Flytec 6040 * User Manual download pdf (Page 19)

Operation Manual FLYTEC 6040 GPS 17

2 Functions

2.1 Altimeter and air pressure

2.1.1 How does an altimeter work?

A barometric altimeter calculates altitude from the present air pressure of the atmosphere.

Air pressure will decrease at increasing height. Due to the fact that air may be compressed,

the pressure decrease is not linear, but indeed exponential. The basis for altitude calculation

in aviation is an international formula which defines a standard atmosphere.

In the CINA- Standard atmosphere the basic pressure on sea level is 1013,25 hPa (Hektopascal) at a

temperature of 15°C. Furthermore it defines a continuous temperature decrease at increasing

height of 0,65°C per 100m ascent.

Therefore is binding: a barometric aviation altimeter displays the precise altitude only if weather

conditions are in exact accordance to the standard atmosphere. In practice, such analoguey is more

likely to be the exception!

Air weight and pressure are strongly influenced by air temperature. If temperature deviates

from standard atmosphere, the display of altitude calculated as per the international formula

is no longer correct. The altimeter displays during summer, when temperatures are higher, indeed

altitude parameters which are too low, and during wintertime it is exactly the contrary!

Flying at lower temperatures is effectively done at lower altitude, and at higher temperatures

flight altitude is higher than the altimeter displays! The deviation of 1 °C per 1000 height meters induces

approx. 4 m error. (This empirical formula is valid for up to 4000m!)

If you are flying during summer through 2000 height meters in an air mass being too warm

by 16 °C compared to standard atmosphere, the altimeter will then display 2 x 4 x 16 = 128m

difference in altitude below real height! Based upon the internationally determined altitude calculation

with standard values, this display error caused by air temperature shall not be

rectified by the instrument.

Air pressure changes in relation to weather conditions. In order to compensate for display

fluctuations, an altimeter always needs to be gauged. This means that the altimeter has to

be set precisely before take-off for any flight to a well-known altitude value. Caution:

the atmospheric pressure may change during the timeline of one day up to five Hektopascal

(for ex. cold front). As a result this is after all the equivalent of more than 40 meters height difference.

There is another possibility to gauge the altimeter which is to enter the current QNH pressure value.

The QNH (Question Normal Height) applied in aviation matches the current local

air pressure, as it would be at sea level, so that the altimeter would indeed display 0m.

Due to this process the local pressure data recorded by the various measurement stations

is area-wide comparable, irrespective of the geographical height.

The QNH-value is subject to be continuously updated and may be read in the flight weather report, or

required by radio from airfields, or by enquiry on the Internet.

The instrument provides5 altitude displays and one QNH pressure indicator.

ALT1 The altimeter absolute altitude shows the altitude over sea level.

ALT2 The altimeter relative altitude shows the altitude towards a reference point.

Alt. Sum Displays the cumulated altitude for one flight.

FL (ft) Flightlevel pressure altimeter display in relation to 1013hPa

ALT GPS GPS altitude

QNH hPa Actual air pressure at sea level in relation to A1 in hPa

QNH inHg Actual air pressure at sea level in relation to A1 in inHg

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Flytec 6040 * User Manual Page 19

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