The RV-6A or P51 are recommended for this achievement. First, you will need to be 3,000 ft above ground level. Then, put your plane into a 45 degree dive. Once there, you need to invert the plane by holding left or right till the aircraft is upside down. Once it's there, you will need to pull the plane up to the sky whilst keeping it inverted. Press the up arrow (or the same control that you would use to make the plane dive if it were the right way up) and the achievement will pop.

~Int