Comment by robocat

20 hours ago

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...

8 comments

robocat

Reply
imglorp  16 hours ago

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.

laxpri  17 hours ago

What does “inches” mean here

  • hansvm  14 hours ago

    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."

    • givc  13 hours ago

      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