This is copied from my guide on the Xbox side:
This is acquired during the story-related mission called Fly South.
Once you have had the "hallucination" scene where you fight the Ink Monster, have "relations" with Citra, kill Vaas, tell her you're staying on the island, and telling your friends goodbye, you will get a call from Willis telling you he's leaving for Russia. Jason wants Willis to wait for him for a ride so he can take out Hoyt, but Willis essentially says "you'd better hurry or I'm leaving without you", which starts a timed sequence where you have to drive to the airstrip where he is.
You'll first have to rescue him there, then you'll have to defend him against several waves of pirates while he repairs the plane. Once that happens, you'll get a cutscene in the air where you'll jump out of the plane and have access to the wingsuit and parachute. At this point, the achievement will pop.