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Ground Proximity Detection For Transponder

David Paule

Well Known Member
I was looking over the installation manual for a transponder. Sure, it's getting a bit ahead of things, since I'm still on the empennage, but that's okay. The transponder has the capability to respond to surface movement radar on the ground. There's a manual switch setting for that, but if the airplane has a squat switch then it's automatic. RVs don't seem to have squat switches.

I used to play with robotics. There are ultrasonic proximity sensors readily available and for all I know, laser ones too. You can even build your own - here's a link to a schematic and article. This one detects up to about six feet, and three or four feet ought to be enough.

The idea is that something like this can be hardwired and installed, and trigger that mode of transponder operation when on the ground, automatically. I suppose it would also tell the transponder that it's time to switch to altitude reporting, too.

Dave
 
Please be careful. Transponders are certified, and a device that automatically turns it on and off for you would need to be certified as well. You're in fuzzy territory making a device that does this.

When Dynon wanted to do this auto air/ground we were going to do it in SkyView using airspeed and GPS, but we learned that was against the TSO, and thus we needed to do it inside the fully certified transponder. The FAA really cares about things like transponders that can affect safety of other aircraft.

Just trying to keep the experimental community on the right side of the regs...

--Ian Jordan
Dynon Avionics
 
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Are you saying that an airframe squat switch on an experimental aircraft would need to be TSO'd to be used in this purpose?

Because that's all a device such as this would be.

Dave
 
I am not an expert on what the TSO requires or how the FAA interprets it. However, what you are building is a "derived" squat switch, and that was what we were prevented from building based on the interpretation of people with certification expertise. We were going to derive it from GPS and AHRS data.

It's pretty easy to show that a mechanical switch gets pressed when the gear is compressed and the failure cases are easy to identify, so I'd imagine you could show a squat switch on your experimental "meets the requirements of the TSO". It's not so easy to prove that your system doesn't have a bug in the code or won't turn off when it flies over something noisy. Remember, the ONLY thing you have to have certified in your homebuilt is the transponder, so that tells you how the FAA cares about it.

It's up to you, I'm just giving you a heads up on our experience. You might want to call a DAR and see what they think before you spend too much time.

--Ian Jordan
Dynon Avionics, but speaking from personal experience, not as an opinion of Dynon's
 
I do appreciate the comments and the viewpoint you brought to this discussion. Thanks!

In my own case, I do not plan to build or integrate such a device. I'm building an RV-3B and hope to keep it relatively simple, or at least to keep the number of circuit breakers down. Still, looking at a transponder manual gave me the idea and I wanted to throw it out and see what anyone thought of it.

Thanks again,
Dave
 
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