Avoiding damage from HABs to other aircraft


(this is a continuation of this topic that was introduced in the HF beacons on US launched balloons thread)
When I learned about this aspect of our wonderful hobby, I immediately became concerned about the risk that these balloons pose to other aircraft.  As I thought about it, I realized that the issue of an airplane running into a balloon or a jet engine ingesting one has probably already been addressed from the perspective of encountering birds in flight.  Per prior posts, it seems that there's an airplane windshield test that involves hitting them with flying (and not frozen) chickens.  I hope jet engines are also designed and tested to accommodate such hazards.  What I also found in the USA NOTAM rules was that there's a density limitation on UAV payloads; that they cannot exceed a particular mass per area facing whatever might fly into it.  I assume that this rule also exists to protect airplane surfaces and engines.  As long as my HABs are much less mass overall than a medium-sized bird and don't contain a dense component, I'm confident that they won't harm other aircraft.  I understand that the upcoming QRP Labs U3B product will be a couple orders of magnitude below the threshold of risk.  And birds should be able to fly around them because they can see them.  What I don't understand is why the big meteo balloons and their payloads that could seriously impair an airplane are permitted.
Halden NR7V

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