When I've flown in helicopters, the kit was in a big duffle in the cargo compartment, which isn't very big anyway. I haven't flown too often in small fixed wing aircraft, but it is possible that alot of cargo could get in the way.

Flying in Canadian military aircraft north of 55 degrees latitude, all passengers are required to have a complete winter kit with them right at their seat. This regulation was implemented after an incident near Alert (83 N), where a Hercules aircraft went down in a blizzard.
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“Expectation strolls through the spacious fields of Time towards Opportunity.” Umberto Eco