GPS based FL - Flight Level

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GPS based FL - Flight Level

Postby kitercuda » 09 Jun 2019, 17:46

FL level is used for flying over transition altitude and for determining if you are inside of airspaces (I mean airspaces which vertical borders are defined by FL)
If your device has Baro (pressure) sensor and your aircraft has not pressurized cabin, you can display Baro flight level at your screen (long tap at the compass rose and select it). It is QNH independent, application set for calculating QNH to 1013 hPa (29.92 inHg).
Altitude strip (blue background) displays Baro altitude. So if you want to see flight level at altitude strip, you must set QNH to 1013 hPa (29.92 inHg).

If your device has not baro sensor, and you want to see GPS based flight level, you must set correct QNH. GPS flight level displays correct value only if you are flying in area, where is QNH 1013 hPa (29.92 inHg). If there is QNH different, GPS flight level displays incorrect value. But if you set correct QNH to application (long press to FL window), application calculates the difference and uses it for displaying flight level and airspace warnings.
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Re: GPS based FL - Flight Level

Postby RogerF » 09 Jun 2019, 18:18

I juste re-tested with the latest version and the QNH input is now possible.
Thanks for this new function!
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Re: GPS based FL - Flight Level

Postby navvegante » 19 Jul 2020, 01:30

Great TIP, appreciated
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Re: GPS based FL - Flight Level

Postby RogerF » 20 Jul 2020, 12:23

Hi Petr,

I have a suggestion to improve the calculation of the flight level from the GPS altitude + QNH.

The QNH is not enough because altimetry and areas top/bottom are purely based on pressure while the GPS provided a true altitude.
Therefore, the temperature deviation (with respect to the standard atmosphere) is required to improve the conversion.

A simple estimation of the correction a for temperature deviation of +1°C is -0.4% of the pressure-altitude.

For example:
- QNH=1013, but temperature at MSL = 30°C (instead of 15°C)
- Purely based on the QNH correction, flying at 6000ft AMSL displays FL060.
- However, FL060 is actually located 15°C x 0.4%/°C x 6000 ft = 360 ft higher.
- This means that FIF should display FL=56 instead of FL=60.
- As you see, the correction can be equivalent to a QNH correction of 12mb.
- At FL120, it would even be twice bigger.

- complement the FL data menu with a "Temperature deviation °C" button to define it.
- enhance the METAR/TAF button to calculate it automatically, based on airfield temperature, QNH and airfield elevation, taking into account that the standard atmosphere correspond to +15°C at the 1013mb level.

The good news is that all data is available in the navigation database.
However, as usually, it is easier to propose a suggestion than to implement it ;-).
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