ATA 26 Fire protection
1 - APU fire bottle activated spontaniously. 737-6/7/8/900
2
- Cargo
fire test fails in the morning, but is ok after some flights. 737-ALL
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1 - APU fire
bottle activated spontaniously.
737-6/7/8/900
More than once the APU fire extinguisher bottle was activated spontaniously, without any fire-handles pulled or automated fire extinguishing actions taken place. It even happened without any electrical power applied to the
aircraft !
Troubleshooting revealed 25-28 volt was present at pin 3 and 4 of plug D594 on the APU bottle squib, even with elec power removed from the a/c.
Further troubleshooting led us to the plug on the remote APU fire control panel in the wheel well, plug D48080. This plug was corroded internally causing a short between several pins, causing hot battery elec power going directly to the APU bottle squib.
Despite wheel well connector inspections and corrosion preventive actions, this plug
is vulnerable to corrosion due to the un-friendly environment of that location.
So next time you have a spontaniously activated APU fire bottle, check out this plug first. See picture for location of this plug.
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2 - Cargo
fire test fails in the morning, but is ok after some flights. 737-ALL
Very often after power-up of the a/c in the morning, after a cold night, the cargo fire system test fails, and in one loop a defect detector is found. Switching the selector to the good working loop-side, allows you to dispatch the a/c iaw the MEL. But often after some flights, if you do the cargo fire test again, all detectors are ok again. This is often due to the presence of moisture / damp inside one of the detectors due to the cold and damp air. Warming up of the a/c and air inside solves the problem. But then it is not possible to find the "bad" detector again, which will certainly fail the next morning too..
The tip here is to go to the bite box inside the fwd cargo compartment ceiling, and during the test of the detectors, the LAST light to come on for the test is often the most suspected detector. Even if the bite shows they are all ok (all lights on), replace this detector first, and probably your early morning fault will be gone, giving you more time for that wake up coffee.