Fortuitous fire protection

The story of the May 13 cargo-hold fire on an Air Canada B767-300 cargo-hold fire drew a comment from a reader who seems to know how the fire-extinguishing system works (see ASW, June 10). He observed that because the aircraft was only ten miles from touchdown it luckily got the benefit of the remainder of the Halon extinguishing agent in one fell swoop upon touchdown - as per system design. In addition, the ground-air sensing would have taken the power off the heater ribbon on touchdown (recalling that it was one of the few circuits not powered down when the extinguishers are armed - and so would still have been electrically stoking the intense fire). He conjectures that the outcome might have been completely different if the aircraft had been over the ocean on an ETOPS (extended twin-engine operations) sortie and wholly reliant upon the continued 195-minute metered feed of the two remaining Halon bottles, particularly if the fire had pierced the hull and the "designed" 3 percent concentration of Halon could not have been maintained. Thought-provoking. He also promises some more insightful observations on an apparent long-term anomaly in that cargo-hold smoke system for next week. Stay tuned.

--------------------------------------------------------------------------------