Narrative:

I saw on the company employee website reports of 'aircraft picking up very light background music on departure and or arrival' 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 transmitters nearby, they have nothing to do with what we're hearing! It's always a -300 or -500 aircraft. At least one, usually both, navigation switches are down. (The same phenomenon occurs elsewhere as well, but since las has the most operations with both navigation switches down, it tends to happen there most often.) with one or both radios automatic-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 navigation receiver is tuned to 108.0 and you're close to an FM station on that frequency, 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, pilots rarely notice this. On some -300 and -500 aircraft (some are worse than others), there is bleed-through of the NAVAID audio into the pilots' headsets, even without NAVAID audio selected. Almost always, it is the first officer who hears the bleed-through, not the captain. At least, I've asked capts I've flown with, 'are you hearing that' and they all say 'no.' (my surmise is that the first officer'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 navigation receivers (most often, through #1 navigation receiver is the one I hear). This is not specific to LNAV operations, it's just a condition with some aircraft, 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 engines running tend to mask it somewhat. When both of these conditions coincide, then I hear music in-flight. Invariably, literally always, I look down and see one or both navigation radios automatic-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 automatic-tuning has gotten lost.' easy fix: tune a VORTAC and FLIP my navigation 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 navigation selector switch to navigation, and (for a time, at least) the automatic-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 automatic-tuning has 'gone stupid,' note both 'off' flags in the RDMI, see the 'anp' invariably increasing (the system 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' position update), but a few moments after the #1 radio automatic-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 automatic-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 navigation 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.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 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.