All airports where only flights are made within the area of one or several Schengen Area member states are considered to be internal borders. No customs control is made on such borders.
But if, in cases of force majeure or immediate danger, or under the direction of relevant bodies, a plane inbound from a third country is to land on an aerodrome which is not a customs crossing, this airliner may continue flying only after it has obtained a permission from customs officers and customs authorities. The same applies when a third-country airliner wants to land with no permission. In each such case a customs control is to be made in line with the Articles 6 to 13 of the Schengen Borders Code.