Exnomad,
The Hunter has a pretty good wing and a reasonable motor. At sea level, from 380kts, it would be fairly easy to gain height - 4G up, 5G down; topping out +4,500ft ish above base with a minimum speed of maybe 140kts.
It would do this all day, until the fuel ran out.
The reported performance point of apex at 100kts and 2900ft (maybe approx page 3 of this thread) suggests to me that the parking brake was on