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| kosmonooit:
But aren't the hold and the cabin pressures linked always? I think generally set to 6000' equiv at altitude, so how could there be a big differential in cargo - cabin pressures & temps? Aren't most Li Ion cells protected with a small circuit board ? I've seen 'em smoking / exploding on YouTube but only once the protection is removed, and after a serious 'battering'. |
| pickle9000:
Electrical /chemical fires need to have the sources removed. Turn off the power or remove the fuel. Breakers and fusible links are there for a reason. |
| kosmonooit:
--- Quote from: tom66 on August 03, 2015, 07:55:29 pm ---SeanB's scenario seems quite plausible. An example of cockpit confusion due to fire is the case of Swissair 111. The pilots are confounded by systems failing one by one as the fire destroys wiring and system avionics. The autopilot fails and they have to take control, but the fire begins destroying the mechanical control cables and control of the aircraft is lost. If the fire began near the aft electronics bay, it's quite possible that the only initially noticeable symptom would be gradual loss of key systems. I'm not sure where the transponder system is, but perhaps if this is located in the aft electronics bay this could be one of the first systems to fail. One question - is there any fire suppression system for the cargo hold or aft electronics bay, and if so is this sufficient to extinguish a lithium battery fuelled fire? --- End quote --- There are also many other instances with pilot confusion in the midst of a panic and wrong waypoints entered (like in Columbia where the aircraft flew into a mountain) - there must be clues in what worked and what didn't in terms of circuits failure, I think there is triple redundancy on the 777? In terms of fire suppression I think there is lots in the cargo hold, but not much you can do about the toxic smoke. Also that was the problem the Helderburg 747 Combi, inadequate fire suppression in the cargo section of the main cabin. |
| SeanB:
You have pressurised and unpressurised cargo areas, the pressurised cargo shares the cabin air and ventilation, as it is often used for transporting livestock, fish and pets.. Thus a fire there will lead to smoke in the cabin, followed by equipment failures as it typically is just behind the avionics bays, or even between them for certain things like gyro's, which have to be mounted at the mass centre of the aircraft. The cargo holds also have the main wiring and hydraulic lines running through them, in bundles either under the floor in a cable tray or in loops of wiring bundled up and laced hanging from the support structure of the floor above. Fire protection used to be a remote discharge halon unit, but they have had to change them out, so now it is only possible to have a dry powder or a CO2 canister, and the main fire suppression method is to depressurise the cabin and close of the air input, to starve it of O2 while aiming for the nearest runway at speed. seeing as the Lithium cells are capable of burning even in low oxygen conditions and they will continue to generate toxic gas so long as they have charge, and thus will fill the interior with toxic or irritating gas for a long time, even if you stop the fresh air inlet, which will prevent the gas from being removed by the cabin purge valves. Triple redundancy does not protect against physical damage, only that the software, firmware is not all going to fail at the same time. Note as well the tiny matter of the firmware counter being found to have a time dependant bug which caused it to fail had after being powered for a few months when an internal counter would overflow and crash. Reboot fixes it till next time, but you really do not want to reboot on approach or take off. The protection circuit may protect against overcharge, but it doers nothing about internal cell failure. Sony and others had massive recalls of battery packs that had a ferrous contaminant in them from the sealing process, which led to some rather spectacular battery fires in laptops. A cell in the pack may short to another if the casing is damaged, and this would be enough to trigger a fire in the pack, just from the connector tabs getting red hot within seconds. |
| AG6QR:
--- Quote from: kosmonooit on August 03, 2015, 08:01:42 pm ---But aren't the hold and the cabin pressures linked always? I think generally set to 6000' equiv at altitude, so how could there be a big differential in cargo - cabin pressures & temps? --- End quote --- Yes, the fuselage of the aircraft is one big pressure vessel, and that's why it's roughly cylindrical in shape with rounded ends. Pressure vessels require significant reinforcement (meaning significant weight) to make sharp corners, or even flat surfaces. The floor of the cabin is not capable of sustaining a significant pressure differential between the cabin above and the hold below. There is enough ventilation to insure that no big pressure differences build up. Temperature differences aren't always so well controlled. --- Quote from: SeanB on August 03, 2015, 08:43:55 pm ---You have pressurised and unpressurised cargo areas, the pressurised cargo shares the cabin air and ventilation, as it is often used for transporting livestock, fish and pets.. --- End quote --- There is NO unpressurized cargo area in a modern airliner. There are areas that lack forced-air ventilation, with no temperature control, and these areas are unsuitable for pets and other live animals. But they are at the same pressure as the cabin. |
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