RADIO COM STUCK MIKE DURING AN IFR TRAINING FLT IN A SMA.

Narrative

AFTER TAKING OFF FROM ADDISON ARPT UNDER AN IFR CLRNC TO F54; ON A REGIONAL APCH FREQ; MY STUDENT AND I EXPERIENCED AUDIO RECEPTION FAILURE. THIS OCCURRED DURING A TIME WHEN WE WERE 'WITH' APCH; BUT NOT CURRENTLY IN DIALOGUE OR ATTEMPTING TO XMIT AT ALL. IN THE MIDDLE OF ONE OF THE CTLR'S DIRECTIONS TO ANOTHER ACFT; OUR AUDIO WENT DEAD. WE INITIALLY ATTEMPTED TO CLR BOTH PUSH-TO- TALKS; THINKING THAT MAY HAVE BEEN THE PROB; BUT GOT NO CLICK OR RESPONSE OF ANY KIND WHEN WE DID SO. I THEN ENTERED SQUAWK 7600 IN THE XPONDER (CALLING OUT; I BELIEVE; AS I PASSED THE VARIOUS NUMBERS TO GET TO 7600; WHICH IS USUALLY MY PROC); OPENED MY 'LOST RADIO' CHKLIST TO VERIFY FLT CONTINUATION PROCS; OPENED THE LOW ENRTE CHART I WAS USING A LITTLE WIDER; PLANNED TO FOLLOW LOST RADIO PROCS; AND INSTRUCTED THE STUDENT TO HOLD LAST ASSIGNED HDG AND ALT IN ORDER TO FLY THE MOST DIRECT RTE TO ARLINGTON; WHERE WE WOULD SHOOT THE APCH UNDER IFR IF VFR WAS NOT AVAILABLE TO US. AS SHE DID AS DIRECTED; I CONTINUED TO TRY TO TROUBLE-SHOOT THE RADIO PROB; TRYING TO GET ANYTHING I COULD FROM NAV/COM 1 OR 2 ON EITHER HEADSET OR SPEAKER ON A VARIETY OF KNOWN ACTIVE FREQS BRIEFLY. I CHKED INTERCOM SWITCHES; ALL RADIO SWITCHES INCLUDING AVIONICS PWR; HEADSET AND MIKE LEADS. WHEN NOTHING WORKED; WE IMMEDIATELY DIALED IN THE ARLINGTON FREQ; IN ORDER TO NOT TIE UP AN APCH FREQ ANY LONGER IF OUR PUSH-TO-TALK WAS THE PROB. WHEN I COULD NOT GET MORSE ON ANY NAVAID FREQ; I SUSPECTED THAT THE PROB WAS INDEED; IN OUR AIRPLANE. WE COULD NOT AFFORD TO TURN OFF THE COM RADIOS; WHICH WERE NOW APPARENTLY XMITTING INTERMITTENTLY FOR VARYING PERIODS OF TIME; AS PWR TO OUR NAV RADIOS IS COUPLED TO THE COM OFF/ON SWITCH. THE VECTOR WE HAD BEEN GIVEN PRIOR TO AUDIO FAILURE WAS A VERY GOOD ONE FOR GETTING US ALIGNED WITH THE RADIAL NEEDED FOR THE ARLINGTON VOR APCH. AS WE INTERCEPTED THE APCH OUTBOUND; I SAW THE GND BENEATH US AND DSNDED TO VMC IN ORDER TO TERMINATE THE FLT AS QUICKLY AS POSSIBLE. WE ENTERED VMC JUST AMI OR SO S OF THE RWY AT ARLINGTON. I SAW AND CALLED TFC TO MY STUDENT; WHO ENTERED THE PATTERN APPROPRIATELY. STILL ATTEMPTING TO CORRECT THE MOST LIKELY PROB; I PUT HARD PRESSURE ON 1 SIDE OF THE STUDENT'S YOKE MOUNTED 'PIN' PUSH-TO-TALK AND; AFTER 8-10 SECONDS OF PUSHING HARD FROM 1 SIDE; IT CLRED AND WE HAD AUDIO AGAIN. AFTER RELEASING THE BUTTON; A FEW SECONDS LATER; PLANE VIBRATION CAUSED THE PUSH TO TALK TO RECONNECT AND XMIT AGAIN; SUGGESTING A SHORT BURIED SOMEWHERE DEEP IN THE YOKE HANDLE. WE LANDED; CLRED THE XPONDER BY RETURNING IT TO 1200; AND I IMMEDIATELY CALLED FLT SVC TO RPT CANCELLATION OF IFR. 5 MIN LATER I CALLED FLT SVC AGAIN TO VERIFY IFR HAD BEEN CANCELED; WHEN I RECEIVED A CALL FROM XXX AND ANOTHER GENTLEMAN WITH THE FAA; WHO TOLD ME THAT WE HAD BEEN XMITTING A 7500 SQUAWK. WHAT WAS ACTUALLY SET IN THE XPONDER IS A MATTER OF SOME CONJECTURE AND CONCERN TO ME; UNVERIFIABLE AFTER THE ACT; BUT I DO KNOW THAT MY INTENT WAS TO SQUAWK 7600; AND AFTER SETTING IT; I BELIEVED THAT WAS WHAT WAS IN THE XPONDER. SOME ADDITIONAL CONCERNS INCLUDE: WE WERE IN THICK IMC IN DIM LIGHT AS SUNSET APCHED DURING THE ENTIRE FLT; BUT IT WAS NOT YET DARK ENOUGH FOR INTERNAL CABIN LIGHTING TO HAVE MUCH AFFECT. I AM ONLY ABLE TO VERIFY WHAT I BELIEVED I SAW SET IN THE XPONDER. MY STUDENT WAS NERVOUS DURING THE SIT (THIS WAS ONLY HER 2ND TIME IN ACTUAL) AND WAS HAVING SOME TROUBLE HOLDING ALT AND HDG; PERIODICALLY SLIPPING INTO SHALLOW TURNS AND DSCNTS AS SHE GREW DISTR BY MY ATTEMPTS TO REGAIN COM. OVERALL SHE MAINTAINED WELL; BUT I WAS HAVING TO PAY CONSIDERABLE ATTN TO HER ACTIONS. I WAS VERY CONCERNED THAT WE NOT STRAY TO THE SE OF COURSE; AS OUR ASSIGNED 3000 FT MSL ALT GAVE US MINIMAL CLRNC OVER AN ANTENNA FARM SE OF ARLINGTON ARPT. I NEEDED TO AGGRESSIVELY MONITOR OUR EXACT POS. FORTUNATELY; WE WERE ON A GREAT VECTOR AND HAD AN ACCURATE WINDS ALOFT RPT; SO WE NEVER CAME NEAR THE ANTENNA FARM. WE WERE GETTING PERIODIC INTERMITTENT NAV FLAGS ON THE VOR RECEIVERS ANDCOULD NOT YET BE SURE THAT IT WAS BECAUSE WE WERE XMITTING OR BECAUSE THERE WAS A DEEPER AVIONICS CONCERN. AS THOSE RECEIVERS ARE NECESSARY FOR THE APCH AT ARLINGTON; WE WANTED 'LIVE' FLAGS ON THE RECEIVER AND WERE WARY OF POTENTIAL RECEIVER FAILURE. WE WERE NEVER IN A HIJACKING SIT OF ANY KIND; AND I INDICATED THIS TO THE GENTLEMEN WITH THE FAA; WHO WERE BOTH VERY DECENT AND SUPPORTIVE IN THEIR INTERACTIONS WITH ME. MR X INDICATED THAT HE HAD ONCE HAD A SIMILAR PROB WITH A YOKE-MOUNTED PUSH-TO-TALK IN ANOTHER CESSNA 172; AND HE BELIEVED MY THEORY WAS CORRECT ABOUT THE SHORT BEING OUT OF REACH WITHIN THE HANDLE OF THE YOKE. HE INDICATED THAT; BECAUSE OF THE 7500 CODE BEING XMITTED; HE WOULD NEED TO WRITE A RPT; BUT THAT HE WAS UNSURE IF THE SIT WOULD WARRANT ANY INTERACTION BTWN THE LCL FSDO AND ME. I GAVE HIM MY ADDRESS AND PHONE NUMBER TO PASS ON TO THE FSDO IF NECESSARY; AND NOW; SOME 20 HRS LATER; I HAVE NOT HEARD FROM ANYONE CONCERNING THE OCCURRENCE. I ALSO ASKED MR X IF THERE WAS ANY FURTHER ACTION I NEEDED TO TAKE TO CLARIFY AND CLOSE THE SIT; AND HE SAID NO. THIS PARTICULAR ACFT HAS HAD INTERMITTENT PLT'S SIDE PUSH-TO-TALK PROBS DURING THE PST MONTH; BUT THE PROB HAS BEEN THAT IT WOULD NOT XMIT; RATHER THAN IT STICKING IN THE XMIT POS. I HAD SQUAWKED THE PROB PREVIOUSLY; BUT SINCE OTHER PLTS FLYING IFR HAD NOT EXPERIENCED XMISSION FAILURES; THE SQUAWK WAS PULLED FROM OUR SQUAWK BOOK; AND MY STUDENT AND I WERE FLYING THE ACFT WITH THE UNDERSTANDING THAT A PROB DID NOT CONTINUE TO EXIST.