Couldn't the 24 month mandatory Transponder test be accomplished in the air with ATC?
 

I keep wondering when I am flying VFR and getting "flight following" with ATC and my transponder is accurately reporting my altitude, why it is necessary to spend the time and money to get a transponder check every 24 months. I never fly under IFR but I do fly through and in Class C and B airspace often. If the purpose of the transponder check is verify the accuracy of your transponder, couldn't that be accomplished by checking in with ATC and verifying their altitude readout matches your actual altitude? Or am I missing something else that makes the transponder check by a qualified avionics tech necessary?

If I am not missing something that affects flight safety, I am thinking that the transponder check could be accomplished by the pilot and logged much like the required VOR check every thirty days for IFR operations. I realize that would require a change to the FAR's which is probably impossible.

I can just see a whole bunch of people falsely confirming the altitued reported..., thereby validating (?) their transponder reported altitude. I DON'T want to be flying around in a cloud with one of those people within 20mi. of me.

There's already enough errors in the system
 

I agree with Scott. Wouldn't want to take out a calibrated standard and rely on ATC for best accuracy, even if the pilot is telling the truth. ATC is reading pressure altitude from your transponder and converting based on pressure at a particular ground reference point. In a steep gradient, pressure can vary widely, especially if the sector they're working covers a wide geographic area. More Accuracy = Greater Safety. Now, if I could just hold altitude as well as an autopilot...
Terry, CFI
RV-9A N323TP

For VFR use, the 24 month transponder check is called up in FAR 91.413, and it points to FAR 43 Appendix F for the details of the check. If you look at FAR 43 Appendix F you will see that the whole point of the check is to confirm the transponder is transmitting on the correct frequency, has the correct power etc. There is nothing in the check that confirms the altitude is reported correctly.

Bottom line - the check you propose has nothing to do with the tests called up in the FARs.

The transponder check verifies the following parameters are within specified limits:

Transmit frequency (+/- 3 Mhz)
Recevier sensitivity level
Min and Max Output power
Squawk codes
Alltitude data correlation
Interrogation reply rate
Side lobe suppression

The integrity of the ATC system depends on this data being correct and without causing interference to other systems or aircraft.

Personally I think there should be a comm check too, the sound of some of the radios I hear is horiffic and they will never get fixed until they outright quit in most cases but in the meantime they disrupt normal communications.

Same thing would happen to the ATC system if there weren't some required tests to make sure transmitting equipment is performing properly.

Transponder check every 24 months
 

Thanks for the details on the purpose of the transponder check. I withdraw my dumb idea.

Most radar sites have no idea what your actuall altitude is. That is what your transponder gives them. If it is wrong what they show you at will be wrong as well.

I know there are some radar sites that do determine altitude but it is my understanding these are military sites.

Here tucked in the mountain valley I'm doing my phase 1. No radar coverage and most days no one else even in the area. I did a couple radio checks with others I heard...all clear. 14hrs or so into my phase 1, I ventured to KBZN. at 15miles out I called them up. They responded, "is there something wrong with your transponder?, we're not showing you on radar".:eek:I didn't even know they had radar(new since I was hiding in the shop)...I said "apparently, it must be broke, here I'll cycle the breaker" tada a few minutes later they had me, but no ALT readout. I hadn't configured the Dynon output properly.

Amazing how well they work when you turn them on. I still need to get the official checks completed. But last week everything was turned on and they picked up as I came over the mountains at the correct Alt. Nice to know it's ok, until I can get somewhere that can do the cal check.

Is there any way to legally test your transponder while the plane is still in the garage? (I don't mean a calibrated test, I just want to know if I've wired the thing correctly, especially since it runs through the Dynon encoder module.)

If you have a digital transponder - say, a GTX 327 or GTX 330 - you can do a basic verification of your encoder wiring by looking at the pressure altitude readout on the display. If it says something like "FL052" and you are at 5000 feet, then it's probably hooked up right. If it says "FL___" then it isn't getting an altitude input.

mcb

I thought it was a good question. I'll bet more people will take the check more seriously now that they know what's involved!

This is a good question, however I don't think it's a bad idea to occasionally ask ATC what altitude your transponder is reporting. If you have a standalone altitude encoder there is no easier way of knowing that it is reporting correctly except doing a full IFR-style pitot/static certification. Be sure you are using the same altimeter setting as the controlling agency though (we should all be doing that anyway, right?).

Thanks Matt. Unfortunately, I have a GTX320A. Any similarly easy ways to get a basic test on that?

For that style of transponder, you pretty much have to use a transponder test set.

mcb