Airplane Seat Power Ground Loop?

Run a ground wire and ask different passengers to hold it. Have you tried it without the DAC? 

It is probably not a ground loop because there can not be more than one path to ground so it is a defective component that is the source and cause of your problems.

clearthink the problem does not occur outside the airplane ie when it’s plugged into a land-based power source.

Also, I used another dac (a THX one) and got the same problem except the problem doesn't go away when I touch it.

I don't know the answer but your first world problem made me smile.  And reminds me of having to hold the TV antenna when I was a kid.

exactly what I thought

Have you tried a mophie or other brand portable battery charger to extend the laptop life?

Interesting. 

When I worked on sensitive equipment, I had to use an AntiStatic Wristband to ground me to the workstation.  It looked like the Twist 'o Flex watchband. Connected that wire to your dac.  Some antistatic band did have a like a 100M ohm resistor in it, which we had to check every so often.  Idea is to Bleed off the charge.  Not current thru you.  Or maybe put a ground lead from your headphone out to the padding of your headphone.   I'll choose my wrist thank you.

Why?  I don't know.  Guessing on it.  The USB power ground might be floating and not part of aircraft ground.  Like that 100M ohm resistor, your butt is it.  You could take a jumper lead from your dac straight to a metal surface of the airplane.  I you get a smoke from your laptop or dac or a big arch and the airplane stop working... hey! sacrifices must be made in the name of Science.

@tomask6 

Airplanes have so many electronic devices working the hum could be many things, but I’m guessing when you hold your device, you are grounding it.  Apple or many others offer external batteries.  I have a couple of them.  The Apple one is about the size of a deck of cards only square and the No name one is the width of a deck of cards only almost twice the length.  Either keeps my iPad charged for 36+ hours, when doing a lot of video calls.

All the best.

Audioengine wrote me back said the airplane power isn't properly grounded so I am the ground. Will try the wristband.
Incidentally, unrelated to this post, I flew a United 787 last year and when my bare arm touched any metal surrounding the armrest, I swear I felt current running thru it to the point where it got uncomfortable after many hours.
We're talking 15 hour flights here, not short hops, so it's nice to play locally stored hi res music on my laptop while I work.