647jc
Well Known Member
I have a GTX327 XPNDR and it receives pressure altitude information from my DYNON EFIS. While flight testing today I was at an indicated altitude of 5,000 ft. The pressure altitude displayed on the GTX327 was 4,900 which was probably correct since the barometric pressure today was not exactly 29.92. I contacted a local airport approach (approximately 40 miles away) and asked for a XPNDR check. They had me squawk a specified code and told me they saw me "bright" which I assume is good and they also said they saw me at 5,400 ft altitude. My XPNDR was displaying 4,900 ft so I don't understand why the tower saw me at an altitude 500 ft higher. Is there an adjustment in the GTX327 that can affect this? I tend to think in digital terms so I don't understand this descrepency. Perhaps the XPNDR converts the digital pressure altitude it receives from the EFIS to some analog equivalent before or during transmission and that is where the descrepency is. Any comments or expainations regarding this behavior, sure doesn't seem right to me?