The Engine Was Burning. The Man on the Wing Kept Filming.

Flames stream beside an aircraft engine. On the wing, a man holds a movie camera and records the test.

Doug Slocum identifies him as his father, Charles William Slocum, who worked on thermal devices for the Edison company. Charles later told Doug about investigating fuel vapors accumulating inside engine compartments—and developing a temporary system to detect and extinguish a fire before it spread.

But even recognizing the fire posed a surprisingly tricky problem.

How do you detect fire in a place that’s supposed to get hot?

Edison’s detector watched how quickly the temperature rose. Its September 1945 brochure describes a “rate-of-rise” system that compensated for normal changes in atmospheric and engine temperatures, rather than relying solely on a fixed temperature threshold.

A fixed temperature threshold creates a tradeoff. A high setting can delay a warning when the compartment starts cold; a lower setting can produce unwanted warnings when its surroundings are already hot. Edison’s brochure explicitly presents rapid-rise detection as a way around those problems.

The company advertised a response to flame within half a second to one second. That is the manufacturer’s stated performance, not a measurement we can independently recover from these photographs.

The system connected thermocouples—sensors that generate a small voltage from a temperature difference—to a sensitive relay. The relay could operate a warning light, horn or buzzer.

Another detail in the brochure matters in a compartment containing combustible vapors: Edison said battery current did not enter the detector wiring inside the nacelle, the housing around the engine. The company presented that arrangement as eliminating ignition from a spark caused by a broken wire in that circuit.

The detector therefore had two jobs: recognize dangerous heating promptly, and avoid introducing an electrical ignition source through its sensing circuit.

The photographs show an aircraft supported above the ground, an opened engine cowling and a panel labeled for gas, oil, ignition and extinguishing channels. Those details make the controlled test visible; they do not tell us its complete procedure.

Charles Slocum told his son that his investigation identified a problem involving accumulated fuel vapors and that he designed a temporary electronic detection-and-extinguishing solution. That account comes directly from Doug’s conversations with his father. The surviving brochures establish Edison’s commercial detection approach, but do not establish that Charles designed that particular product.

Doug later donated six documents and 28 photographs to Thomas Edison National Historical Park in West Orange, New Jersey. The park’s June 1, 2016 acceptance letter identifies Edison Instrument Division documents and photographs of aircraft tested at the location it calls Wright-Patterson Air Force Base. The collection’s accession number is EDIS-00829.

The engineering idea remains striking: the useful question was not simply “Is it hot?” It was “How fast is it getting hotter?” And the family photograph preserves a man standing beside the experiment, making sure its behavior was recorded.

Primary materials supplied by Doug Slocum: Edison’s Aircraft Fire Detection System brochure, Publication No. 3003 (September 1, 1945); aircraft-test photographs 183680 and 182135; National Park Service donation acknowledgment and deed, accession EDIS-00829 (June 1, 2016). Charles William Slocum’s individual role and temporary extinguishing solution are attributed to his son’s recollection of conversations with him. Photograph creators are unidentified. No AI imagery.

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