Where There’s Smoke, There’s Not Always Fire
NASA Glenn Research Center has successfully tested a new design concept for fire detection in commercial airliners. These aircraft are equipped with fire detectors and extinguishers in the baggage and cargo compartments. The current detectors sense the smoke particles coming from a fire. But, when they sense smoke, there is not always fire. The sensors can be fooled by dust and other aerosols that sometimes form in the cargo compartments. The smoke detectors can mistakenly signal the pilots to needlessly initiate emergency procedures for a fire. NASA’s newly tested concept is a multi-sensor package built to read a wider picture of fire emissions, not just smoke. NASA’s detector looks for increased concentrations of combustion gases along with the smoke, thus reading a more complete fire signature. The multi-sensor package compares various gas concentrations and smoke particle sizes to those values characteristic of an actual fire. The result is a fire detector system that works to effectively recognize the presence of fire while screening out false alarms.
NASA Glenn applied its expertise in MicroElectroMechanical Systems (MEMS) chemical sensors to this new fire detection concept in response to the significant number of false alarms generated by the current fire detectors used on aircraft. Current fire detectors use either photoelectric or ionization sensor to detect the smoke particles produced by a fire. These sensors work well at detecting the presence of fire, but it has been estimated that as many as 200 false alarms occur for every actual fire. In-flight fires are rare events. However, with no way for the pilot or crew to visually verify the presence of a fire in the under-floor cargo compartment, any alarm has to be treated as an actual fire. The pilots must discharge on-board extinguishing equipment, declare emergency priority over other air traffic, and, as soon as possible land the aircraft at the nearest airport. Prior to takeoff, a false alarm will require that the pilots return the aircraft to the gate and delay the flight until mechanics can repair the problem.
Designing a fire detection system for an aircraft cargo compartment is a challenge. Unlike the conditions seen by the smoke detector in your house, aircraft cargo compartments present an array of challenges from widely varying humidity and air temperatures and pressures causing condensation, to dust, dirt and aerosols from the cargo ramp environment disturbed by turbulence and vibration.
Tests recently conducted at the Federal Aviation Administration (FAA) Cargo Compartment Fire Testing Facility in Atlantic City, NJ, successfully demonstrated the miniaturized, combined chemical and particle sensors approach. First, the multi-sensors were subjected to dust and high humidity levels which would regularly cause conventional smoke detectors used in cargo bays to false alarm. Over a series of exposures, the NASA MEMS multi-sensor package had a zero false alarm rate. Then, the MEMS multi-sensor package was mounted alongside conventional smoke detectors in the facility’s Boeing 707 under-floor cargo bay. Over the entire test series, the multi-sensor package sensed the onset of actual fire equally as well as the conventional smoke detectors.
The MEMS sensor system developed by NASA Glenn includes a miniaturized CO and CO2 sensors, a smoke particle detector, and integrated software. The on-board processor evaluates the response of the complete sensor system and gives an indication of whether there was a fire condition. Interpreting the fire signature has been shown to be an effective approach to fire detection while minimizing false alarms.
The development and use of MEMS-based chemical sensing and particulate technology for fire detection has been on-going for a number of years as part of a joint NASA-FAA program. NASA and the FAA have also funded and managed development of analytic fire models by Sandia National Labs. Sandia’s modeling tool is intended for fire detection system designers to help determine effective sensor locations and determine effective set points to activate cockpit warnings. The tool is also intended to replace, where possible, the need for destructive testing using actual fire to support design and certification.
These activities have also been coordinated with NASA developments related to safety of its International Space Station. False alarms are also of concern in Space Station operations and for future Exploration Missions. Possible use of this new fire detection approach is being considered for those applications.
These tests have shown the feasibility of an integrated multisensor fire sensor system. There are still possible improvements needed to the sensors, packaging and software/interface, long term durability and drift effects of the sensors, and FAA certification criteria for these new multi-sensor detectors. But these tests have successfully demonstrated a new generation of aircraft cargo compartment smoke detectors that look for the fire’s comprehensive signature rather than just its smoke.
