Not launched yet. This is a preview. Do not file a real claim or pay.
Two minutes. Everything the airline desk will ask for.
The Property Irregularity Report is the airline's official record of the problem. Everything else (tracing, letters, a claim) builds on it.
No PIR yet? Get one at the airline's baggage desk before you leave the airport. It's much harder afterwards. What to do at the carousel
Which flight?
The carrier on your boarding pass, which can differ from who sold the ticket.
