Based on the file format, I suspect that adding new fields will be relatively flexible. There is a header row with one row for Labels/units and a row for datafield names.
It has to be flexible because different installations will have different fields. For example CHT 1-4 vs 1-6.
I suspect there is simply something wrong in the header rows that is causing the issue.
The solution I have now will will upload to fly.garmin and looks great on the web interface, but there is something wrong with the data.
When I try to open the log on the iPad or iPhone app, the app crashes closed.
I suspect there must be some minor bit of formatting that is missing or incorrect from the file.
When the app tires to load the data, it encounters an error, something is missing or doesn't match the format it expects, throws an exception, that is not handled and the app crashes.