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In a commercial airliner at FL370, in the event of rapid (explosive) decompression, will the cabin air fog? And can that fog trip smoke detectors?

Pressurized aircraft can have a cabin altitude of 6,000 ft, or so. What is the differential pressure at 37,000 ft? Again, would an explosive decompression of 8-9 PSI, fog the interior environment?

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    $\begingroup$ It would probably fog, but not long enough to trip any detectors even if they would alarm on condensation, which is doubtful. Same reason your smoke detector in your house wouldn't go off if you held it above a pot of boiling water in the "fog". $\endgroup$
    – Ron Beyer
    May 22, 2016 at 21:01
  • $\begingroup$ lower pressure could cause overheating for air-cooled components $\endgroup$ May 22, 2016 at 22:34
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    $\begingroup$ Explosive decompression would require the loss of a door-sized piece of the fuselage. Surely the smoke detectors going off would be the last thing you would be concerned about, and that's assuming you can even hear it over the outrush and then wind noise. $\endgroup$
    – paul
    May 22, 2016 at 23:33
  • $\begingroup$ If the cargo fire detectors went off, that WOULD get the pilots' attention, as well as needlessly complicating the troubleshooting process. Never heard of that happening, though. Maybe even because of engineering so that the detectors reject exactly this sort of "false positive" indication. $\endgroup$
    – Ralph J
    May 23, 2016 at 0:44
  • $\begingroup$ @RonBeyer That all depends. If I have a long hot shower and open the bathroom door before the extractor fan has dealt with most of the steam, it sets my smoke alarm off. $\endgroup$
    – Simon
    May 23, 2016 at 6:39

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In an explosive decompression the cabin air may fog, this is because of rapid cooling causing the moisture in the air to condense. How thick it is and how long it may last depends on a number of factors.

There are two types of smoke detectors, these are optical particle detection devices and ionization devices. Both types are used on commercial aircraft, some detectors have both methods built in. Ionization devices are tripped by changes in the electrical conductivity of the air flowing through them and are not very sensitive to steam. Optical smoke detectors work on a different principle, detecting light being scattered by particles in the air. Optical devices are susceptible to false readings from moisture, in fact manufacturers of home devices recommend they are installed away from bathrooms to avoid false alarms.

However, airplane certified units are designed and built to a very high standard, the FAA states in the Aviation Maintenance Handbook that requirements for fire protection systems include:

No false warnings under any flight or ground condition.

So it is very likely that the manufacturers have taken steam into account, after all cabins sometimes fill with steam when the air conditioning has been turned on when if the air is very moist. An extreme example is this flight at Shenzen. That did not set off smoke alarms, and although I've never seen it that bad I have personally been in the cabin when it's noticeably filled with steam and there were no smoke alarms.

So yes, the cabin can fill with steam in an explosive decompression, but it is unlikely - but possible - that the smoke alarms would go off.

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