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2 year transponder recertification


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#1 Roger Lee

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Posted 18 September 2010 - 11:13 PM

Hi All,

Just a note.

Do not forget to re-certify your transponder. It is due every two years. I have seen a few come through out of date. Yes, it is required if you fly in B,C, or D air space. It was part of your original equipment so don't let it slide. Here in Tucson it cost about $85, but can vary quite a bit and be much higher so ask around before you pick an avionics guy to do this. It does not take long and is done right on the plane
Roger Lee
Ryan Airfield (KRYN)
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#2 Doug in IL

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Posted 07 June 2012 - 01:29 AM

Generally Roger, does the radio guy have to hook up to your transponder or what kind of hands-on does he have to perform to test...? Also, was told Mode-S cost more..dj

#3 Roger Lee

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Posted 07 June 2012 - 01:32 AM

Hi Doug,

Yes he hooks up to do the test, but he doesn't have to take anything apart to do it. My guy charges $85 for a transponder recert. I'm not sure about the extra for mode S.
Roger Lee
Ryan Airfield (KRYN)
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#4 Doug in IL

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Posted 07 June 2012 - 01:35 AM

Thanks! How does he hook up? He must have to slide it out to hook up to the rear connector, I guess, or does he test the antenna signal???

#5 Roger Lee

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Posted 07 June 2012 - 01:48 AM

Doesn't have to slide anything out. test it just like it sits in the plane.
Roger Lee
Ryan Airfield (KRYN)
Tucson, AZ.
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Authorized Rotax Repair Center - Heavy Maint Rated
520-574-1080 Home Try home first
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#6 Doug Hereford

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Posted 07 June 2012 - 10:58 AM

Doug/Roger,
The check that you guys are referring to is commonly referred to as the "VFR" check. It is required by FAR 91.413 each 24 calendar months when you operate in airspace specified in FAR 91.215. I used to do them and charged $70. All that happens is that the transponder antenna system is coupled to the ramp test equipment via a calibrated coupler that sides over the antenna. There is also new designed equipment that can be user calibrated, and sit some distance from the antenna. Some shops charge more for mode S. My equipment checks both, so I never did. Anyway, here are some thoughts.

FAR 91.413 requires that if any maintenance is done which could introduce data correspondance error (altimeter/encoder/transponder) the integrated system must be checked IAW FAR 43 app E ©. What this actually means is that if you remove and reinstall the transponder, encoder, install new equipment, or repair associated wiring regardless of whether or not you fly IFR, the integrated system must be checked. Also, this check must be performed by an appropriately rated repair station or aircraft manufacturer.

As an appropriately rated repair station, I don't do "VFR" checks any more. The nature of the involved equipment (altimeter, encoder, transponder, and static system) is such that integrated system errors can occur over time. Altimeters are nothing more than calibrated barometric pressure instruments, and can drift out of tolerance, encoders drift as well.Many of the air data competers used in non-certified aircraft are also prone to errors. Static systems easily develope numerous leaks over time, and many maintenance people don't realize that R&R of transponders/ADC's (when they give gray code or AIRINC 429 altitude output) and or encoders or opening of static systems (except drains) can introduce integration error (which requires checks IAW FAR 91.413). What all this boils down to is that you can be flying around VFR, reporting an altitude much different that your actual flight altitude. ATC may be separating traffic based on reported altitudes for VFR and IFR traffic. This could set-up a dangerous situation. I now only do the "IFR checks" for this reason.

My advice is to have the IFR check done every twenty-four calendar months even though the regulations don't require it. It does however, run up to three times as much money (assuming no problems) but it is well worth it.

Doug Hereford

#7 Runtoeat

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Posted 07 June 2012 - 12:34 PM

My avionics tech brought his equipment to my hangar and did my 2 year transponder cert. His equipment didn't require any connection to the airplane and took about 10 minutes. I believe that he sent a test queery code to my CT's transponder and checked the response from this to make sure the unit was correctly reporting the altitude and was on frequency but not totally sure about this. He charged me $75.
Dick Harrison
CTSW N9922Z

#8 Doug Hereford

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Posted 07 June 2012 - 11:25 PM

Runtoeat,
The transponder test checks the following things: Transponder reply frequency, sensitivity, and transmitter power. It also checks to see that the transponder properly suppresses responses to side lobe (false) interrogations. These are the only required checks for the "VFR" certification. Most any ramp test equipment will also display the currently reported altitude from the aircraft's encoding system through the transponder, and mode A squawk code as is dialed in (most of the time it will be 1200). I always run through all possible mode A codes when doing the transponder cert. because some of the older units have wafer type switches for this output. These switches oxidize over time if never moved from 1200, and can cause improper code output. I also check Ident function.

The problem with this check as I have said eariler, is that the only altitude output verified on a "VFR" check is the current aircraft altitude corrected to 29.92". With any gray code transponder (many still are wired this way) there are nine code lines that produce the various codes for each altitude in one hundred feet increments up to 14,800 ft. The only way to verify the presence of each of these lines, is to run the encoder up in altitude to the maximum altitude that the aircraft is certificated to, and compare transonder output to the pilot's altimeter as it is simultaneously ran up. The reported altitude needs to match what the pilot sees (corrected to 29.92") within +- 125 ft. The VFR check does not do this. I have seen numerous cases where on code line is missing which can translate into as much as a three thousand foot error. All this says nothing about altimeter calibration errors which are also possible. Any shop that does the "VFR" check must also verify Airworthiness Directive status of the transponder. There are a number of transponders out there that have AD's against them.

All this is why I don't do "VFR" checks anymore, and recommend that people spend the extra money and have the "IFR" check done. I realized that this probably sounds like me talking out of both sides of my mouth. I do not intend to. A compromise would be to have the "IFR" check done every 48 months, or at every other cert. interval. Just remember that if you, or your mechanic open the static system, remove the transponder or encoder, or make any wiring repairs that could introduce error into this integrated system, the correlation check as I have described above does become required by law (ref. FAR 91.413 (B)).

By the way, I lose quite a few certification jobs each year by refusing to do the "VFR" check for them. I know another shop in my area that will do the check, and I send them to it.

Doug Hereford

#9 Doug in IL

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Posted 08 June 2012 - 12:50 AM

Thank you for the education Doug, Hooooly Smokes! What a science project....We're unworthy! :wacko:

#10 Doug Hereford

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Posted 08 June 2012 - 02:26 AM

All,

One correction to what I posted eariler: Opening of the static system by itself would not require by law, an integration check for VFR ops. It would be prudent to perform a static system leak check though after the system is restored. Here is a scenario:

Mechanic removes some static system plumbing to facilitate other maintenance. Maybe some instrument panel backlighting is bad and access is poor with certain pumbing lines installed. Mechanic repairs the lights and reinstalls the plumbing. Lets say the static plumbing is the white poly-flow tubing that has the little stabilizer ferrules in each connection. In this case the ferrule falls out while the line is disconnected and goes unnoticed by the mechanic. System is restored, and the mechanic knows that the aircraft is VFR only. He is sure that the connection is tight, so no leak check is performed. Poly flow system requires these ferrules, and will almost never seal without them.
Winter comes and the pilot is flying on a long cross-country flight with the cabin heater on high. The aircraft cabin is nice and tight (Like CT's are), and so consequently, the density altitude inside the cabin is considerably higher now than the actualy altitude that the aircraft is flying at. Because the mechanic induced a static leak last summer, and it went undetected during the warm months, no one is suspicious of anything. Now the altimeter and encoder are getting a corrupted static signal from inside the heated cabin, instead of the originally designed static system which samples true atmopheric static pressure. Result is that the aircraft is actually lower than the pilot (and ATC) thinks it is. Maybe it is night time, and the pilot routinely flys over a certain pass that he knows he can clear with four thousand feet of altitude. This could be bad.

Another scenario: Mechanic removes the transponder to facilitate other maintenance. When it is racked back in, it catches on the front of the instrument panel just enough not to fully seat into the rack. The wiring connector is engaged, but the coax connector is not. The latching screw feels tight, and the transponder lights up with the avionics buss, so the mechanic thinks that the ops check is ok. Because the coax connection did not fully engage, a portion of the transmitted RF energy is lost when the transponder replys. Enough signal is lost that mode C (altitude) information is corrupted, and erroneous altitudes are now reported to ATC. This a situation where by law, the owner should have had the integrated system checked (even if they are VFR only).

This may sound like a bunch of bull sh!77, but I have actually been involed in both of these situations many times. Fortunately I discovered my (or someone else's) errors by performing the required (or recommended inspections).

We have to assume that we are all "unworthy", and perform inspections to find unsafe conditions that we or others may have induced.

Doug Hereford

#11 Runtoeat

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Posted 08 June 2012 - 03:46 AM

Doug, thanks for taking the time to provide the technical info. I am better informed as a result of this. I have a question regarding your point about possible incorrect altitude reporting by the transponder. My Garmin GTX327 transponder displays pressure altitude. As a check to insure the transponder is correctly reporting altitude, can one set the altimeter pressure to 29.92 to get pressure altitude and compare this to the transponder readout? And, as a sanity check to insure that the altimeter is working correctly, can it's true altitude indication be compared to my Garmin GPS's altitude reading? I know that my altimeter and GPS are not certified equipment so this is just working with what I've got and making an assumption that a decent probability exists I'm correctly reporting altitude if 3 pieces of equipment agree.
Dick Harrison
CTSW N9922Z

#12 Doug Hereford

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Posted 08 June 2012 - 10:06 PM

Runtoeat,

You are correct. More modern transponders like the GTX-327 show encoded altitude inputs right on the display. In my opinion, it would be safe to assume that that information is actually being transmitted as displayed (assuming that the transponder was certified IAW FAR 43 app. F). You stated eariler that your's was. You are also correct that if you adjust the baro setting on your altimeter to 29.92, you could verify correlation of the altitude reported to altimeter display. Unfortunatly, most transponders still in service (outside of the Garmin line)today don't have this handy feature.
As for GPS altitude vs. the altimeter, I would be careful with this. As I'm sure you know, altimeters are nothing but calibrated barometers. GPS altitude may be actual AGL altitude. I don't know, it may depend on the particular GPS, and/or other factors.

One caution to all: Many panel mounted GPS units get encoded altitude information from the encoder/Airdata computer via ARINC 429, RS232 buss or gray code lines that "Y" off of the encoder. Don't assume that just because the altitude readouts on these GPS units (like GNS 430/530) are the same as is being reported to the transponder. When ever I do a certification, I always pull up the relevant page on the GPS to see encoded altitude reported to it and compare to what the transponder is putting out (the important and legally required stuff). I have seen on a number of occasions where the altitude on the GPS was correct, but who ever installed it did not wire the code lines correctly down stream of the GPS, causing erroneous altitude reported from the transponder. I even recall one installation where the RS232 buss was complete to the GPS, but was never wired to the transponder, hence no altitude reported at all. This should have been readily apparent, but the required certification check had apparently not been done. We were actually performing an Annual inspection on this aircraft, and noticed no altitude displayed (GTX 327). After further investigation, we found the omitted wiring.

One other statement that I have heard repeated over the years: "There is nothing like seeing the actual information radiated from the antenna." Its like visualizing the tube pass through the cords (Roger will know what I mean).

Doug Hereford

#13 Roger Lee

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Posted 08 June 2012 - 11:51 PM

Hi Doug,

I do not what you mean about watching the tube pass through the cords. Were you a medic?
Roger Lee
Ryan Airfield (KRYN)
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#14 Doug Hereford

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Posted 09 June 2012 - 12:04 AM

Roger,
I still am. Driver/Medic on One's Pumper/Heavy Rescue in Kansas City (IAFF Local 64)for 22 years. We run ALS pumpers, and also the transport service. The FD is sadly stress relief from the idiosyncrasies [scarsasm] of running an aircraft maintenance business.

#15 Roger Lee

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Posted 09 June 2012 - 12:29 AM

30 years for me and retired 4 years ago. I was a Capt., Hazmat Tech and medic. The oldest medic on the job when I left. I was on a paramedic hazmat engine company. (Local 479)
Roger Lee
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#16 Doug Hereford

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Posted 09 June 2012 - 05:09 AM

Roger,

Yep I'm sadly an old head (not as old as you..........good for you , bad for me) too. Knobody knows what PASG or Bicarb means these days. I'm sure you've seen the benefits of both. Now it's therapeutic hypothermia, and CCR. Not unlike SLSA vs. old Standard Airworthiness. Those old Pipers and Cessnas do leak oil sometimes, but not normally until after about 20 or 30 thousand hours TTIS. Where will SLSA be after that kind of tenure? Just to stay on topic, there are transponders and altimeters out ther that have been around just as long. Meet the new boss.........same as the old boss. (Pete Townshend) Who knows?

Doug Hereford

#17 Doug in IL

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Posted 09 June 2012 - 10:23 AM

Doug, are you based in the KC area? I'm over in Alton, IL.

#18 Doug Hereford

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Posted 09 June 2012 - 02:55 PM

Yes, I am based at KMKC (Kansas City Downtown).

#19 Runtoeat

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Posted 09 June 2012 - 05:06 PM

Hmmm, just can't resist....I think there's play on words here if one word is added........to rephrase....."(Pete Townsend)..........(the) Who knows".
Dick Harrison
CTSW N9922Z

#20 Doug Hereford

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Posted 09 June 2012 - 06:09 PM

Runtoeat,
Nicely done!




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