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Altitude as indicated by your altimeter. Suffers from several problems that cause it to indicate incorrectly:
- Instrument error – small imperfections due to manufacture, age and installation
- Position error – occurs due to the location of the static vent.
- Instrument lag – the Altimeter takes time to respond to sudden pressure changes, therefore it may not be reading your correct altitude instantaneously during high-speed climbs or descents.
Our actual height above mean sea level (AMSL).
Indicated Alt and Calibrated Alt vary due to the changes in the pressure lapse rate.
The altimeter measures height above QNH assuming that for every 30 feet of elevation the pressure will reduce by 1 hPa.
If the lapse rate changes from that value, then the calibrated altitude will change, without a change in true altitude.
Pressure Altitude (PA)
Altitude under ISA conditions. If you set 1013hPa in the pressure window of your altimeter this will indicate your pressure altitude.
Many performance graphs are related to pressure altitude.
Density Altitude (DA)
Density altitude allows for the change in the density of the air with changes in temperature.