1.) Get A Helicopter
2.) Roll It Upside Down Towards The 1st Tree In Prison Outdoor Area
3.) When Upside Down, Lent Onto The Tree, Use Health Cheat
4.) Tada You Should Be Under The Map, If You Fly Above The Map You Will Out With Some Damage

PS. Once Or Twice I Have Had 2 Spwn Another Heli Infront Of Me When Upside Down

If This Does Not Work Then Unlucky Because I Can Barely Do It