How do I tell my PhD advisor that I am leaving the program four years into the program?

Honestly, no that is a terrible degree. What can you do with it if you fail a flight physical or simply get laid off? Get a degree in engineering, physics, education, business, finance something you can fall back on if for whatever reason you can't fly!

Better yet get an appointment to the Air force Academy and get a degree in politics, get the AF to teach you to fly, and if you find you can't fly you can become a Senator and rip us all off! (Sorry that was mean wasn't it.) But please don't go aeronautics unless it is Aeronautical engineering and you can design aircraft as a fall back. Airlines DO NOT care what degree you have just have one and have the flight time, certificates, ratings, and training needed to get an interview.

TL is correct you should have a plan B just in case because you never know. I had to fall back on plan B when I was grounded for a medical which worked OK in my case as I had the diploma for electronics and worked in the aviation industry.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.