Planes Internal

The advantage of an internal flight is that you do not have to get from Athens airport to Piraeus (but you do still have to get from your destination airport to the place of your choice). Internal flights cost very roughly the same as the same trip on a High Speed. Taking Athens as an example, there are not many flights, less than one a day. The flights leave at different times each day. The flights are usually fully booked even in mid-season. And you need to consider the luggage allowance on internal flights (lower than on international flights). I believe there is a weight concession if your international flight was with Olympic.