I checked with maintenance regarding my recent post about the fire protection system. Our fire detector is photocell activated. The photocell is sensitive to a particular wavelength of light, supposedly characteristic of fire.
I didn't get to ask the obvious questions about why fire detection systems aren't overwhelmed with false positives from the sun, an awesome source of almost every electromagnetic wavelength, and able at some time of the day or some practical bank angle, to shine through every chink and opening in an engine cowling. An apprentice had a more pressing question about reinstallation of an exhaust manifold, and I was wearing silly girl shoes for a social event, so I teetered back out of the hangar to the office and left the maintenance to the maintainers.
Along with the answer to my question, and before the apprentice needed direction, I received the following story.
A certain air carrier's night maintenance crew (they know who they are)
conducted an inspection on at least one of the carrier's fleet. Their
fire detection system used visible light, and the crew followed the
manufacturer's instructions to test it by shining a flashlight on the
detectors. The system failed the test. The crew was preparing to replace
several thousand dollars worth of aviation grade electronic gear, when
someone realized that that instructions were as old as the system,
twenty years or so, and predated the invention of the LED flashlights
the crew were using. The diodes were not emitting the necessary
wavelength to trigger the detector, because when the instructions said
"flashlight" an incandescent flashlight was assumed.
The Wikipedia article on Fire Detection suggests that our photocell might be tuned to multiple wavelengths,
possibly including UV and/or IR to compare the ratios. This still
doesn't explain how an ordinary incandescent flashlight would trigger
it, but the sun wouldn't, but does make the concept of photometric flame
detection clearer. While looking for more information on flame detectors I discovered that there is a whole new generation of fire detection that uses machine intelligence to recognize the appearance of actual flames, something that is pretty fricking clear to humans. Combined with smoke detectors, something a commenter suggested on the previous post, there are enough different kinds of combustion detectors on aircraft to warrant a survey post on the topic, some time when my working day is shorter.
I was also entertained by the Wikipedia article on
the Flashlight, which explains why Americans call them flashlights. The earliest zinc–carbon batteries
could not provide a steady electric current so when combined with the
inefficient carbon-filament bulbs, the result was a light that
frequently blinked off so the battery could recover.