A B733 FO COMES UP WITH A FIX FOR AUDIO BLEED THROUGH FROM LCL FM STATIONS; EXPERIENCED MOSTLY AT BWI AND LAS; NV.

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I SAW ON THE COMPANY EMPLOYEE WEBSITE RPTS OF 'ACFT PICKING UP VERY LIGHT BACKGROUND MUSIC ON DEP AND OR ARR' TO LAS. I'VE HEARD SIMILAR MUSIC FREQUENTLY; BUT I'M CERTAIN I CAN EXPLAIN THE CAUSE. WHILE THERE MAY BE RADIO STATIONS WITH MISADJUSTED XMITTERS NEARBY; THEY HAVE NOTHING TO DO WITH WHAT WE'RE HEARING! IT'S ALWAYS A -300 OR -500 ACFT. AT LEAST ONE; USUALLY BOTH; NAV SWITCHES ARE DOWN. (THE SAME PHENOMENON OCCURS ELSEWHERE AS WELL; BUT SINCE LAS HAS THE MOST OPS WITH BOTH NAV SWITCHES DOWN; IT TENDS TO HAPPEN THERE MOST OFTEN.) WITH ONE OR BOTH RADIOS AUTO-TUNING; SOMETIMES ONE OR BOTH WILL; FOR LACK OF A BETTER TERM; 'GO STUPID' AND NO LONGER TUNE TO NAVAIDS. WHEN THIS HAPPENS; THE RADIO TUNES TO 108.0 MHZ; AND THE 'ANP' SLOWLY GROWS. THAT 108.0 MHZ IS BOTH THE BOTTOM OF THE VOR/ILS RANGE AND ALSO THE TOP OF THE FM RADIO BAND; (AND IS USED BY SOME STATIONS). ANY TIME A NAV RECEIVER IS TUNED TO 108.0 AND YOU'RE CLOSE TO AN FM STATION ON THAT FREQ; YOU CAN PULL UP THE 'VOICE' AUDIO FOR THAT RECEIVER; AND YOU'LL HEAR THEIR PROGRAMMING; MUSIC; TALK; COMMERCIALS; ETC. SINCE WE DON'T TYPICALLY MONITOR NAVAID AUDIO; PLTS RARELY NOTICE THIS. ON SOME -300 AND -500 ACFT (SOME ARE WORSE THAN OTHERS); THERE IS BLEED-THROUGH OF THE NAVAID AUDIO INTO THE PLTS' HEADSETS; EVEN WITHOUT NAVAID AUDIO SELECTED. ALMOST ALWAYS; IT IS THE FO WHO HEARS THE BLEED-THROUGH; NOT THE CAPT. AT LEAST; I'VE ASKED CAPTS I'VE FLOWN WITH; 'ARE YOU HEARING THAT' AND THEY ALL SAY 'NO.' (MY SURMISE IS THAT THE FO'S GET OLDER PANELS WHICH ARE MORE SUSCEPTIBLE TO BLEED-THROUGH; BUT I DON'T KNOW.) WHAT I HEAR IS INDEPENDENT OF ANY VOLUME SETTINGS ON THE SELECTOR PANELS; AND ALSO THE VOICE/BOTH/RANGE SETTING. IN OTHER WORDS; IN A 'BAD' AIRPLANE; I CAN'T FULLY GET RID OF HEARING; WHATEVER AUDIO IS ON ONE OR BOTH OF THE NAV RECEIVERS (MOST OFTEN; THROUGH #1 NAV RECEIVER IS THE ONE I HEAR). THIS IS NOT SPECIFIC TO LNAV OPS; IT'S JUST A CONDITION WITH SOME ACFT; NOTICEABLE AS SOON AS I PLUG MY HEADSET IN. IT'S WORST IN BWI; WHERE THE ATIS PLAYS OVER THE VOR! SINCE THE BLEED-THROUGH IS FAIRLY FAINT; HIGHER NOISE LEVELS WITH ENGS RUNNING TEND TO MASK IT SOMEWHAT. WHEN BOTH OF THESE CONDITIONS COINCIDE; THEN I HEAR MUSIC INFLT. INVARIABLY; LITERALLY ALWAYS; I LOOK DOWN AND SEE ONE OR BOTH NAV RADIOS AUTO-TUNING TO 108.0 -- TO NO NAVAID; BUT JUST TO THE DEFAULT BOTTOM-OF-THE-RANGE. THE RDMI ALSO SHOWS 'OFF' FLAGS. IT HAS; FOR ME; BECOME VIRTUALLY AN AUDIBLE ALERT THAT 'HEY THE AUTO-TUNING HAS GOTTEN LOST.' EASY FIX: TUNE A VORTAC AND FLIP MY NAV SWITCH UP; FORCING #2 TO THE STATION (USUALLY LAS); AND A FEW SECONDS LATER; THE #1 RECEIVER 'FINDS' ITSELF AND TUNES TO ANOTHER NAVAID. AS SOON AS BOTH DME WINDOWS IN THE RDMI ARE SHOWING DISTANCES AND THE BEARING POINTERS ARE STABLE; I RETURN MY NAV SELECTOR SWITCH TO NAV; AND (FOR A TIME; AT LEAST) THE AUTO-TUNING WORKS NORMALLY; AND I DON'T HEAR FM MUSIC. I'VE VERIFIED MANY TIMES THAT THINGS WORK AS DESCRIBED -- IT'S NOW A PERFECTLY ROUTINE OCCURRENCE: HEAR MUSIC; KNOW AUTO-TUNING HAS 'GONE STUPID;' NOTE BOTH 'OFF' FLAGS IN THE RDMI; SEE THE 'ANP' INVARIABLY INCREASING (THE SYS CAN'T CORRECT THE IRS' DRIFT). WHEN I TUNE LAS; THE 'ANP' MAY OR MAY NOT DECREASE (A SINGLE NAVAID'S RADIAL/DME ISN'T A VERY 'TIGHT' POS UPDATE); BUT A FEW MOMENTS AFTER THE #1 RADIO AUTO-TUNES TO ANOTHER NAVAID; THE 'ANP' NUMBER DROPS TO A LOW VALUE; GENERALLY ABOUT 0.15 TO 0.3 NM. ALSO; AS SOON AS #1 AUTO-TUNES OFF 108.0; I IMMEDIATELY LOSE THE MUSIC FROM THE FM STATION. I'VE DEMONSTRATED THIS TO MULTIPLE CAPTS; INCLUDING HAVING THEM PULL UP THE #1 NAV AUDIO TO VERIFY WHAT I'M HEARING; THEN SHOWING THEM MY FIX; AND NO ONE HAS SUGGESTED ANYTHING TO MAKE ME DOUBT THIS EXPLANATION. WHILE BAD FM STATIONS EXIST; I DOUBT THEY HAVE MUCH TO DO WITH THE MUSIC WE HEAR. AUDIO PANELS SUSCEPTIBLE TO BLEED-OVER AND ALGORITHMS THAT SEND A RADIO TO THE 108.0 DEFAULT ARE THE CULPRITS.