Which of the following is NOT used to detect fires in reciprocating engine nacelles?

In the context of fire detection in reciprocating engine nacelles, smoke detectors are typically not employed. This is primarily because the environment in the engine nacelle presents challenges for the effective functioning of smoke detection technology. The presence of hot gases, fuel, and various engine components can obscure smoke or lead to false alarms.

In contrast, rate-of-temperature-rise detectors, flame detectors, and heat detectors are used because they are better suited for the dynamic conditions present in an engine nacelle. Rate-of-temperature-rise detectors monitor sudden increases in temperature, providing a rapid response to fire conditions. Flame detectors sense infrared or ultraviolet radiation from flames, allowing for quick identification of combustion. Heat detectors are designed to respond to elevated temperatures, triggering alarms when a specific threshold is reached.

In summary, the limitations of smoke detectors in the harsh and variable environment of engine nacelles make them an unsuitable choice, thereby confirming that they are not used for fire detection in this specific application.