Originally Posted by JohnInReno
I am not trying to high-jack this thread, but my professionally installed 335 won't go into ground mode like it should. The shop said it is a Garmin problem and Garmin confirmed. I manually used Standby to pass the rebate test on the second try.
Anyone else experience this?
First off, we encourage you to get newer software (V2.12 is current now) if you have not already. We have made continual improvements in the logic in the GTX 3X5 series transponder that sets the transmitted status to "in air" or "on ground". We provided free updates to V2.12 at Oshkosh this year to anyone with a homebuilt who walked up with a GTX 3X5.
For aircraft with a gear squat switch, it is a no-brainer for the transponder to always know when it is on ground or airborne. For all us us with homebuilts that don't have a squat switch (almost everyone), the transponder has pretty complex logic considering all available data such as airspeed, ground speed, altitude changes, etc to make the best possible decision regarding whether the aircraft is still airborne.
The regulatory authorities never
want to see any transponder reporting an aircraft is on the ground when it is in the air, so it is very important to err on the side of reporting the aircraft is airborne if there is any question.
As you know, some aircraft can not only fly very slowly, but can fly with zero ground speed in some conditions, so the transponder has to be very clever in determining when the aircraft has landed, and when it has not. Again, it can never transmit an on-ground status unless there is good assurance it is really on the ground.
We fly an RV-7A with a GTX 345R, and frequently request ADS-B compliance reports to see what the FAA sees. As you know, these reports have a section called "Air On Ground" which is a comparison of what the FAA computer thinks your air/ground status is compared to what your transponder is transmitting. Unlike the transponder which doesn't have an airport or terrain database, the FAA computer probably knows the airport elevation and can use that information in combination with the transmitted pressure altitude and GPS altitude to determine when an aircraft is on the ground.
We have never seen a problem with late air-to-ground status transitions with our RV-7A (even back to GTX V2.03), and we seldom totally stop the plane on the runway, and certainly don't switch the transponder to standby. It is very common for the Air On Ground % Fail in our reports to be 0% or very close to that.
I will say that like any good "A" model driver, we exit the runway slowly and taxi very slowly all the way back to parking, which gives the transponder plenty of time to determine we have landed and switch to reporting ground mode - typically before even leaving the runway.
In the past when people have seen high % Fail numbers in the Air On Ground section of the report, the plane has often landed and exited the runway pretty fast. Those customers have always been able to get a passing "score" by taking it easy after landing.
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