Narrative:

Flight dispatch release listed an MEL 23-4 stating 'service interphone inoperative.' aircraft log had a crew write-up stating that the passenger and crew call/lav smoke detector circuit breaker had popped and been reset, then popped again. The maintenance sign-off was 'do no use attendant call buttons.' I questioned this write-up and was informed by san diego mx personnel that the service interphone was in fact operative and the only problem was with the attendant call buttons. It was explained that as long as we didn't use the call buttons, the circuit breaker would not pop and thus would have no adverse affect on the lav smoke detectors. Since the attendant call function is not required for flight as per MEL 23-4, I accepted the aircraft and the flight to lax was completed west/O incident. While the explanation I received from mx seemed reasonable at the time, further discussion of this situation has raised the question of the exact status of the lav smoke detectors. I understand that mx later corrected the problem of this aircraft by changing a bulb which had shorted out in the aft attendant pink call light. My question is, does a short circuit in one part of the system render the entire system inoperative? Perhaps the smoke detection system should have its own circuitry. Callback conversation with reporter revealed the following: aircraft was later ferried to home maintenance base because of this problem. Reporter said wasn't until after he flew his leg that while legal on paper he wondered if the lav smoke detector was activated it would also pop the circuit breaker rendering the system inoperative. He wasn't certain if later crew refused the aircraft or if maintenance decided the system was questionable. Suspect it required more than just changing the bulb to fix the circuit.

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Original NASA ASRS Text

Title: PLT CONCERN ABOUT SAFETY OF POWER SOURCE FOR LAVATORY SMOKE DETECTORS.

Narrative: FLT DISPATCH RELEASE LISTED AN MEL 23-4 STATING 'SVC INTERPHONE INOP.' ACFT LOG HAD A CREW WRITE-UP STATING THAT THE PAX AND CREW CALL/LAV SMOKE DETECTOR CB HAD POPPED AND BEEN RESET, THEN POPPED AGAIN. THE MAINT SIGN-OFF WAS 'DO NO USE ATTENDANT CALL BUTTONS.' I QUESTIONED THIS WRITE-UP AND WAS INFORMED BY SAN DIEGO MX PERSONNEL THAT THE SVC INTERPHONE WAS IN FACT OPERATIVE AND THE ONLY PROB WAS WITH THE ATTENDANT CALL BUTTONS. IT WAS EXPLAINED THAT AS LONG AS WE DIDN'T USE THE CALL BUTTONS, THE CB WOULD NOT POP AND THUS WOULD HAVE NO ADVERSE AFFECT ON THE LAV SMOKE DETECTORS. SINCE THE ATTENDANT CALL FUNCTION IS NOT REQUIRED FOR FLT AS PER MEL 23-4, I ACCEPTED THE ACFT AND THE FLT TO LAX WAS COMPLETED W/O INCIDENT. WHILE THE EXPLANATION I RECEIVED FROM MX SEEMED REASONABLE AT THE TIME, FURTHER DISCUSSION OF THIS SITUATION HAS RAISED THE QUESTION OF THE EXACT STATUS OF THE LAV SMOKE DETECTORS. I UNDERSTAND THAT MX LATER CORRECTED THE PROB OF THIS ACFT BY CHANGING A BULB WHICH HAD SHORTED OUT IN THE AFT ATTENDANT PINK CALL LIGHT. MY QUESTION IS, DOES A SHORT CIRCUIT IN ONE PART OF THE SYS RENDER THE ENTIRE SYS INOP? PERHAPS THE SMOKE DETECTION SYS SHOULD HAVE ITS OWN CIRCUITRY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: ACFT WAS LATER FERRIED TO HOME MAINT BASE BECAUSE OF THIS PROB. RPTR SAID WASN'T UNTIL AFTER HE FLEW HIS LEG THAT WHILE LEGAL ON PAPER HE WONDERED IF THE LAV SMOKE DETECTOR WAS ACTIVATED IT WOULD ALSO POP THE CB RENDERING THE SYS INOP. HE WASN'T CERTAIN IF LATER CREW REFUSED THE ACFT OR IF MAINT DECIDED THE SYS WAS QUESTIONABLE. SUSPECT IT REQUIRED MORE THAN JUST CHANGING THE BULB TO FIX THE CIRCUIT.

Data retrieved from NASA's ASRS site as of August 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.