False Alarms

An engineer, who prefers to remain anonymous, believes the frequency of false smoke alarms in cargo compartments is scandalous, and stems in large measure from the regulatory need for an alarm to trigger within one minute (see ASW, July 4). "Time is not important in a warning system," he writes. Rather, he argues, "A warning system must warn before the condition becomes hazardous so timely corrective action can be taken."

The regulations must be amended, he says. Here is the gist of his concept:

"Elimination of false fire alarms and enhancement of aviation safety requires [a change in approach]. Smoke is not hazardous to the cargo compartment structure or its inanimate cargo. Fire suppression tests show greater smoke generation after fire suppression than before as fire smolders. Besides, a clean burning fire that generates little smoke poses a greater hazard than a smoldering fire.

"Smoke is hazardous to animal cargo, but most animals in the cargo compartment get asphyxiated by Halon total flood fire suppression that the flight crew is obligated to activate on fire warning annunciation, real or false. Cargo compartments do not communicate with the passenger cabin. Structural integrity of the cargo compartment is essential for the smoke (or fire) detection system and also for total flood fire suppression system effectiveness. Heat is the hazard to cargo compartment structure and its content. Heat makes smoke, if present, buoyant and so it travels to the smoke detector. Fire suppression systems reduce the heat generation rate (the hazard) and not smoke generation rate. It increases smoke generation. I recommend [the regulations] FAR/JAR 25.858(a) be amended to the following:

" 'The detection system must provide a visual indication to the flight crew before

(i) the air temperature near the cargo compartment ceiling exceeds 131 degrees Fahrenheit (55 degrees centigrade), or

(ii) the differential air temperature near the cargo compartment ceiling between two adjacent cargo compartment sections x feet long (where x is the length of the largest size container or pallet certified for transport) exceeds 5.4 degrees Fahrenheit (3.0 degrees Centigrade).'

"Also, I recommend that FAR 25.858(c) be amended to the following:

" 'There must be means to allow the crew to check in flight

(i) the functioning of each fire detector circuit, and

(ii) to monitor the warning signal (temperature) at each detector location.'

"Technology exists to design fire detection systems compliant with the proposed amended rules."

The focus on heat (vice smoke) may be well taken. Our choice would be for an infra-red (IR) system that would detect heat via overlapping directional detector arrays that wouldn't be sucked into false alarms. Each gridded area of the cargo bay would be covered by two IR detectors, #1 connected to system #1 and #2 connected to system #2. An ALARM would be illuminated if both IR detectors light. A warning (only) would be illuminated if either system's detector lights, and immediate action by the pilot would consist of a loop integrity test of the other (dormant) system, and that system's detector.

If it checked out, the assumption could be that the alarm was false. The tactic would then be to await, but not expect, a heat-rise to be reported by either/both system's detectors immediately adjacent to the area of interest, i.e., an indication of a "spreading" fire would be the illumination of any adjacent detectors' advisory lights.

In many respects, this would be identical to the system previously advocated for detection, localization and mapping of electrical fires behind cabin linings (see ASW, April 26, 2004). The IR system in cargo holds could be backed up by a smoke detector with a high trigger threshold and an IR camera, monitored visually in the cockpit. This approach, with realistic triggering thresholds, would get away from false alarms triggered by volatile fruits, emanations from marine life, and other live cargo (see ASW, July 18). It might also save a few pets from a premature demise.