Thanks to FE Hoppy, I just dug up the following references:
Volume of air entering the cabin from the cargo hold
From which I gather that there is air entering the cabin from the cargo hold, in particular CO2 (Carbon dioxide, not monoxide), from, for example, dry ice, under even normal circumstances. It remains to be calculated just how much dry ice would be needed to attain a lethal level of 3%.
The second point would be, what if it's not just a small escape of this gas, but a large one which may have been triggered by (a) a smouldering fire damaging the dry ice containers, and (b) what would be the effect of extremely cold CO2 mixing with very hot Lithium (-ion or not) fumes plus the normal air in the cargo hold. Perhaps one of the posters has chemistry knowledge and can chime in.
It would also assume that for whatever reason (maintenance or defect) the pressure of the cabin was not higher than the hold as recommended, which I understand may happen, therefore there would be upwards flow from the hold to the cabin.
If, as is posited in this reference, it is possible for CO2 to reach the cabin, what about nitrogen containers instead of dry ice?
Either of these two products (CO2 or Nitrogen) would rob oxygen from the living (whether humans or animals), at the same time as, theoretically, extinguishing any fire. Depending on the contingencies, it may be possible that fire alarms are not triggered because of this.
Boeing Smoke Detection Systems
I do not understand the consequences of the limits of detection of a fire in this article. If a fire occurred in extremely close proximity to a nitrogen container, rupturing it, and if the nitrogen extinguished the fire, would there be enough smoke particles to trigger the dectection systems, while all the nitrogen emptied itself in the hold?
That is assuming that the placement of the goods involved has been made according to regulations - which they may not have been, and assuming also that the distance between the detector and the fire is fairly close which may or not be the case.
This is also assuming that the goods involved are in the hold or baggage compartments - what if they were carried in the cabin? Well maybe not nitrogen or dry ice, too bulky, but some similar hazardous material - one never knows what a passenger has in a bag...
@Max Nightstop
This reference here:
Fire Protection Systems explicitly says that Carbon monoxide (CO) detectors do exist.
What I am learning from this reference is that pilots can control the airflows from hold to cabin.
Quote from page 17-18:
Fire detecting sensing elements are located in many high-activity areas around aircraft engines. Their location, together with their small size, increases the chance of damage to the sensing elements during maintenance.
The following reference
B777 Air Systems - downloadable pdf file details the air flows in a B777. I do not have the knowledge to analyse this document.
@Yawn
Thank you.
@Fred The Red
Is it safe to assume a) CO couldn't enter cabin (either from known (ie. engines) or unknown (cargo hold fire) combustion sources) and, b) if CO were present, a/c are fitted with appropriate detectors?
From the above references,
(a) no.
(b) I don't know for sure but this would need checking - perhaps cost may limit their use?
@others
Thank you for helping examining this theory.