Fire detection systems as they are now configured in aircraft cargo bays do not always meet the current FAA requirements, including a specification that a fire be detected within 1 mm after initiation. A new FAA Airworthiness Directive requires upgraded fire detection systems on all combination passenger/cargo aircraft built by Boeing and McDonnell Douglas. A thermal monitoring/fire detection system that meets the new FAA regulations has been developed. It is based on infrared detection technology coupled with a Fresnel scanning optical system. The system uses a dual computer controller unit, which permits totally redundant thermal monitoring within the cargo bay. The system is responsive to either overheat or fire conditions, has three levels of fire alarm signal, incorporates complete optical coverage and electronic circuitry redundancy, and provides fire location information. It is highly reliable and immune to false alarm stimuli. Each sensor is designed to cover a portion of the cargo bay approximately 20 ft in diameter from 12 ft above. A system description is presented, and performance characteristics for fire/overheat and false alarm immunity are discussed.