Date: 201908
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Toilet Smoke; Overheat; Fire Detectors & Warning
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Captain; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe; Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: Pre-flight
Result: Aircraft Equipment Problem Dissipated; Flight Crew FLC Overrode Automation; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: I arrived to the gate along with one other flight attendant. The airplane had been moved [to this gate from another] with little notice so everyone was making their way to the jet. When we got onboard; the jet was far too hot to allow passengers on; so I ran up and told [Customer Service] to hold off on boarding until the CA (Captain) told her it was ok and ran back to start the APU. The aircraft was in the terminate checklist configuration so I closed the E. Exit Light switch and Battery switch and started the APU. Once I placed the APU generator on the busses; I got a loud alarm that sounded like an EE overheat alarm. I quickly shut off the INS and checked all of the fans running and in Normal. The alarm did not abate. I took the APU off line and put the plane on Ground Power. The alarm continued. I disconnected ground power and turned the battery switch off and EE Lights switch off; essentially returning the aircraft to the terminate configuration. This was the only configuration that made the alarm stop. By this time the Captain was onboard; along with the rest of the flight attendants. The Captain entered [a digital maintenance] report and verbally notified maintenance. On the radio; maintenance asked if the alarm was coming from the floor and we said it was hard to tell but it sounded like an equipment cooling alarm. We did try placing the aircraft on the Ground Service Bus; but that didn't change the alarm status. As long as it was on GP or APU; the alarm continued. I went outside to do the walk around and did not notice the alarm outside which I reported when I returned. By this time the first maintainer came onboard and began troubleshooting. He did all of the same things I did; and was unsuccessful. After 10 minutes; he called for backup. During this time; the flight attendants; the pilots and the maintainer were all onboard listening to the constant alarm with nobody able to identify it. Three more avionics technicians came onboard and did a sweep of the aircraft and finally figured out it was the forward lav fire alarm. Obviously there were no secondaries and nobody really knows why it got set off. It was reset and the flight continued with no further incident.


Second narrative: The First Officer arrived at the aircraft which was excessively hot. Although conditioned ground air was connected at [this] gate; the AC had arrived at [another gate] and been towed over. The AC; sitting in the full sun with an ambient temperature of 97 degrees; was heat soaked. As a result; the FO (First Officer) told the gate agent to hold off boarding until I arrived and could assess the situation.The FO then began normal preflight procedures; including starting the APU in order to improve the AC cooling and aligning the IRS units. Almost immediately; a VERY loud aural warning began to emit. The FO tried several trouble-shooting steps but was unable to silence the warning or definitively determine the source. I was at the podium checking in at the time; the FO called my cell phone to notify me of the alarm so I immediately went down to the airplane. As soon as I entered the cockpit; I independently assessed the situation and tried the same trouble-shooting steps as the FO. The only thing that initially seemed to silence the warning was to completely depower the airplane; including turning the battery switch off. For this reason; along with the timing of the alarm with the IRS alignment process; we misdiagnosed the problem as being an E&E bay equipment overheat. Although I am familiar with the sound of the horn when the Equipment Cooling Supply and/or Exhaust fans are unpowered; I thought the constant higher pitch warning was an indication of an overheat (FM language states 'If an over temperature occurs on the ground; alerting is provided through the pilot call horn in the nose wheel well.')I called maintenance on the radio to inform them of the issue and entered a write-up via ACARS. While waiting for maintenance to arrive at the AC; in an attempt to keep air circulating in the E&E bay and hopefully prevent any equipment damage; we disconnected ground power while holding the Ground Service switch at the forward Flight Attendant panel (previously a standard practice when completing the Termination Checklist). This process silenced the warning horn while powering AC lights; the battery charger and various other AC powered equipment.The first mechanic who arrived at the AC; fully re-powered the electrical system and the warning horn immediately restarted. He seemed to run through all the same trouble-shooting steps we did without success and checked inside the E&E bay. He did not ascertain the source of the alarm nor was he able to silence the warning for approx. 25 min. He stated that an avionics mechanic was on his way to assist. He then (again) completely depowered the AC; including the Emergency Exit Lights and Battery Switch.Three additional mechanics arrived at the AC to assist. After their own troubleshooting; one of them determined the warning horn was the FWD LAV SMOKE detector alarm. By pulling the Lavatory Smoke Circuit Breaker they were able to silence the warning. The mechanics were then able to successfully reset the alarm. The mechanics stated that they would need to review what was allowable regarding a potential deferral. Their guess was that a smoke detector may not be deferrable. If it was allowed; the LAV would need to be taken out of service. We had a group discussion about the validity of the alarm and whether the detector was working as designed (in other words; if there was an unseen threat/problem) or the detector itself was malfunctioning. Regarding a LAV deferral; I expressed some reservations about leaving an equipment hub with a deferred LAV; 3:18 flight time and 170 pax plus 6 crew. When all four mechanics left the aircraft; my understanding was that one of them would return to discuss the resolution. I was surprised that the [maintenance release] was signed off within 3 minutes of them leaving and then sent to the cockpit 5 minutes later. Although I had been expecting a follow-up discussion; the FO and I agreed that since the reset had been successful without a subsequent alarm;the system was working normally. In hindsight; after further analyzing the unfolding of events; there are a number of factors I find concerning. First; out of 2 pilots; 4 flight attendants; and 4 mechanics none of us were able to immediately identify the sound of a Lav Smoke Detector Warning. Second; 3 flight attendants; 2 pilots and 1-4 mechanics were on the aircraft with an active smoke warning on/off during the trouble-shooting process for almost an hour. After the first mechanic arrived at the AC and depowered the airplane; including the Battery Switch and Emergency Exit lights; we had 3 flight attendants; 2 pilots and 1 mechanic sitting on an airplane without egress lights available.Several suggestions based on lessons learned: 1. Add a demonstration of the sound of a Lav Smoke Detector Warning during initial aircraft training for both pilots and flight attendants. I was really surprised how loud it was; so any demonstration should be live; not via computer based training. 2. In the future; if I am ever involved with any maintenance being done on any system related to fire detection or overheat; I will insure that only essential personnel are on the AC during that process. This should be incorporated into company policy.3. Finally; our company procedures do not prohibit turning off the Emergency Exit Lights AND Battery Switch with people on board (including customers!). Although I do not engage in this practice; many pilots complete the Termination Checklist then leave the aircraft. (The logic being that the flight attendants can use the switch at the panel in the aft galley which will over-ride the cockpit switch in the event ground power drops off the aircraft with customers still deplaning).


Synopsis: Flight crew reported that; on aircraft power up; the Forward Lavatory fire alarm began to sound and was mistaken for an aircraft equipment cooling alarm; resulting in a delay.