Flytec 6030 * - Manual (Page 46)

Flytec 6030-GPS
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9 Appendix
9.1 Altimeter
General inforation
How an altimeter is it functioning?
An altimeter is really a barograph because it doesn’t directly measure height, but air
pressure. Height is calculated from changes in air pressure. The pressure at sea level is
used as zero point height for the calculation of real height (after the international height
formula).
Why does pressure change with height? Pressure at any given point on the earth is created
by the weight of air in the atmosphere above it. Therefore, pressure reduces with height –
there is less air above you. A change in pressure of one millibar (mb) at 500 metres above
sea level is a height difference of about 8 metres.
In practice, it is not as simple as that because of the many other factors that influence air
pressure. Therefore, air pressure is also depending on temperature and of course, on
weather conditions. On a stable day, temperature induced differences of 1mb mean a height
difference of +/-10 metres. Depending on the weather, air pressure at sea level (QNH) may
vary from 950 mb to 1050 mb. In order to eliminate the influence of the weather, the
altimeter has to be calibrated at certain intervals. This means the altimeter has to be set to a
known height and to show this height.
During rapid weather changes (e.g. passage of a cold front), the air pressure can change by
5 mb during one day. This means a height difference of 40 metres!
Another way to calibrate an altimeter is to set it to QNH.
What is QNH? General air traffic needs a common zero point. This means that at a certain
height all aircraft show the same height on the altimeter. The reference point for this is QNH.
The QNH is the actual pressure calculated back to sea level (1hPa=1mb). It is calculated
several times a day and can be taken from the weather forecast for aviation or it may be
requested by radio from airfields.
9.2 Variometer
9.2.1 Gross- Netto - Vario
In contrast to the standard Vario which shows the vertical speed of the glider wing, the Netto
Vario indicates the rising and sinking of the surrounding air mass. How does the instrument
provide this performance? The prerequisite for this is a correctly entered polar curve and a
speed sensor, of course. Let’s assume that a pilot is flying at 50 km/h through the air.
The FLYTEC 6030 GPS determines from the polar curve that at 50 km/h there would be a
sink rate of 1,1 m/s to be expected. In our example, the normal Vario could only show 0,5
m/s, therefore the surrounding air has to rise by 0,6 m/s in order to reach this pair of values.
If however, in our example the normal Vario would show a sink rate of 2 m/s, the air mass
would have to sink correspondingly by 0,9 m/s. This means, with the correct polar curve and
in calm air the Netto Vario would have to show a value of 0 at all speeds. Or, the other way
round, we are hereby in position to check our polar curve entry if we are certain that the air is
absolutely steady. If in the upper speed range the Netto Vario would indicate continuously
rising air with 0,3 up to 0,5 m/s, then we would know that our wing is better than the saved
polar curve and the effective sink rate is by about 0,4 m/s lower than the polar display.
This fact can be corrected.