GTX330 Automatic Air to Ground Mode Switching
Firstly, this doesn't work from the KLN94, even though the GTX330 installation manual shows the KLN94-GTX330 wiring. Garmin washed their hands of support on this when I had it installed and the avionics shop could not get it to work.
Secondly, you can end up with an inadvertent Enhanced Mode S installation. The concept of Elementary and Enhanced is a purely Eurocontrol invention which was done without considering what products actually exist, and since the GTX330 radiates any parameter sent to it via RS232 or ARINC the resulting installation becomes "Enhanced Mode S" which - on an EASA aircraft - is a Major Mod. So, with the GTX330, one needs to avoid sending it any data, which means that the ground/air switching needs to be done with an airspeed sensor or a squat switch. Or - if using a GPS which it actually works with - simply ignoring this stupid ruling which affects EU-reg aircraft only anyway. The reference to this is here (local copy). Some UK avionics shops have got so worried about this that they are reportedly ripping out the GPS-transponder connections on previous installations. Garmin don't talk about this issue. It appears that the GTX328 avoids the problem by never radiating anything, but it doesn't have the GTX330 functionality in other ways.
Page last edited 29th July 2008