Answer
r = 7600 meters
Work Step by Step
We find the speed of the plane:
$v=(340)(1.8)=612m/s$
We know that the centripetal acceleration cannot exceed 5g's, for we must subtract one from 6g's since gravity is acting downward. Thus:
$5g =\frac{612^2}{r} \\ \fbox{ r = 7600 meters}$
