I originally had a Terra transponder and radio in my RV-4. After trying different things for a long time, performance was still not what I wanted, so about two years ago I upgraded to a Trig TT31 and the Garmin GTR200. Much better, now I could actually be seen and heard for ATC flight following. But in my view, now it was beginning to be time to do something about ADSB Out, since the location of my airport in Seattle will require it. I am not overly interested in the information IN aspect of ADSB, so I needed only the WAAS GPS input for the TT31. What I really wanted was a unitized antenna and GPS like Dynon has, but it doesn?t play with the TT31, and nobody else was producing a similar unit that would. I considered the Trig TN70, which at about $1750 included a separate antenna and might qualify me for the $500 rebate, but then I would have to dance to the FAA?s fiddle to get the money. I decided instead to go with the Garmin 20A with a GA35 antenna which wouldn?t qualify me for the rebate, but would meet the 2020 standard; and guess what, the cost was almost exactly $1750 - $500.
My plan was to put the 20A in the back of my baggage compartment, on the raised part, and between the strobe unit and 406 ELT already there, then put the GA35 antenna above the baggage compartment, on the rear deck of the cockpit but under the canopy. I could run the COAX from the antenna back to the 20A inside the compartment along the top and side so it wouldn?t impact any baggage, run the wires to the TT31 under the floor, and supply the extra position output from the 20A to the ELT for more accuracy in the event of a crash. I created a form fit fiberglass gasket to handle the curve of the rear deck for the GA35, and the installation went exactly as planned, and looks rather neat. Since the antenna was under the canopy, I did not make up a backing plate for it as recommended. I then programmed the TT31 with the additional information it needed for the ADSB Out. The RS232 input #1 was already used by the serial altitude input from the Dynon D10A, so the RS232 input #2 was used for the GPS. The TT31 automatically recognized the Garmin+ protocol. I did not believe a squat or airspeed switch would be necessary, reasoning that air to ground switching would be handled from the GPS information, so an airspeed switch was not installed, and as described in the install manual the 6.1.6 Squat Switch Source input selection was left at the default ?Not Connected?.
Flight Testing: The FAA makes available a Public ADSB Performance Report if you request one from the following website:
https://adsbperformance.faa.gov/PAPRRequest.aspx
You go out flying, then anytime after you can request the report. It is quite simple to request and you receive it within 30 minutes although there is a little ?gotcha? where the 2nd GPS description is called for. You have to enter ?not installed? in the pull down field, or the form will just sit there and stare at you when you try to file it.
I rechecked connections, and turned the power on to the avionics buss. I left the rear seat back off so I could see the two LEDs on the 20A, each has a different flashing or steady message, and they seemed to be sequencing normally. The TT31 has a function selection that shows the coordinates when they are being sent out to ADSB. It was dashes for awhile, and then the coordinates came up, everything seemed to be working normally. So I buttoned her up, went flying locally out of AVQ, then landed and requested the performance report. It came back working well for all the airborne data, but I had the dreaded Red Mark for air to ground switching, failing 12% of the time. Unfortunately the TT31 shows the coordinates being transmitted, but as far as I noticed doesn?t tell you if the air message or the ground message is being sent, like the Dynon setup apparently does. OK says I, I will just taxi in GND position on the TT31, switch to ALT on the runway before takeoff, and back to GND on the rollout. So off I went, and tried it that way. The performance report came back exactly the same as the first time, great in the air, Red Mark for air to ground switching, with a 38% fail this time. Apparently GND and ALT do not mean ground messages and airborne altitude messages are being transmitted.
OK, I guess I have to install an airspeed switch for the TT31. It is used instead of the squat switch on aircraft without retractable gear to signal the change from taxi to airborne and back. It goes in the pitot line and around 40 mph (adjustable) the pitot pressure makes the switch and essentially connects pin #5 of the TT31 to ground. So I installed an airspeed switch, just under $50 from Aircraft Spruce. The big hiccup came when connecting to pin #5. I had purchased the wire harness for the TT31 when I bought it, however there was no wire in the harness to pin #5. Remember the TT31 was designed to replace the KT76 to the extent of using the same Molex connectors, but these did not look like any Molex I had seen. Some research turned up the Molex part number 08-05-0302, but also the fact that they were obsolete, Molex doesn?t make them any more and they are getting rare as hen?s teeth. I needed one lousy connector and I had to buy ten on ebay for $25 and was glad to find them.
Electrical connection finally made, now it is time to program the TT31. Step to the Squat Switch Source (6.1.6 in manual) <enter> Step through selections: ?Low when airborne? that?s what I want, (Low means connected to ground through the switch) OK, it?s in. But when I went through the rest of the selections there was also an ?auto on GPS? option. Could I have avoided the whole Airspeed switch requirement if I had entered this option? It wasn?t mentioned anywhere in the manual and there really wasn?t any reason to program a switch I didn?t have, the default ?not connected? seemed appropriate. Oh well, It?s done. Buttoned it up, flew locally for 15 minutes, Report came back, Clean Bill of Health!
I must say I really like the TT31, the display, switchology, and so far the reliability. The manual is well written and has excellent diagrams, but seems like it needs some updating.
My plan was to put the 20A in the back of my baggage compartment, on the raised part, and between the strobe unit and 406 ELT already there, then put the GA35 antenna above the baggage compartment, on the rear deck of the cockpit but under the canopy. I could run the COAX from the antenna back to the 20A inside the compartment along the top and side so it wouldn?t impact any baggage, run the wires to the TT31 under the floor, and supply the extra position output from the 20A to the ELT for more accuracy in the event of a crash. I created a form fit fiberglass gasket to handle the curve of the rear deck for the GA35, and the installation went exactly as planned, and looks rather neat. Since the antenna was under the canopy, I did not make up a backing plate for it as recommended. I then programmed the TT31 with the additional information it needed for the ADSB Out. The RS232 input #1 was already used by the serial altitude input from the Dynon D10A, so the RS232 input #2 was used for the GPS. The TT31 automatically recognized the Garmin+ protocol. I did not believe a squat or airspeed switch would be necessary, reasoning that air to ground switching would be handled from the GPS information, so an airspeed switch was not installed, and as described in the install manual the 6.1.6 Squat Switch Source input selection was left at the default ?Not Connected?.
Flight Testing: The FAA makes available a Public ADSB Performance Report if you request one from the following website:
https://adsbperformance.faa.gov/PAPRRequest.aspx
You go out flying, then anytime after you can request the report. It is quite simple to request and you receive it within 30 minutes although there is a little ?gotcha? where the 2nd GPS description is called for. You have to enter ?not installed? in the pull down field, or the form will just sit there and stare at you when you try to file it.
I rechecked connections, and turned the power on to the avionics buss. I left the rear seat back off so I could see the two LEDs on the 20A, each has a different flashing or steady message, and they seemed to be sequencing normally. The TT31 has a function selection that shows the coordinates when they are being sent out to ADSB. It was dashes for awhile, and then the coordinates came up, everything seemed to be working normally. So I buttoned her up, went flying locally out of AVQ, then landed and requested the performance report. It came back working well for all the airborne data, but I had the dreaded Red Mark for air to ground switching, failing 12% of the time. Unfortunately the TT31 shows the coordinates being transmitted, but as far as I noticed doesn?t tell you if the air message or the ground message is being sent, like the Dynon setup apparently does. OK says I, I will just taxi in GND position on the TT31, switch to ALT on the runway before takeoff, and back to GND on the rollout. So off I went, and tried it that way. The performance report came back exactly the same as the first time, great in the air, Red Mark for air to ground switching, with a 38% fail this time. Apparently GND and ALT do not mean ground messages and airborne altitude messages are being transmitted.
OK, I guess I have to install an airspeed switch for the TT31. It is used instead of the squat switch on aircraft without retractable gear to signal the change from taxi to airborne and back. It goes in the pitot line and around 40 mph (adjustable) the pitot pressure makes the switch and essentially connects pin #5 of the TT31 to ground. So I installed an airspeed switch, just under $50 from Aircraft Spruce. The big hiccup came when connecting to pin #5. I had purchased the wire harness for the TT31 when I bought it, however there was no wire in the harness to pin #5. Remember the TT31 was designed to replace the KT76 to the extent of using the same Molex connectors, but these did not look like any Molex I had seen. Some research turned up the Molex part number 08-05-0302, but also the fact that they were obsolete, Molex doesn?t make them any more and they are getting rare as hen?s teeth. I needed one lousy connector and I had to buy ten on ebay for $25 and was glad to find them.
Electrical connection finally made, now it is time to program the TT31. Step to the Squat Switch Source (6.1.6 in manual) <enter> Step through selections: ?Low when airborne? that?s what I want, (Low means connected to ground through the switch) OK, it?s in. But when I went through the rest of the selections there was also an ?auto on GPS? option. Could I have avoided the whole Airspeed switch requirement if I had entered this option? It wasn?t mentioned anywhere in the manual and there really wasn?t any reason to program a switch I didn?t have, the default ?not connected? seemed appropriate. Oh well, It?s done. Buttoned it up, flew locally for 15 minutes, Report came back, Clean Bill of Health!
I must say I really like the TT31, the display, switchology, and so far the reliability. The manual is well written and has excellent diagrams, but seems like it needs some updating.
