Date: 201907
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Dusk
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Cargo Compartment Fire/Overheat Warning
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Pre-flight
Result: General Maintenance Action
Contributing factors: Aircraft; Company Policy
Primary problem: Aircraft

Narrative: During preflight preparation and testing of systems I noted that the Cargo Fire Detection system did not test correctly. 'FWD' Warning light would not illuminate. I waited a few seconds and retested the system again. This time neither the 'FWD or the 'AFT' Warning light illuminated. After waiting a few moments; I again retested the system and this time the 'AFT' Warning light did not illuminate. We called out Maintenance to address the issue. They troubleshot the system and swapped the cockpit control panel with a panel from another aircraft with all the same symptoms reoccurring. Please note that every so often the system would test normally; but on subsequent testing all the same symptoms would re-appear. When a normal test would take place; the Mechanic seemed inclined to want to sign off the aircraft immediately and send us on our way. This was unacceptable to me since further testing always led to another incomplete test.I find this alarming that this Technician would send the aircraft on its way under these circumstances. Considering the potential hazards we have traveling on our aircraft in checked baggage; for example; e-Cigarettes; PEDs (Personal Electronic Devices); lithium batteries; and smart bags; we need to be exceptionally cautious that the Cargo Fire Detection system is FULLY functional. Obviously Company is concerned about the potential hazard of these items ending up in our cargo compartments since the flight attendants are required to ask passengers checking bags at the jetway if they have any of these items contained in their checked bags; as per Flight Attendant Manual Customer Care. During this particular situation; I dealt with three Mechanics and Maintenance Control and all gave me a different reason why we were getting faulty tests.I was told if we test it more than three times the circuit would overheat. I was also told that we needed to give it 15 seconds between each test. Thirdly; I was told if you test it more than two times you would get a faulty reading. Nowhere in our manuals are any of these stated. If any of these statements are true why is it not listed in our manuals? Why would a system be designed that you could only test twice before it presents a fault? I seriously doubt this is the case. This was the second aircraft with faulty Cargo Fire Tests in this week's four-day pairing. A Maintenance Technician in [another airport] told me he is now seeing more of the Cargo Fire Test issues than ever. The Cargo Fire Warning System needs to be investigated. I have experienced numerous faults during testing over the last year and I have spoken with other Company Pilots experiencing the same issues. Mechanics need to be instructed not to be so quick to sign off the logbook when dealing with these issues of this system. If any of the explanations they gave to me regarding the faulty test results were true; Company needs to include this information in our manuals.




Synopsis: B737-700 Captain reported multiple instances improper Maintenance procedure regarding the Malfunctioning Cargo Fire Warning Systems on multiple aircraft.