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.

IGC data loggers record an uncorrected (29.92 assumed) pressure altitude in an integer number of meters. They may also record a GPS altitude as a separate item. Scoring software calculates a fixed correction based on the known surface height at takeoff that is applied throughout the flight. A correction computed at landing is used in the US at the finish if more favorable.

In the US competition environment, GPS altitude is not considered unless baro altitude is not available from the logger (i.e. it is not an IGC logger but is an IGC "position recorder").

If the logger is not plumbed to the aircraft static, meaningful differences can occur between your "steam gauge" altimeter and the logger. This is a function of how your cockpit pressure varies up or down relative to outside (mostly) with airspeed.

Differences also occur because the calibration curves of the "steam gauge" and the IGC logger are not the same.

Knowing how to monitor (while flying) the corrected-to-takeoff-elevation baro altitude being recorded in you logger is a very worthwhile endeavor and not always easy depending on your equipment.

This difference bites folks most often in two ways:
1. Busting the 17,500 or 17,900 MSL ceiling of airspace
2. Being too high or too low when crossing the finish cylinder boundary

Hope this helps.

Scorer with the scars to prove it,
QT