Narrative:

At XA58; apr/fri/05; we departed ZZZ on an IFR flight to our destination; a brief 50 mins away. The preflight; taxi; takeoff checks and communications were all normal until attempting to contact departure control. Upon calling departure; we briefly encountered an audio feedback making the assigned heading and altitude difficult to understand. To the best of my recollection; the controller requested that we read back the assigned heading and altitude a second time. The annoying audio situation subsided momentarily; and we reconfirmed our clearance including the comment that someone had a stuck microphone. We immediately investigated the possibility that the stuck microphone may be our own. Accordingly; we verified all audio switches and selector position; and ultimately switched transmitters. During a period of improved communication; we copied our handoff to ZHU. Again; we encountered the annoying feedback and were advised by the new controller that someone had a stuck microphone. Almost simultaneously another flight crew commented that the controller's transmission was blocked. After several communications with other flts; the controller returned to us and advised that he could hear our cockpit conversation; confirming it was our aircraft with the audio problem. We confirmed our new clearance to FL230; and advised the controller that we had experienced an audio situation on a prior frequency. The PF continued climbing and leveled off at our assigned altitude of FL230. As I was the PNF; I continued to investigate all options of isolating the audio feedback. These included temporarily switching from headsets to hand mikes; confirming boom microphone versus oxygen microphone selector switches; unplugging the oxygen mask; isolating the flight deck interphone system; checking squelch; verifying volume controls; yoke microphone switches and again switching between our newly installed collins transmitters. For a brief moment; I caught a glimpse of the green transmitter light on the vox interphone controller which is positioned on the throttle pedestal. The low altitude sector controller then handed us off to a higher altitude controller who cleared us to FL260. Upon leveling off at FL260; or shortly thereafter; the controller advised that he was not receiving our mode C. I informed him that we had experienced avionics difficulties prior to handoff and that I would switch to our #2 transponder. Approximately 90 seconds later; I inquired if our mode C appeared to be operative. The response was affirmative. The remainder of the flight to our destination and the return flight to ZZZ was routine and without incident. Immediately upon completing our shutdown checks; we visited the local avionics facility and requested a thorough investigation of our audio system; both xponders and the autoplt. The audio complication was duplicated and determined to be an internal malfunction of a microphone switch on the copilot's yoke. A new switch was installed and has resolved that problem. Avionics personnel were unable to duplicate the mode C failure. However; they suggested the antennae may have been temporarily blocked by the fuselage position. Both transponder antennae are located on the lower fuselage centerline. The autoplt also performed satisfactorily during a ground test.

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Original NASA ASRS Text

Title: AN IAI1124 FROM TKOF; CLB TO FL230 EXPERIENCED LOSS OF COM CAUSED BY A FAILED SWITCH ON THE FO'S YOKE.

Narrative: AT XA58; APR/FRI/05; WE DEPARTED ZZZ ON AN IFR FLT TO OUR DEST; A BRIEF 50 MINS AWAY. THE PREFLT; TAXI; TKOF CHKS AND COMS WERE ALL NORMAL UNTIL ATTEMPTING TO CONTACT DEP CTL. UPON CALLING DEP; WE BRIEFLY ENCOUNTERED AN AUDIO FEEDBACK MAKING THE ASSIGNED HDG AND ALT DIFFICULT TO UNDERSTAND. TO THE BEST OF MY RECOLLECTION; THE CTLR REQUESTED THAT WE READ BACK THE ASSIGNED HDG AND ALT A SECOND TIME. THE ANNOYING AUDIO SIT SUBSIDED MOMENTARILY; AND WE RECONFIRMED OUR CLRNC INCLUDING THE COMMENT THAT SOMEONE HAD A STUCK MIKE. WE IMMEDIATELY INVESTIGATED THE POSSIBILITY THAT THE STUCK MIKE MAY BE OUR OWN. ACCORDINGLY; WE VERIFIED ALL AUDIO SWITCHES AND SELECTOR POS; AND ULTIMATELY SWITCHED XMITTERS. DURING A PERIOD OF IMPROVED COM; WE COPIED OUR HDOF TO ZHU. AGAIN; WE ENCOUNTERED THE ANNOYING FEEDBACK AND WERE ADVISED BY THE NEW CTLR THAT SOMEONE HAD A STUCK MIKE. ALMOST SIMULTANEOUSLY ANOTHER FLT CREW COMMENTED THAT THE CTLR'S XMISSION WAS BLOCKED. AFTER SEVERAL COMS WITH OTHER FLTS; THE CTLR RETURNED TO US AND ADVISED THAT HE COULD HEAR OUR COCKPIT CONVERSATION; CONFIRMING IT WAS OUR ACFT WITH THE AUDIO PROB. WE CONFIRMED OUR NEW CLRNC TO FL230; AND ADVISED THE CTLR THAT WE HAD EXPERIENCED AN AUDIO SIT ON A PRIOR FREQ. THE PF CONTINUED CLBING AND LEVELED OFF AT OUR ASSIGNED ALT OF FL230. AS I WAS THE PNF; I CONTINUED TO INVESTIGATE ALL OPTIONS OF ISOLATING THE AUDIO FEEDBACK. THESE INCLUDED TEMPORARILY SWITCHING FROM HEADSETS TO HAND MIKES; CONFIRMING BOOM MIKE VERSUS OXYGEN MIKE SELECTOR SWITCHES; UNPLUGGING THE OXYGEN MASK; ISOLATING THE FLT DECK INTERPHONE SYS; CHKING SQUELCH; VERIFYING VOLUME CTLS; YOKE MIKE SWITCHES AND AGAIN SWITCHING BTWN OUR NEWLY INSTALLED COLLINS XMITTERS. FOR A BRIEF MOMENT; I CAUGHT A GLIMPSE OF THE GREEN XMITTER LIGHT ON THE VOX INTERPHONE CONTROLLER WHICH IS POSITIONED ON THE THROTTLE PEDESTAL. THE LOW ALT SECTOR CTLR THEN HANDED US OFF TO A HIGHER ALT CTLR WHO CLRED US TO FL260. UPON LEVELING OFF AT FL260; OR SHORTLY THEREAFTER; THE CTLR ADVISED THAT HE WAS NOT RECEIVING OUR MODE C. I INFORMED HIM THAT WE HAD EXPERIENCED AVIONICS DIFFICULTIES PRIOR TO HDOF AND THAT I WOULD SWITCH TO OUR #2 XPONDER. APPROX 90 SECONDS LATER; I INQUIRED IF OUR MODE C APPEARED TO BE OPERATIVE. THE RESPONSE WAS AFFIRMATIVE. THE REMAINDER OF THE FLT TO OUR DEST AND THE RETURN FLT TO ZZZ WAS ROUTINE AND WITHOUT INCIDENT. IMMEDIATELY UPON COMPLETING OUR SHUTDOWN CHKS; WE VISITED THE LCL AVIONICS FACILITY AND REQUESTED A THOROUGH INVESTIGATION OF OUR AUDIO SYS; BOTH XPONDERS AND THE AUTOPLT. THE AUDIO COMPLICATION WAS DUPLICATED AND DETERMINED TO BE AN INTERNAL MALFUNCTION OF A MIKE SWITCH ON THE COPLT'S YOKE. A NEW SWITCH WAS INSTALLED AND HAS RESOLVED THAT PROB. AVIONICS PERSONNEL WERE UNABLE TO DUPLICATE THE MODE C FAILURE. HOWEVER; THEY SUGGESTED THE ANTENNAE MAY HAVE BEEN TEMPORARILY BLOCKED BY THE FUSELAGE POS. BOTH XPONDER ANTENNAE ARE LOCATED ON THE LOWER FUSELAGE CTRLINE. THE AUTOPLT ALSO PERFORMED SATISFACTORILY DURING A GND TEST.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.