# Altitude calculation

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• #31134

Please can you help me understand how to get to the answer of

P-MET-AP-0018 I got an answer of -130 so i don’t understand how to get to the correct answer.

P-MET-ALT-0004 I got an answer of -1074ft

#31136

P-MET-AP-0018

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

P-MET-ALT-0004

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

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