Narrative:

I was captain on flight from ZZZ. The first officer was flying while I was PNF. The time off the ground was XA03Z. After climbing through 10000 ft; the flight attendant called the cockpit and told me of an alarm that was sounding in the cabin. I asked if it was the lavatory smoke alarm; she said they were not sure where the sound was coming from or which alarm it was. Every time she tried to make a PA announcement this alarm would sound. I had them try the aft cabin PA; and the alarm sounded then as well. By this time the flight was climbing through 20000 ft; while I tried to diagnose the problem. There was no evidence of smoke or fire; either in the cabin or cockpit indications; as I ruled this out immediately. I contacted maintenance control; and told them the situation. They said uh; we think you should come back. I asked; what else could this be? What alarm could they be talking about? And do you have any other ideas? The response was again; if it's still going off; then come back. We coordinated an air return with ATC; and ZZZ operations; etc. The time back on the ground was XB27Z; 1 hour 24 min flight time. After landing while taxiing to the gate; a passenger stood up and went into the lavatory. I heard the flight attendant shouting situation down; situation down; I brought the aircraft to a stop. The flight attendant called the cockpit and said someone was in the lavatory. I heard more shouting from cabin; but could not really hear exactly what was said. At this time I assumed the passenger; who came out of the lavatory was arguing with my flight attendants. As a precaution; I ask the first officer to call operations to have police standing by at the gate; in case this was a passenger disturbance. After we blocked into the gate; and opened the cockpit door; I was informed the shouting I heard was our flight attendant yelling information to the passenger; because she still thought the PA was malfunctioning and unusable as it was in-flight. After everyone deplaned; and I talked with maintenance to try to troubleshoot the problem further. It was suggested as a possibility that the ignition was set to override accidentally (instead of off); and sound of the igniters firing was being picked up by the PA system; and amplified throughout the cabin. Thus; sounding like the alarm that was being reported. I cannot say with 100% certainty that this was the case. However; it is a plausible explanation. During final approach on the air return; during the landing checklist; I did notice the ignition switch feel a little different than usual (normal feel to go from off to on is 1 detent). This time; it felt like 1 plus a little more detents; perhaps 1.5 detents. I thought this was strange; but the priority at this time was landing the airplane. We swapped airplanes and completed the round trip uneventfully. Suggested corrective actions: this has definitely been a learning experience for me; and this will never happen to me again. I would like to see in the future additional thoughts and input from maintenance control while providing assistance.

Related reports:

Google
 

Original NASA ASRS Text

Title: MD80 PA SYSTEM BROADCASTS AN 'ALARM' SOUND EACH TIME IT IS USED. UNABLE TO IDENTIFY THE SOURCE OF THE ALARM; THE FLT CREW ELECTS TO RETURN TO DEP ARPT.

Narrative: I WAS CAPT ON FLT FROM ZZZ. THE FO WAS FLYING WHILE I WAS PNF. THE TIME OFF THE GND WAS XA03Z. AFTER CLBING THROUGH 10000 FT; THE FLT ATTENDANT CALLED THE COCKPIT AND TOLD ME OF AN ALARM THAT WAS SOUNDING IN THE CABIN. I ASKED IF IT WAS THE LAVATORY SMOKE ALARM; SHE SAID THEY WERE NOT SURE WHERE THE SOUND WAS COMING FROM OR WHICH ALARM IT WAS. EVERY TIME SHE TRIED TO MAKE A PA ANNOUNCEMENT THIS ALARM WOULD SOUND. I HAD THEM TRY THE AFT CABIN PA; AND THE ALARM SOUNDED THEN AS WELL. BY THIS TIME THE FLT WAS CLBING THROUGH 20000 FT; WHILE I TRIED TO DIAGNOSE THE PROB. THERE WAS NO EVIDENCE OF SMOKE OR FIRE; EITHER IN THE CABIN OR COCKPIT INDICATIONS; AS I RULED THIS OUT IMMEDIATELY. I CONTACTED MAINT CTL; AND TOLD THEM THE SITUATION. THEY SAID UH; WE THINK YOU SHOULD COME BACK. I ASKED; WHAT ELSE COULD THIS BE? WHAT ALARM COULD THEY BE TALKING ABOUT? AND DO YOU HAVE ANY OTHER IDEAS? THE RESPONSE WAS AGAIN; IF IT'S STILL GOING OFF; THEN COME BACK. WE COORDINATED AN AIR RETURN WITH ATC; AND ZZZ OPS; ETC. THE TIME BACK ON THE GND WAS XB27Z; 1 HR 24 MIN FLT TIME. AFTER LNDG WHILE TAXIING TO THE GATE; A PAX STOOD UP AND WENT INTO THE LAVATORY. I HEARD THE FLT ATTENDANT SHOUTING SIT DOWN; SIT DOWN; I BROUGHT THE ACFT TO A STOP. THE FLT ATTENDANT CALLED THE COCKPIT AND SAID SOMEONE WAS IN THE LAVATORY. I HEARD MORE SHOUTING FROM CABIN; BUT COULD NOT REALLY HEAR EXACTLY WHAT WAS SAID. AT THIS TIME I ASSUMED THE PAX; WHO CAME OUT OF THE LAVATORY WAS ARGUING WITH MY FLT ATTENDANTS. AS A PRECAUTION; I ASK THE FO TO CALL OPS TO HAVE POLICE STANDING BY AT THE GATE; IN CASE THIS WAS A PAX DISTURBANCE. AFTER WE BLOCKED INTO THE GATE; AND OPENED THE COCKPIT DOOR; I WAS INFORMED THE SHOUTING I HEARD WAS OUR FLT ATTENDANT YELLING INFO TO THE PAX; BECAUSE SHE STILL THOUGHT THE PA WAS MALFUNCTIONING AND UNUSABLE AS IT WAS INFLT. AFTER EVERYONE DEPLANED; AND I TALKED WITH MAINT TO TRY TO TROUBLESHOOT THE PROB FURTHER. IT WAS SUGGESTED AS A POSSIBILITY THAT THE IGNITION WAS SET TO OVERRIDE ACCIDENTALLY (INSTEAD OF OFF); AND SOUND OF THE IGNITERS FIRING WAS BEING PICKED UP BY THE PA SYS; AND AMPLIFIED THROUGHOUT THE CABIN. THUS; SOUNDING LIKE THE ALARM THAT WAS BEING REPORTED. I CANNOT SAY WITH 100% CERTAINTY THAT THIS WAS THE CASE. HOWEVER; IT IS A PLAUSIBLE EXPLANATION. DURING FINAL APCH ON THE AIR RETURN; DURING THE LNDG CHKLIST; I DID NOTICE THE IGNITION SWITCH FEEL A LITTLE DIFFERENT THAN USUAL (NORMAL FEEL TO GO FROM OFF TO ON IS 1 DETENT). THIS TIME; IT FELT LIKE 1 PLUS A LITTLE MORE DETENTS; PERHAPS 1.5 DETENTS. I THOUGHT THIS WAS STRANGE; BUT THE PRIORITY AT THIS TIME WAS LNDG THE AIRPLANE. WE SWAPPED AIRPLANES AND COMPLETED THE ROUND TRIP UNEVENTFULLY. SUGGESTED CORRECTIVE ACTIONS: THIS HAS DEFINITELY BEEN A LEARNING EXPERIENCE FOR ME; AND THIS WILL NEVER HAPPEN TO ME AGAIN. I WOULD LIKE TO SEE IN THE FUTURE ADDITIONAL THOUGHTS AND INPUT FROM MAINT CTL WHILE PROVIDING ASSISTANCE.

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.