The helicopter is climbing to a Flight Level or Pressure Altitude, which means that the pilot has to set the standard pressure (1013 hPa), whenever necessary during the climb phase.

Local QNH is 995 hPa. So, in order to find the height difference, the pilot has to either convert the airfield elevation into pressure altitude or the ToC pressure altitude into indicated altitude with local QNH.

For this question, we will convert the airfield elevation into pressure altitude:

1. When we increase the QNH setting => the altimeter reading increases (this increase is at a rate of approx. 30 ft per 1 hPa).
2. Airfield pressure altitude: 2 500 ft + (1013 hPa - 995 hPa) x 30 ft/hPa = 2 500 ft + 540 ft = 3 040 ft.
3. Height difference between airfield and ToC: 10 500 ft - 3 040 ft = 7 460 ft.
4. Thus, the flight time from airfield to ToC is: Height difference ÷ RoC = 7 460 ft ÷ 500 ft/min = 15 min approximately.