Our groundspeed is determined by how long it takes us to fly a certain distance:
We can use this to find our actual groundspeed and if it has changed, we can find a revised estimated time of arrival (RETA).
You are flying from Lydd Airport to Rochester Airfield, a straight line distance of 29nm. You departed Lydd at 1100 zulu and your ETA at Rochester was 1117 zulu. At 1108 zulu, you pinpoint your location as 10nm along your track to Rochester. What is your new estimated time of arrival at Rochester?
Again, you can use the above formula and a standard calculator to find the groundspeed if you prefer, or you can use the CRP-1:
1 Align the time taken (8 minutes) on the inner scale with the distance flown (10nm) on the outer scale
2 Find the total distance (29nm) on the outer scale and read off the time on the inner scale (23 minutes)
23 minutes is the total time it will take to you fly from Lydd to Rochester
3 Add the total en route time (23 minutes) to the departure time (1100)
1100 + 23 = 1123
RETA at Rochester Airfield is 1123 zulu
4 To find your actual groundspeed, locate the 60 minutes/1 hour mark on the inner scale and read off the groundspeed from the outer scale (75 knots)
Make sure to read the question carefully! Some questions will ask for a revised estimated time of arrival (RETA) and some will ask for a revised or actual groundspeed.