The Altimeter; Altimeter; Setting The Altimeter - Flytec 3040 Operation Manual

3.

The Altimeter

How does an altimeter work?
A barometric altimeter calculates altitude (elevation) from the actual air pressure of the
atmosphere at a given location. Air pressure decreases with increasing elevation; however,
since air is compressible, the pressure change is exponential not linear. Altimeters designed for
aviation use the CINA (Commision International de Navigation Aérienne) formula to derive
altitude from air pressure. In this calculation the CINA–atmosphere is used where standard
atmospheric pressure at sea level is 1013.25 hPa (Hecto-pascal) at a temperature of 15°C.
Temperature also decreases with increasing altitude and must also be considered in the altitude
calculation. A constant temperature decrease of 0.65°C per 100 m ascent is also assumed in
the CINA equation. Because of these assumptions with respect to pressure and temperature, a
barometric aviation altimeter only indicates the actual altitude when the weather conditions
correspond to the standard atmosphere and lapse rate. In reality the atmosphere rarely
corresponds to the CINA standards.
To further complicate matters, the air pressure over a given location changes as weather
systems move across the area. To compensate for pressure changes induced by changes in
the weather, an altimeter must be adjusted prior to each flight. This can be done by setting the
altimeter to a known elevation (e.g., launch). Another method of setting an altimeter is to enter
the current QNH pressure value. QNH is the barometric pressure at a measuring station
reduced to sea-level. If an altimeter was set to the QNH at a measuring station (regardless of
elevation) and then brought to sea-level it would read zero. The QNH value is constantly
updated and can be obtained from flight service stations or requested from airfields over an
aeronautical radio. Keep in mind that the atmospheric pressure can change up to five millibars
over the course of a day (such as with the passage of a cold front), corresponding to a change
in elevation of more than 130 ft.
3.1

Altimeter

The 3040 is equipped with three altitude dispays:
ALTI ft Absolute altimeter in feet
QNH QNH pressure in inHg or hPa
ALTI m Absolute altimeter in meters
Short-press the yellow altimeter key to alternate between displaying ALTI ft,
ALTI ft
QNH, and ALTI m. The arrow indicator on the left side, in the upper line of
QNH hpa
the lower display window, indicates the selected altitude function.
ALTI m
3.2

Setting the Altimeter

The altimeter indicates the absolute altitude above sea level in ft or m. When you adjust the
altimeter, the QNH value is automatically adjusted. The QNH display shows the barometric
pressure reduced to sea level. Consequently, if your current altitude is unknown it can be set by
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setting the QNH available from weather reporting stations or
flight service. When you change the value of QNH, the
altimeter is automatically adjusted.
To set the altimeter, select the desired altitude value with short
presses of the ALTI ft. / QNH hPa / ALT m. key (altitude in feet
in the example). Long-press the same key to enter Set-mode,
and the current altitude will begin to flash.
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