13 hour flight 7 weeks post BBL.. Will I damage any fat cells?

You should wait at least 2 weeks before sitting without a support cushion and sleeping on your back. Sitting with no support too soon can harm the newly moved fat cells.

Hello and Great Question.  I appreciate the time you are spending researching this procedure.  My name is Dr. Matthew J. Nykiel.  I am a Board Certified Plastic Surgeon, a Fellowship Trained Cosmetic Surgeon, and a Butt Augmentation Specialist.
You shouldn't have any restrictions 7 weeks post op.  If you do have any questions, I would recommend that you contact your plastic surgeon before traveling.  
I hope that helps!  Best of Luck!
Matthew J. Nykiel, MD
#CoreAesthetic
Butt Augmentation Specialist
Liposuction Specialist
Board Certified Plastic Surgeon

I think at 7 weeks out from surgery there shouldn't be a risk of fat loss from sitting.  However you should confirm with your surgeon.

Dear wababa3,I instruct my patients not to sit for 2 solid weeks. We provide them a specialized pillow that our patients can sit on during that time. They can sleep on their stomachs or sides. Fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area.Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

By 7-weeks, the fat grafts should have stabilized or “taken.”  You should be fine to fly after 7-weeks.  I would continue using your BBL pillow for as long as possible.  It’s also important to avoid shear on the buttock.  That is, when you sit, you must lower and raise yourself from a seated position as opposed to sliding into the seat.  Since you are fresh after surgery, walking or doing calf exercises every 2-hours are important to avoid DVT.  You should probably take an aspirin and wear compression hose for a long flight.

Hello dear!!
Thanks for the question and provided information as well.
Yes, you should be perfectly fine, flying for 10+ hours, 7 weeks after surgery. 
If you have any concerns, please follow up close with your plastic surgeon.
Good luck :)