Flytec 6020 * - Manual (Page 46)

Operating Manual Flytec 6020
9 Appendix
9.1 Altimeter
An altimeter is really a barograph because it doesn’t directly measure altitude, but air
pressure. Altitude is calculated from air pressure data. The pressure at sea level is used
as zero point altitude for the calculation of absolute altitude (according to the international
altitude formula).
Why does pressure change with altitude? 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 1 mbar at 500 metres local altitude
above sea level is a height difference of about 8m.
However, in practice, it is not as simple as that because more factors have influence on
air pressure. Therefore, air pressure is also depending on temperature and of course, on
weather conditions. On a stable day, temperature induced air pressure variations of 1mb
can occur, which means 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 again at certain intervals. This
means the altimeter has to be set to a known height and it needs to display this height.
During rapid weather changes (e.g. passage of a cold front), the air pressure can change by
up to 5 mbar during one day. This means a height difference of 40 m!
Another alternative to calibrate an altimeter is setting it to the actual QNH.
What is QNH? General air traffic needs a common zero point. This means that at a certain
altitude all aircraft show the same altitude on the altimeter. This common reference basis
is the QNH. The QNH is the actual air pressure in hPa ( 1 hPa=1mbar ) calculated back to
sea level. 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 Speed
9.2.1 True or Indicated Airspeed - TAS or IAS
In general aviation it is customary to measure the airspeed by use of a pitot tube as a dynamic
pressure speed (=IAS) and also to display it as such. The advantage of this method is the fact
that at any altitude level the maximum admissible speed or the stall is marked (flight safety) at
the same position on the scale. Furthermore it is the same for the speed of best glide for any
altitude which is on a fixed position on the scale (flight performance).
It is however the disadvantage of this system that all geographic calculations, such as for
distance, required flight path angle, and all final approach computing, necessitate the true
air speed to perform the calculation.
Another disadvantage is the fact that the indicated speed is correct only at one certain
altitude (usually at sea level). The higher one climbs, the glider will fly increasingly faster due
to the air getting thinner, without the display screen following this fact. At approx. 6,500 m the
air weighs only half of that at sea level, therefore the air speed will increase by 1.41 times
(as a radix of 2).
- 46 -