On Monday, March 5, 2018 at 2:48:06 PM UTC-5, wrote:
How do current certified loggers determine altitude? Being some are plumbed to external pitot/static and some are cockpit pressure only, what differences are expected?

Does your mechanical altimeter (especially 57mm size) read close to your "glide caculator" altitude?

Are loggers solely dependent on pressure altitudes or is GPS altitude comparison ever included in the algorithms?

Seems one would be more likely to "bust" min/max altitudes using cockpit pressure loggers.

Read chapter 4 "pressure altitude recording" of http://www.ukiws.demon.co.uk/GFAC/do..._spec_gnss.pdf for the first question. If you check the Flight Recorder approval documents, they say, for example, for PowerFLARM: "The recorder case is not pressure-sealed and "cockpit static" pressure is recorded on the IGC file." For CNv-IGC (which you might assume uses plumbed static pressure) it is the same (not pressure sealed, "cockpit static" is recorded. An assertion that some FRs use plumbed sources is incorrect (you can go through the 59 Approval Documents if you want to check your specific one at http://www.ukiws.demon.co.uk/GFAC/igc_approved_frs.pdf ).

Second has nothing to do with flight recorders since you put QNH into glide computer and altimeter (neither are FRs; Flight Recorders use QNE as a reference). Mine agree reasonably closely (50' or so) since I had my winter altimeter (57 mm) recalibrated at the factory. They recommend every couple of years. Before calibration - many hundreds of feet out. I find my V3 vario (set QNH) and CNv Flight Recorder (set QNH) read very closely to each other..

Third: Pressure alt to 15,000m/49,213'; above, approved "High Altitude Flight Recorder", see Sporting Code 3b, Chapter 2.1. http://www.ukiws.demon.co..uk/GFAC/d...sc3b(2015).pdf .

Last question - mentioned in Grand Prix threads - open cockpit vents at start (max alt), close cockpit vents on Vne finish (min alt). Minor pressurization effect, but harsh penalty after 5m in GP. You could do a couple of tows and do short final glide at Vne with vents open on the first, closed on second, to determine whether it's worth it to you. The effects are essentially the same for all finishes (around same speed, around same altitude) so it balances out somewhat. The altitudes are compared to the baseline altitude after landing too.

Dan