An aircraft flies from aerodrome “A”, where QNH is given as 1020Hpa, to aerodrome “B”, where the QNH is given as 999Hpa. Aerodrome “A” is 800ft above mean sea level and aerodrome “B” is 500ft above mean sea level. If the altimeter sub scale is not changed from 1020, what is the altimeter indication on landing? (ASSUME 1hPa = 30ft)
We can work this out exactly the same way that you would pressure altitude, except we would use 1020 instead of 1013
Airfield elevation + ((1020-999) X 30)
500 + (630) = 1130ft
If the elevation is 900ft, the QNH is 998. OAT is -7 What is the Density Altitude?
Step 1: Work out the Pressure Altitude
Pressure Altitude = Elevation + ((1013 – QHN) X 30)
Pressure Altitude = 900 + ((1013 – 999) X 30)
Pressure Altitude = 1320ft
Step 2: Work out the ISA Temperature at the airfield elevation
ISA Temp = 15 – ((Elevation/1000) x2)
ISA Temp = 15 – ((900/1000) x2)
ISA Temp = 13.2C
Step 3: Work out the ISA Dev and apply 120ft per degree C
ISA temp = 13.2C
OAT = -7
ISA DEV = -20.2C
-20.2 X 120 = – 2424
Step 4: Apply the calculation from step 4 to pressure altitude, as it the ISA DEV is – we will subtract it
1320 – 2424 = -1104ft
This reply was modified 2 weeks, 5 days ago by Louise Hahn.
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