Altitude Calculator
Altitude Calculator
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This WILL BE IN YOUR PPL EXAM!!!!! In fact you will likely have 3-4 questions on this.
RPL you will definitely need to know Pressure height!!!!
See worked examples in these expander boxes and some fully worked examples at the bottom of the page.
Pressure Height = 1380 ft
Density Height = 2340 ft
1. Find pressure height = Elevation + (1013-QNH) x 30 e.g. 1800 ft at QNH 1027, = 1800+ (1013-1027) x 30 = 1800+ -420 = 1380 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft
3. Find ISA temp = temp – 2 x the thousands of feet (at the PH) e.g. 20 C at 1380 = 15 – 2 * 1.5 = 12
4. Take ISA temp away from OAT e.g. 20°C to get ISA deviation = 20° – 12° = 8° ISA dev
5. Multiply ISA deviation by 120 to get temperature correction adjustment e.g. 120 x 8 = 960 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 1380 ft + 960 ft = 2340 ft
Pressure Height = 3290 ft
Density Height = 3530 ft
1. Find pressure height = Elevation + (1013-QNH) x 30 e.g. 3500 ft at QNH 1020, = 3500 + (1013-1020) x 30 = 3500+ -210 = 3290 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft
3. Find ISA temp = temp – 2 x the thousands of feet (at the PH) e.g. 10 C at 3290= 15 – 2 * 3.5 = 8
4. Take ISA temp away from OAT e.g. 10°C to get ISA deviation = 10° – 8° = 2° ISA dev
5. Multiply ISA deviation by 120 to get temperature correction adjustment e.g. 120 x 2 = 240 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 3290 ft + 240 ft = 3530 ft
Pressure Height = 2460 ft
Density Height = 5220 ft
1. Find pressure height = Elevation + (1013-QNH) x 30 e.g. 2400 ft at QNH 1011, = 2400 + (1013-1011) x 30 = 2400 + 60 = 2460 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft
3. Find ISA temp = temp – 2 x the thousands of feet (at the PH) e.g. 33 C at 2460 = 15 – 2 * 2.5 = 10
4. Take ISA temp away from OAT e.g. 33°C to get ISA deviation = 33° – 10° = 23° ISA dev
5. Multiply ISA deviation by 120 to get temperature correction adjustment e.g. 120 x 23 = 2760 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 2460 ft + 2760 ft = 5220 ft
Pressure Height = 890 ft
Density Height = 1250 ft
1. Find pressure height = Elevation + (1013-QNH) x 30 e.g. 500 ft at QNH 1000 , = 500 + (1013-1000 ) x 30 = 500 + 390 = 890 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft
3. Find ISA temp = temp – 2 x the thousands of feet (at the PH) e.g. 16 C at 1280 = 15 – 2 * 1 = 13
4. Take ISA temp away from OAT e.g. 16°C to get ISA deviation = 16° – 13° = 3° ISA dev
5. Multiply ISA deviation by 120 to get temperature correction adjustment e.g. 120 x 3 = 360 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 890 ft + 360 ft = 1250 ft
Pressure Height = 25000 ft
Density Height = 25600 ft
Note at 1013 hpa Elevation/Altitude = Pressure Height
1. Find pressure height = Elevation + (1013-QNH) x 30 e.g. 25000 ft at QNH 1013 , = 25000 + (1013-1013 ) x 30 = 25000 + 0 = 25000 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft
3. Find ISA temp = temp – 2 x the thousands of feet (at the PH) e.g. -30 C at 25000= 15 – 2 * 25 = -35
4. Take ISA temp away from OAT e.g. -30°C to get ISA deviation = -30° – -35° = 5° ISA dev
5. Multiply ISA deviation by 120 to get temperature correction adjustment e.g. 120 x -5 = 600 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 25000 ft + 600 ft = 25600ft
Density Altitude Worked Examples
Example 1 has pressure and temp both higher than ISA.
Given QNH 1027 Elev/Alt 1800 OAT 20 Find Pressure and Density Altitude
Pressure Height = 1380 ft
Density Height = 2340 ft
1. Find pressure height = Elevation + (1013-QNH) x 30
e.g. 1800 ft at QNH 1027, = 1800+ (1013-1027) x 30 = 1800+ -420 = 1380 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft (because 500ft will find temp to nearest 1°C).
e.g. at 1380 ft , nearest 500 ft is 1500 feet, sol lets use 1.5 in the next part.
3. Find ISA temp = 15 – 2 x the thousands of feet (at the PH rounded to 1.5 as per above.)
e.g. 20 C at 1380 = 15 – 2 * 1.5 = 12 (This is what the standard temp should be)
4. Take ISA temp away from current OAT to get deviation from ISA
e.g. 20°C to get ISA deviation = 20° – 12° = 8° ISA dev
5. Multiply ISA deviation by 120 to get pressure correction height e.g. 120 x 8 = 960 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 1380 ft + 960 ft = 2340 ft
Note:
This last answer allows for both the pressure and temperature difference from ISA.
Example 2 has pressure and temp both lower than ISA.
Given QNH 1009 Elev/Alt 1800 OAT 6 Find Pressure and Density Altitude
Pressure Height = 1380 ft
Density Height = 2340 ft
1. Find pressure height = Elevation + (1013-QNH) x 30
e.g. 1800 ft at QNH 1009, = 1800+ (1013-1009) x 30 = 1800+ 120 = 1920 ft
2. For the next part of the calculation ONLY, round pressure height to nearest 500ft (because 500ft will find temp to nearest 1°C).
e.g. at 1380 ft , nearest 500 ft is 1500 feet, sol lets use 1.5 in the next part.
3. Find ISA temp = 15 – 2 x the thousands of feet (at the PH rounded to 1.5 as per above.)
e.g. 20 C at 1380 = 15 – 2 * 2 = 11 (This is what the standard temp should be)
4. Take ISA temp away from current OAT to get deviation from ISA
e.g. 20°C to get ISA deviation = 6° – 11° = -5° ISA dev
5. Multiply ISA deviation by 120 to get temp correction adjustment e.g. 120 x -5 = -600 ft
6. Add pressure correction height to the temp correction adjustment to get density altitude e.g. 1920 ft + -600 ft = 1320 ft
Notes:
ISA is the international standard atmosphere at AMSL of 15°C and 1013.2 hPa. For every 1000 ft up in altitude temp decreases by 1.98°C, but lets call it 2°C. (Sidenote: Therefore every 500ft is 1°C). For every 1hPA up or down equates to 30 feet.
Steps 1 to 2 deal with the adjustment due to pressure difference from ISA.
Steps 3 to 5 deal with temp difference from ISA.
Step 6 adds the two together so both the pressure and temperature difference from ISA are accounted for to arrive at “density altitude”.