In the lesson CRP-1: Calculating Heading & Groundspeed we learnt how to calculate our groundspeed when planning a flight from A to B.
Using this groundspeed and the distance from A to B, we can calculate the flight time from A to B and an estimated time of arrival (ETA) at B:
You are flying from Lydd Airport to Rochester Airfield, a straight line distance of 29nm. Your groundspeed is 103 knots. If you depart Lydd at 1100 zulu, what is your estimated time of arrival overhead Rochester Airfield?
You can use a standard calculator and the formula above to find the time taken if you prefer, or you can use the CRP-1:
1 Align the 60 minutes/1 hour mark on the inner scale with the groundspeed (103kts) on the outer scale
2 Find the distance (29nm) on the outer scale and read the time figure aligned with it off the inner scale (17 minutes)
You will need to have a rough idea of the en route time to know where the decimal point should be (i.e. is the answer 1.7 minutes, 17 minutes or 170 miuntes?)
3 Add the en route time (17 minutes) to the departure time (1100)
1100 + 17 = 1117
ETA at Rochester Airfield is 1117 zulu.