path a 3D random walk with exponential dropoff which dominates the inverse square geometric dropoff.

Sr90 beta decays emitting an electron becoming daughter decay product Yttrium-90. Yttrium-90 then beta decays (after a half-life of a few days) emitting more energetic electrons than Sr90, and those electrons travel a mean distance through STP air of maybe meters and effectively don't travel further than say 10m.

At long distance through air the Bremsstrahlung xray radiation dominates? It is generated by the fast electrons curving when passing close to nuclei. Xrays through air are inverse square? I guess it depends on the absorbtion characteristics of air.

AFAIK the above is mostly correct but I haven't checked anything properly sorry.

Basically anything to do with radiation is usually deeply complex and you need to look at all the different factors before making assumptions.

The video explains it pretty well. There's a spring loaded plunger in that IBIS sensor that can be seen visually by mechanics on the ground. When the pressure drops, the plunger retracts, showing a visual indication. At the same time, it also removes the rad source from its shielding when it does that, increasing emissions for the in flight sensor to see.

Radioactivity has nothing to do with it.

Was solved in Mi-6/Mi-8/Mi-26 blades without it.

Would be actually interesting to know how other helicopters solved this. :)

I saw they moved to composite blades but why don't they need such a warning system?

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.

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.

Chesterton's Fence

Given the forces involved, the need to avoid wireless communication, the complexity of any bearing with associated electrical contact, and so on this seems like the least complicated solution. You can rely on radioactive elements to emit radioactivity above background, you can build a pretty reliable pressure indicator, and you can fill a cavity in the blades with gas pretty reliably. You sidestep all of the complicated mechanical linkages in favor of a solution that's not in the rotor hub at all.

I though it sounded pretty clever. Now, that can be good or bad depending on context. I mean it in a good way. It seems simple conceptually (I have no idea if it adds a lot of complexity to the manufacturing) and rather foolproof. I guess the radiation shield could jam and not move when it should.

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?

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...

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.

> Exposure to a nuclear blast doesn't necessarily make things radioactive

Isn’t this neutron activation?

It is, but a) it's a probabilistic process and b) most of the products decay rapidly, down to the background level in a matter of days.

Generally they are going to airburst to cause maximum damage over an area which means what free neutrons hit are gasses which dispurse pretty quickly. Ground detonation does cause more fallout though.

Strontium-90 is a major component of fallout and strongly in the ‘really not good for you’ category.

Nothing I wrote refutes that

fallout is present in dust in the air. it’s radiation would not be coming only from below the helicopter.

I guess in this extremely contrived hypothetical where a Navy helicopter flies through heavily-irradiated dust cloud from a nuclear detonation, yes, the blade alarms might go off spuriously. Well done.

what helicopter do you expect EXCEPT a navy helicopter to be flying through the fallout from a nuclear explosion?

especially with all the nuclear tests they were doing for decades?

Ambulance and rescue helicopters, if there are any left, and the crew didn't just take the helicopter and run as far as possible...

Which will almost certainly be military, because civilian helicopters and staff aren’t equipped or trained for that kind of environment except for perhaps a every couple of years hazmat/mass casualty training.

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.

All nuclear things fall off much faster than square of the distance unless you're in vacuum. The atmosphere is nowhere near transparent to any of them. An addition to the inverse square issue the behavior of the various types is complex:

Gamma: Halving distance in the low hundreds of meters.

Alpha: Goes a few centimeters.

Beta: Goes a few meters per million electron volts. Most sources don't go above a few million electron volts.

Neutron: The only one with good penetration, halving distance around a kilometer for high energy neutrons. But note that there are few sources of neutron radiation other than fission.

Thus unless you're flying into fallout it's not much of an issue.

Gamma is technically a kind of electromagnetic emission, so it falls off with the square of the distance. The others fall off as you say.

Everything falls off as inverse square in 3D. His point is that the atmosphere attenuates it as well.

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.

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.

At that point a blade crack might be the best option

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.

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

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...)?

You must not hang out on there. Those are some pretty standard grade Hackaday comments. I love the site, read every day, but the comments section can be... rough.

On the bright side, now we can have another comparison scale, for how many banana-copters worth of radiation a given item contains.

Occasionally I browse the comments when something I've worked on is misunderstood on there.

You should never read the comments.

  Safe for humans as long as they do not breathe it.

We're flying. In the air, and there is a rotating strontium spreader above us. How we're not going to breathe it in?

Wait until the general public learns about the tech behind most of the currently fielded smoke detectors.

Also, this has long-since been obviated with active acoustic/vibration detection in modern rotor drive-trains.

All that said, every registered civilian and military chopper has high duty-cycle manual inspection of rotor blades too. They use both their Mark I eye-balls and ultrasonic transducers.

> 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

If it hasn't been retrofitted out, then it's still in use today. That sentence would only be incorrect if the new solution was retrofitted on all aircraft.

> is so elegant it’s still being used today

I think it's the "so elegant" part that's wrong. It's not still being used today because it's so elegant. It's still being used today because the old helicopters haven't replaced it.

I think elegance has to do with the technology of the time. Some clever design that is cheaper/better than you would otherwise think possible given the era. There are probably many elegant aspects to Roman aqueduct engineering, but they would still get replaced with modern materials during a retrofit.

if it was less elegant from a function/cost perspective, it would have been replaced a long time ago.

Terrible explanation

Hackaday isn't the end-all, be-all authoritative source for anything. It's a blog that, for the most part, links to neat stuff with a summary write-up. If something interests you then it's on you to follow the links.

This is how the web used to work.

I miss those days.

Sorry, all out of the good old days. All we have are people working in tech too ashamed to admit they're a willing and complicit part of the surveillance state under the auspices of working in something they also won't admit is marketing or advertising tech, but is 100% just that.

Reply on news.ycombinator.com