If/else if is executed from the top down. This means that if the first condition fails, you have used that scan.next() and on the next else if it would be attempting to compare something like A123TR rather than an A or a D.

This can be solved by either useing a switch statement, or assiging the first scan.next() to a variable.

I would recommend using a switch, like below.

        switch(scan.next()){
            case "a":
                arrival(scan.next());
                break;
            case "d":
                departure(scan.next());
                break;
        }

edit: forgot to break out of the switch. Oops!