Are the two GTN boxes fed from separate antennae?
The comm frequencies to check are:
121.150, 121.175, 121.185, 121.190, 121.200, 130.285, 131.250, 131.275, 131.290 & 131.300 MHz. They are harmonic to the GPS frequencies.
Tune each frequency on each comm, and transmit for at least 20 seconds, but better, hold the transmit until the radio automatically cuts itself off transmitting (usually about 35 seconds). Do these transmissions while observing the satellite status bars for each GPS. A little wavering of the status bars during this is normal, but if your bars crash to the bottom, that’s the problem. Of course, be certain that such transmissions will not interfere with local ATC.
I do this test regularly following electrical system changes on aircraft, and have personally witnessed three fails over the years. In each case (two planes, one helicopter), it was necessary to relocate wiring.
Such a failure is not necessarily a comm nor GPS wiring error, it can simply be a wire, unrelated to either of those, which passes close enough to both the comm antenna and GPS antenna, or their wiring, so as to form a signal path between them when a transmission is made. Every time a wiring change is made in the plane, which adds wire anywhere near the comm and GPS antennas, this test should be done. Pilot/owners can and should do this test themselves post installation, if if there’s a problem, take the plane back to the shop for warranty on the wiring – the shop should be doing this test before they deliver the plane to you.
Peter wrote:
Are the two GTN boxes fed from separate antennae?
GTN’s have their own dedicated antennas for GPS. It is not an uncommon failure mode for a defective antenna to break into oscillation and kill GPS in the vicinity. Over tightening the antenna and cracking it can do this or a bad water seal. If one system is bad and it breaks into oscillation, both systems will show all satellites as gone, as the errant system acts as a jammer. If this is the case, one can isolate which system is causing the issue by turning one system off at a time. It usually takes several minutes before the non failing system to recover and reestablish a position, so you have to be patient when testing this.
Pilot_DAR wrote:
121.150, 121.175, 121.185, 121.190, 121.200, 130.285, 131.250, 131.275, 131.290 & 131.300 MHz.
Garmin lists six additional frequencies to check. But maybe your frequencies are the most important?
The list of frequencies to test came from Transport Canada Avionics Engineering, following their accidentally finding this problem with a maritime Dash 8 (Swedish, if recall – hang on, I think I have a thread drift photo of it).
If Garmin gives different frequencies, I would certainly test them to – but I have first hand experience that the TC frequencies will produce the conditions to indicate a defect in wiring. I do this test for just about every wiring change, and have done it more than sixty times for AWI’s Polar 5&6 research planes in Germany. They’ve always been fine, but they do change wiring in the cabin a lot!
My photos during testing of the liferaft drop modifications (unrelated to the GPS, but the same plane.
That’s me in the red running the Fire Department Zodiac with some trainees, to recover the dropped liferafts. Photo by the Swedish observer on shore.
Sorry for the thread drift, some thoughts just lead to others!
