TrueAlt -   Cold Weather Altitude Corrections

How Altitude in an aircraft is measured 

The altitude of an aircraft is determined based on the measurement of atmospheric pressure: the greater the altitude - the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is called a pressure altimeter or barometric altimeter.

The altimeter is calibrated to follow the mathematical model of the International Standard Atmosphere (ISA).

When the ambient temperature at altitude is less than the ISA model, the surfaces of constant pressure (isobars) in the atmosphere are brought closer to each other. This effect causes that for nominal geometric height change the number of isobars crossed is increased, and thus the indicated height or altitude change on the altimeter - the altimeter will indicate a higher altitude or height than the actually flown. The opposite will occur in case of the ambient temperature is more than the ISA model - the altimeter will indicate a lower altitude or height than the actually flown.

Therefore the proverb: “If temperature is low, look below!”

For temperatures colder than ISA model, is therefore necessary perform the required corrections in order to preserve the required terrain altitude/obstacle clearances.

What this App does

Intended to be used by aircraft pilots, this app helps to calculate the altimeter error when the OAT (Outside Air Temperature) is below ISA temperatures, providing a corrected altitude when parameters as airport elevation and airport temperature are inserted.  

The calculations and results provided by this application are based on the International Civil Aviation Organization (ICAO) document 8168, Vol 3, Chapter 4.3.

The layout is intended to be as simple as possible, where significant points of an approach chart can be inserted and identified in an easy way.

For any query contact goheld@twilightpro.net