KLM Airlines' usually permits check-in via their website or mobile app up to 30 hours prior to departure. Airport check-in desks open several hours prior to departure; airport-specific opening times apply. KLM Airlines Check-In Policy Travelers who miss the check-in deadline risk not being allowed to board the aircraft. After check-in, baggage drop is required for checked luggage. Possession of the required travel documents—passport and visa—is imperative. Go to the KLM website or get in touch with customer service for the most up-to-date information.