High Temperature Fire & Overheat Sensor Elements (FOSE) for aircraft Fire & Overheat Detection System (FODS)

One of the 21st century major challenges is to answer the growing mobility by a carbon saver solution. For aeronautics, improvement of engine architecture (NIPSE European project) permits to reduce its volume and so to get more space for passengers & freight (15%), to save weight and fuel consumption (2-3% i.e. 1% CO2 emissions). This optimization hinges on integrating the engine and the nacelle into single assembly (Ultra High-Bypass Ratio turbofan engines or UHBR) that means equipment close to engine (calculator, valves, but also sensors and their extension cables…) have to resist higher temperatures. The Fire and Overheat Detection System (FODS) is a critical component for safe working of the aircraft as it controls and prevents from overheat and fire all those electrical equipment and the engine itself. THERMOCOAX, manufacturer of Mineral Insulated Cable (MIC) for over 60 years, is a privileged partner of the aeronautics industry (anti-icing for air inlet, pitot sensor, drain mast and thermal measurement from thermocouples to complete systems such as harnesses). For 25 years THERMOCOAX has manufactured Fire and Overheat Sensor Elements (FOSE) - the sensitive part of FODS - and as a specialist has taken part in this innovative project. The sensing principle of FOSE lies on electrical insulation resistance (IR) drop as the cable is exposed to heat. A wide range of research was made to find technical solutions for the new thermal specification. The difficulty comes from having IR with good sensitivity and linearity at temperature, covering a large temperature range: overheat threshold of either 473K ( 200°C) or 873K ( 600°C) depending on the location of the sensor. Also, the FOSE has to remain functional after 2 fires at 1453K ( 1180°C) during 5 min each, so the sensor sensitive insulant must not be deteriorated. Several solutions based either on thermistor powders or on percolation effect powders have been tested by miscellaneous trials: repeatability, thermal cycling, fire resistance test, local response at different room temperatures (from RT to 723K) and homogeneity along the cable. Finally, a new insulant solution has been selected to be integrated to the FOSE future generation. The other aspect of those new temperature rates is the electrical connectics that should not deteriorate the FOSE response, especially when the aircraft is in overheat or fire situation. So, a new design of 3 contacts ceramic-metal connector has been developed to resist more than 2 fires without generating parasite responses and to fit with aeronautics standards. Severe environment requested by NIPSE project forces THERMOCOAX to develop a new technology of FOSE based on insulant advanced studies and specific connectors to resist high temperature.