The answer for HPN is wHite PlaiNs. Some city codes do not always seem logical, until they are explained to you. Take Fort Walton Beach, Florida, city code VPS. You're thinking, how do they get VPS out of Fort Walton Beach? The airport that serves FWB is actually in Valparasio, just outside of FWB. How airports get their codes is when they apply for their city code, they submit several choices (just like an airline does with airline codes), and if the code they wish to use is available, then they can use it. An interesting quirk in the city code system is that 'X' codes are normally reserved for U.S. Navy airfields, but when the new airport opened up in Fayetteville, Arkansas opened up, they got the city code XNA, the 'x' has nothing really to do with the name, but the NA part is short for Northwest Arkansas (Regional Airport). Some other cities whose codes don't make since intially, TYS (Knoxville, TN Tyson Field), BNA (Nashville, TN Berry Field), SWF (Newburgh, NY Stewart Field), and last, but not least, RSW (Fort Myers, Florida Southwest Florida Regional Airport).