The rate at which an aircraft climbs or descends is typically expressed in feet per minute (for example, a 500ft/min rate of climb).
A given rate of climb is usually achieved at a given airspeed. For example, an aircraft may have a maximum rate of climb of 500ft per minute at an indicated airspeed (IAS) of 70kt.
Often when we need to make climb or descent calculations, they are in relation to a distance over the ground. Let’s look at an example:
You are flying at 2000ft with a 20kt tailwind when you commence a 500ft/min climb at an IAS of 70kts. What altitude will you be at after flying for 6nm?
1 First, we need to know our ground speed so that we can work out how many minutes it will take to fly 5nm
Indicated Airspeed = 70 knots
Wind = 20 knot tailwind
Ground Speed = 70 knots + 20 knots = 90 knots
2 Set the ground speed (90 knots) in the CRP-1
A ground speed of 90 knots means we will fly 90 nautical miles in 60 minutes. So we align 90 on the outer scale with the 60 minutes mark on the inner scale.
3 Find the distance (6nm) on the outer scale and read off the time taken (4 minutes) on the inner scale
4 Calculate the altitude the aircraft will climb in this time
Rate of Climb = 500ft per minute
Time = 4 minutes
Altitude Gained = 500 x 4 = 2000ft
5 Add the altitude gained to the starting altitude
Climb Started = 2000ft
Altitude gained = 2000ft
Final Altitude after 6nm = 4000ft