Refer to figure.
First we will determine the distance covered to reach cruise altitude and the distance covered to reach the departure airfield altitude, then we will subtract them to obtain the climb time.
1. CONVERSION FROM INDICATED ALTITUDE TO PRESSURE ALTITUDE
Since the cruise altitude is a pressure altitude (7500 ft), which is referenced to 1013 hPa, whereas the airfield elevation is referenced to MSL (QNH 1009 hPa). Since the two altitudes are based on different pressure references, they must first be converted to the same datum. Therefore, a pressure correction must be applied and hence, we will convert our airfield elevation (indicated altitude) to pressure altitude.
Pressure Correction:

• Pressure difference: 1013 hPa - 1009 hPa = 4 hPa
• Height correction for pressure: 4 hPa x 30 ft/hPa = 120 ft
• Pressure altitude: 1380 ft + 120 ft = 1500 ft*

* The local QNH (1009 hPa) is lesser than the standard pressure of 1013 hPa. Therefore, PRESSURE ALTITUDE > INDICATED ALTITUDE
Note: You can either convert the airfield elevation (1380 ft) to pressure altitude or convert the pressure altitude (7500 ft) to indicated altitude.

2. DISTANCE TO REACH CRUISE ALTITUDE
We need pressure altitude and Outside Air Temperature (OAT) to calculate the time. However, since the OAT is given as an ISA deviation, we must first determine the actual OAT.
Step 1: Calculate the actual OAT

• The lapse rate at which temperature decreases is approximately 2°C/1000 ft up to 36 080 ft (tropopause)
• ISA temperature at 7500 ft: +15°C – (2°C x 7.5) = 0°C
• Hence, OAT at 7500 ft: ISA temp + ISA dev = 0°C + 12°C = +12°C

Step 2: Calculate the distance to reach cruise altitude
The steps are marked in blue.

1. Enter the graph from bottom left at OAT +12°C.
2. Move vertically up to the pressure altitude of 7500 ft (halfway between 7000 ft and 8000 ft).
3. From that point, move horizontally to the right until you intersect the “DISTANCE - NM” line.
4. Then move vertically downward to read a distance of 20 NM.

3. DISTANCE TO REACH DEPARTURE AIRFIELD ALTITUDE
We need pressure altitude and Outside Air Temperature (OAT) to calculate the time. Then we can follow these steps.
The steps are marked in green.

1. Since the OAT is standard (STD), start at the point where the 1500 ft pressure altitude line intercepts the “STD TEMP” line.
2. From that point, move horizontally to the right until you intercept the “DISTANCE - NM” line.
3. Then move vertically downward to read a distance of 4 NM.

4. DISTANCE COVERED DURING CLIMB
We can calculate the distance covered during the climb by subtracting the distance to reach the departure airfield altitude from the distance to reach the cruise altitude.

• Distance covered (NM) = Distance to reach cruise altitude (7500 ft) - Distance to reach departure airfield (1500 ft)
• Distance covered = 24 NM - 4 NM
• Distance covered = 20 NM