Indicated Altitude- Direct Reading from Instrument ( Whatever indicated, that's always error involved.)
Absolute Altitude- Height over Terrain ( Measure by Radio Altimeter) Not necessary be Airport elevation
True Altitude- TRUE, Actual elevation above mean sea level (Local QNH setting) Pressure Altitude- Keyword: Standard Datum Plane 1013.2hPa (正常係睇完FAA definition都唔會明)
Density Altitude- For Performance
Correct for non-ISA condition
*Transition Layer varies in different region.
In general, Pressure Altitude同true altitude 係pilot 同ATC ge altitude
最明顯用ge例子: aircraft operate below transition altitude required to set Local QNH. And traffics operate above Transition altitude are required to set standard (1013hPa) to Pressure Altitude; ensure all traffics operate on the same reference (standard) datum plane.
以上所有ge altitude, 都係pilot concern altitude; 但係隻飛機係唔鬼理~如果喺隻飛機ge角度睇， 佢只係care DENSITY altitude.
空氣密度影響左幾多performance?
Engine/propeller/wing performance差左幾多?
(仲飛唔飛得起?~)
But how to find Density altitude
e.g. imagine we are in Hong Kong, a hot afternoon with low pressure around.
VHHH elevation 28ft (consider sea level) 28.8 inches of Hg, temp 35⁰c
From the chart on the right, first correct the pressure, 28.8"of Hg result in 1053ft Pressure altitude.
1053ft PA with 35⁰c resulted in 3500ft (roughly) in Density altitude.
Although aircraft located at sea-level; however, aircraft performs as it is at 3500ft, a big drop in terms of performance.
Aside from reading from Chart, we can use PA and ISA Temperature Deviation to find Density Altitude.
1) Density Alt = Pressure Alt + (ISA deviation x 120ft)
from the previous example:
Density Alt = 1053 + (35⁰c - 15⁰c) x 120ft = 3453ft
Another formula we can use to find out True Alt
2) True Alt = QNH Alt + (ISA deviation x 0.4% x Height above Met station)