An inbound flight from outside the EU normally requires an eligible EU operating carrier for EU261 to apply. Korean Air's branding or a round-trip ticket alone does not establish coverage.
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What to check
The route and operating carrier matter as much as the number of hours. Check which rule covered the flight on its travel date before using a compensation amount.
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What to do next
Read the applicable country rule first, then send the airline the facts that support that specific claim.
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Official sources
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Your route and travel date may change the answer. Check the linked official rule.