So the confusion here is about the term 'knock.' In this context I believe it's being used to indicate the nature of the mechanism of entry. It's a 'knock' bec

So the confusion here is about the term 'knock.' In this context I believe it's being used to indicate the nature of the mechanism of entry. It's a 'knock' because the entry of the code is used to indicate to the people in the cockpit that someone outside wishes to enter. It is the people inside who can then decide if the person is allowed entry or not, even if the code is entered correctly. So in that way it's similar to knocking on a door, just one that is authenticated with a code.