The Research on the Diagnostics of the Overheat Detection System Fault

Based on the requirements of a certain type of regional aircraft, the overheat detection system is designed to monitor bleed air leakage of the engines and APU (Auxiliary Power Unit). The detection system contains a series of overheat sensing elements, each of which is an inconel tube with center wire conductor embedded in the eutectic salt. The overheat alarm of the detection system can be triggered when there is a certain decrease in the value of the resistance between the center wire and the ground. Due to the fact that the sensing elements are connected in series circuit as a detective loop, it is thus difficult to distinguish the defective element from the whole loop, when a short or open fault message comes out and identifies the fault condition of the overheat detection system during the line maintenance. This result can cause extra maintenance hours and a consequential delay of flight. In order to facilitate the fault isolation procedure and speed up aircraft dispatch, this paper probed into an applicable solution to map the defective sensing element in the overheat detection system. Based on the already-developed method of fault diagnostics, this paper adopted the following methods to further identify and locate defective element: (1) Obtain the resistance and capacitance factors of the sensing element and the electric wiring in normal conditions. (2) Calculate the short and open circuit location based on the electric bridge method and capacitance method. (3) Establish the relationship between the defective sensing element and the location code. (4) Present the location code via CMS (Central Maintenance System) for short event, and prepare the result referencing tables for FIM (Fault Isolation Manual) in open event. Through the observation of its application in actual work, the above methods have achieved great advantages in mapping the defective element and improving the maintenance convenience capacity of the aircraft.