Deductible = you pay it all.
$500 bill and $1000 deductible you pay $500

Coinsurance = cost share AFTER meeting deductible

$1500 bill, $1000 deductible and 5% coinsurance
You pay $1000 plus 5% of the amount over

At some point coinsurance caps out. A million dollar bill may only cost you $3k or something

The IRS restrictions are on deductibles. I suspect the core hdhp is a “high” deductible (by IRS standards), high premium, generous co-insurance plan aimed at high income pilots who want a tax shelter