Narrative:

While taxing into position and hold in mia; the flight attendant in first class (the flight leader) called and reported that the smoke detector in the first class lavatory was going off and that she had checked the first class lavatory for any signs of smoke or fumes and found no sign of anything wrong. Knowing that the flight leader had checked for smoke and found nothing; and that we had just taxied through an active and dusty construction zone; and that a B737 had just run up its engines in front of us to turn on to the runway; I (with the agreement of the first officer) elected to continue the takeoff. My go decision was based on two factors: the first was assurance from the cabin that there was no smoke; the second was prior experiences with malfunctioning smoke detectors. Based on experiences in other aircraft; I believed that the smoke detector would stop sounding shortly after we pushed the power levers up; which would increase airflow through the aircraft. Because of the increased engine noise at takeoff power; I could no longer hear the smoke alarm sounding in the forward lavatory and I believed that my assumption regarding increased airflow terminating the alarm was correct. Takeoff was uneventful. I checked with the flight attendant on climb out and was told that the detector was still sounding. I made a PA to the passengers to say sorry about the noise; that one of our lavatory smoke detectors was malfunctioning and we would have the problem fixed soon. A male flight attendant came up from the back of the aircraft and disconnected the smoke detector battery. I made another PA at cruise explaining the reasons that smoke detectors sometimes give erroneous warnings and that conditions on the ground in mia were conducive to this type of erroneous indication. I called maintenance in flight and advised them of the situation. The flight attendants blocked off the forward lavatory per maintenance instructions. The flight leader periodically rechecked the lavatory to ensure that she could not smell any smoke or fumes. The rest of the flight was uneventful. While the passengers were deplaning; the flight leader advised me that she had been standing up when she heard the engines spool up for takeoff. She assumed that we would be holding on the ground to deal with the malfunctioning smoke detector. I have certainly learned from this experience. Faced with a similar situation (or any abnormal while taking the runway); I would pull off the runway rather than assuming that things would be fine after we got some air going through the aircraft. I would also coordinate better with the cabin crew to find out the exact state of affairs in the cabin; rather than assuming that because we had already received the cabin is ready for takeoff call; everything was set to go.

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Original NASA ASRS Text

Title: MD80 Flight Crew is informed by the lead Flight Attendant that a lavatory smoke alarm is sounding but no smoke is present; as the aircraft is taxied into position on the runway. The crew elects to depart and deal with the smoke warning in flight; which catches the lead Flight Attendant standing for the takeoff.

Narrative: While taxing into position and hold in MIA; the Flight Attendant in First Class (the Flight Leader) called and reported that the smoke detector in the First Class lavatory was going off and that she had checked the First Class lavatory for any signs of smoke or fumes and found no sign of anything wrong. Knowing that the Flight Leader had checked for smoke and found nothing; and that we had just taxied through an active and dusty construction zone; and that a B737 had just run up its engines in front of us to turn on to the runway; I (with the agreement of the First Officer) elected to continue the takeoff. My go decision was based on two factors: the first was assurance from the cabin that there was no smoke; the second was prior experiences with malfunctioning smoke detectors. Based on experiences in other aircraft; I believed that the smoke detector would stop sounding shortly after we pushed the power levers up; which would increase airflow through the aircraft. Because of the increased engine noise at takeoff power; I could no longer hear the smoke alarm sounding in the forward lavatory and I believed that my assumption regarding increased airflow terminating the alarm was correct. Takeoff was uneventful. I checked with the Flight Attendant on climb out and was told that the detector was still sounding. I made a PA to the passengers to say sorry about the noise; that one of our lavatory smoke detectors was malfunctioning and we would have the problem fixed soon. A male Flight Attendant came up from the back of the aircraft and disconnected the smoke detector battery. I made another PA at cruise explaining the reasons that smoke detectors sometimes give erroneous warnings and that conditions on the ground in MIA were conducive to this type of erroneous indication. I called maintenance in flight and advised them of the situation. The Flight Attendants blocked off the forward lavatory per maintenance instructions. The Flight Leader periodically rechecked the lavatory to ensure that she could not smell any smoke or fumes. The rest of the flight was uneventful. While the passengers were deplaning; the Flight Leader advised me that she had been standing up when she heard the engines spool up for takeoff. She assumed that we would be holding on the ground to deal with the malfunctioning smoke detector. I have certainly learned from this experience. Faced with a similar situation (or any abnormal while taking the runway); I would pull off the runway rather than assuming that things would be fine after we got some air going through the aircraft. I would also coordinate better with the cabin crew to find out the exact state of affairs in the cabin; rather than assuming that because we had already received the cabin is ready for takeoff call; everything was set to go.

Data retrieved from NASA's ASRS site as of April 2012 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.