If the flight attendant says something about your CPAP being another carry-on, let him or her know that it's a CPAP machine, and under the Americans with Disability Act, it's not counted as an added carry-on. Also, through normal luggage handling procedures, your machine can easily be broken.

Under the Air Carrier Access Act (ACAA) and Department of Transportation (DOT) regulations, a CPAP machine is classified as a "medical assistive device." This means that when it is carried in its own dedicated case, it does not count toward your carry-on or personal item limit. For example, if your airline allows one carry-on bag and one personal item, you can bring both plus your CPAP bag without being charged or forced to check a bag. It is a federal requirement for airlines operating in the U.S. to allow these devices on board. When going through security, you must remove the CPAP machine from its carrying case and place it in a bin for X-ray screening (though you can keep it inside a clear plastic bag to keep it clean). While you do not need a doctor's note just to carry the device, you should ensure it has an "FAA-approved" sticker on the bottom if you plan to use it during the flight. It is also wise to notify the airline 48 hours in advance if you intend to use it in-flight, as they may need to confirm your seat has a working power outlet or that you have sufficient battery life.