Altitude and Altitude IGC

[the_gle...@msn.com]

There are two pressure Altitude Navboxes in LX Styler - Alt and Alt IGC. I presume that Alt IGC is what will be saved in the IGC file. Does anyone know:

What is the difference between the two?

Which is set or confirmed on starting the instrument, or is it both?

What is adjusted in Settings/QNH?

What is used for final glide etc calculations and for height AGL?

I am looking at this because on a couple of recent flights I have confirmed my field elevation on startup, but the Alt Navbox has shown something a bit different by the time I have taken off. I believe I should look at Alt IGC as my altitude, since if I keep out of airspace using that then I expect there will be no danger of the IGC file showing an infringement.

Thanks for any input. If no-one here knows the answer I will email the question to LXNAV.

Mark Burton, London Gliding Club, UK

[Benjamin Meier]

Hi Mark

Alt: Altitude above MSL [Probably based on whatever you have set in below mentioned setting or always pitot-static, this is not mentioned and I haven't tested that]
AltIGC: Altitude above MSL( [based on] IGC [sensor using cockpit air pressure])  
RawIGC: Raw altitude from IGC pressure sensor (Above 1013 level) 

  5.1.1.4 Altitude Source The system has two pressure altitude sensors. One is built into the main display unit and is used for the IGC recorder and the second one is built into the vario unit and is connected to the glider pitot-static system. Changing the altitude source defines which sensor is used for pressure altitude used in program.  

Above are excerpts from the LXxxxx Manual (lx90xx-80xxUserManualEnglishVer802rev41) [with my comments in square brackets]. 

1. Difference between the two is stated above, normally you would want to have the altitude used to be from the pitot-static system, as this has less errors than the cockpit pressure. Depending on your cockpit ventilation you get a slight over or under pressure compared to static and this will change your AltIGC a slight bit.

2. Both Alt and AltIGC are set when powering on the instrument, actually it is the QNH that is set that way.

3. The same setting is adjusted with Settings/QNH.

4. You can choose, normally you would take pitot-static.

The IGC file records "RawIGC" and GPS-Altitude, corrections for QNH are done in post processing and might differ between platforms.
Now the question remains how the airspace infringements are calculated in the software used. But in general, yes even if you have an offset from cockpit ventilation the trace should not show an infringement if you use AltIGC.
My take on this however is to use normal Alt and calculate some extra safety margin in to allow for the difference from Alt to AltIGC, you could also test this difference in various configurations (Ventilations open and closed, sliding window open and closed, different angles of sideslip, at different airspeeds).

Benjamin Meier, SGPilatus, Switzerland

[the_gle...@msn.com]

Thanks Benjamin, that was helpful and some of it was new to me.

I know you are correct in saying you should take an extra safety margin to keep out of airspace, but sometimes it is tempting to go close. For example recently I was in a thermal under an airspace ceiling of 2,500 foot, about 2,000 foot or 600m over the ground and I wanted to take the maximum height possible. (Because I couldn't climb higher there in the end I had to start my engine). Obviously the results should be close but if the IGC file showed that I had gone even a few feet too high then I would not be able to put it online. That is why I worry about the altitude shown in the IGC file.

I have source set on the QNH page to IGC - I presume that means that is what is used for final glide and for height above ground and I should probably change it to Vario. That will be more accurate for the flight computer and will make the front seat and back seat use the same data for final glide. On the other hand the Navboxes I see are set to Alt not to Alt IGC - and I should probably change them to Alt IGC for if I go close to airspace.

BTW - what started me worrying about this was what happened on a couple of flights. I confirmed my field elevation on switch on, but by the time I took off the Alt display had changed a bit. Then when I looked at the IGC file on SeeYou my take-off was shown at the correct airfield elevation. I wonder why the Alt changed away from airfield elevation - perhaps there is a problem with my vario unit. Now I understand where the info comes from I will monitor it; I think I will show both Navboxes on the next flight.

Thanks again, Mark

[the_gle...@msn.com]

I have now had a couple of flights showing on my default screen both Alt and Alt IGC. They are both always identical. I have not noticed any further odd behaviour.

As mentioned before source is set to IGC

Mark Burton