Lots of dupers do well traveling by air, and being prepared increases your chance for success. I surveyed the Dup15q Parent support page for suggestions on what works:
Read a social story about airports/airplanes to prepare your child. Create a picture schedule. See if your airport has a Wings for Autism program. Familiarity will reduce anxiety.
Practice using earphones/earbuds.
Download apps, music, ibooks, and movies onto devices.
Bring doctor documentation of disability to get a free National Park Access Pass in case there are national parks/monuments/sites near your destination. (Or order online for $10 handling fee: https://store.usgs.gov/access-pass)
Get a letter from the doctor confirming diagnoses and list of current medications.
Call the TSA Cares helpline 72 hours prior to travel at 855-787-2227 (8am-11pm ET, 9am-8pm ET weekends/holidays). You can print out your own notification card to present to TSA officer for accommodations on www.tsa.gov.
Schedule a direct flight if possible.
Book a hotel with a pool.
Ask for handicapped assistance right when you get to the airport – you can ride in a cart, get a passenger support specialist, and/or facilitate the TSA inspection.
Tell TSA your child ‘has special needs’ or ‘has autism’ and/or show the notification card.
Keep ipad/electronics on top to easily get out for the scanner, and keep meds in the original labeled container. Declare any liquid medicines and formula to the TSA agent before screening; these can exceed the 3 oz limit, but may be tested for explosives.
Run your child around the airport to get out excess energy before boarding. Consider saving eating for an activity to do on the plane.
Change diaper or take to the bathroom just before boarding. Most airports have family bathrooms or companion rooms.
Wheel your stroller/wheelchair right up to the plane and gate check it.
Preboard when the airline announces boarding for people who need extra time. Seat your duper next to the window.
Bring a carseat on the plane if your child is used to traveling with a carseat. It will be familiar and keep your child safe. Some use five-point travel vests.
Bring a backpack with favorite snacks, favorite toys, books, fidgets/sensory items, and iPad with headphones (some recommend CozyPhones or Bluetooth clip on speakers). “Keeping them busy definitely helps.” Also bring medicines, diapers/wipes, and perhaps a lap pad and change of clothes.
Some people put multiple diapers with slits cut in them to serve as filtration to the main diaper.
Keep fluids up to help with ear pressure (although some recommend limiting liquids to reduce diaper changes.)
Sit at the bulkhead or have someone from your party sit in the seat in front of your duper to minimize annoyance due to kicking the seat.