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Posted By:
Michael Nault
Young Eagles Pilot or Volunteer
#1 Posted: 11/11/2009 21:21:46

I purchased a yellow tagged GTX-327 transponder and a new Ameri-King AK-350 encoder for my Rans S-6ES. They're bench wired and ready to install in the plane and I'll be the installer. My question is: do I need to or am I required to have these bench tested together by an avionics shop before installing? Or, do I install them then fly to an avionics shop that can test them once installed? And, once tested, how do I do an in-flight ops check? Request clearance for the nearby Class B space and hope for confirmation that I'm squawking from ATC?

Mike Rans S-6ES N208M LXT
Chris Holloway
Homebuilder or Craftsman
#2 Posted: 11/12/2009 23:29:26

Hi Mike

I believe the transponder needs to be tested (interrogated) in the airplane. This wasn't always the case, until issues arose from  units being  tested, reinstalled in the aircraft and then failing to work properly. The avionics shop wil apply a vaccuum to the encoder and take it up to 20000 feet (in the hangar) to test it. I believe, under the FAR's, with prior permission you can enter class B airspace to land and have the unit tested. Even though the unit may work perfectly after you have installed it, it is "unserviceable"  until interrogated by the avionics shop.

This is the rule in Canada (where I am) and I imagine the same in the US.



Victor Thompson
Homebuilder or Craftsman
#3 Posted: 12/21/2009 18:39:41

Chris, the Canadian reference for this is:



Canadian Aviation Regulations 2009-2

Content last revised: 2007/12/30

The performance of Air Traffic Control (ATC) transponders can be tested using either a bench check or portable test equipment and shall meet the standards of airworthiness requirements elaborated in relation to section 571.02 of the CARs and listed in paragraphs (a) through (k) of this appendix. The test required by paragraph (k) is a system integration test to verify the accuracy of the data transmitted by the system as a whole, and as such shall be conducted on the aircraft. In order to prevent interference with the Air Traffic Control Radar Beacon System (ATCRBS) and airborne aircraft equipped with TCAS, portable test equipment shall be used with the appropriate precautions and operated at a rate of 235 interrogations per second. An additional 3 decibel (dB) loss is allowed to compensate for antenna coupling errors during receiver sensitivity measurements conducted in accordance with subsection (c)(1) when using portable equipment.

VJ Thompson Thorp T-18 C-GIRQ
Michael Nault
Young Eagles Pilot or Volunteer
#4 Posted: 12/21/2009 19:39:50

Thanks for the response Chris.  My CFI and I did just what you suggested during my cross-county last month and ATC identified my transponder within 30' of the altitude my GPS was reading.  And, it has checked good on several flights since then.

Mike Rans S-6ES N208M LXT
Jim Hann
Vintage Aircraft Association MemberHomebuilder or CraftsmanAirVenture Volunteer
#5 Posted: 12/22/2009 11:49:51


The full answer lies in the regs.  If you have a transponder installed (and operative) you required to operate it.  In order to operate it you must comply with 14CFR91:

F.A.R.  91.413


(a) No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or Sec. 135.143(c) of this chapter unless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with Appendix F of part 43 of this chapter; and

(b) Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), Appendix E, of part 43 of this chapter.

(c) The tests and inspections specified in this section must be conducted by--

(1) A certificated repair station properly equipped to perform those functions and holding--

(i) A radio rating, Class III;

(ii) A limited radio rating appropriate to the make and model transponder to be tested;

(iii) A limited rating appropriate to the test to be performed;

[(iv) deleted]

(2) A holder of a continuous airworthiness maintenance program as provided in part 121 or Sec. 135.411(a)(2) of this chapter; or

(3) The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer.

Amdt. 91-269, Eff. 4/6/2003

Some folks will bring up 91.411 about the pitot/static system but it pertains to IFR

Sec. 91.411
Altimeter system and altitude reporting equipment tests and inspections.
(a) No person may operate an airplane, or helicopter, in controlled airspace under IFR unless--

Good luck, be safe.


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