Air ducts are removed for inspection or trouble shooting in aircraft maintenance. When the inspections are completed and the fault is resolved, these air ducts are reinstalled then the system related to the duct is operated. Ducts are checked with hand whether there is any leakage in the related area. There are sealing elements names as combo seals at the joints of these air ducts.
These elements lose their properties over time due to the effect of temperature and leakage is observed in the area. If leaks can not detected, they may damage the surrounding components and cause problems in the system by sending less air to the systems it feeds.
The leak detection system is used to eliminate these problems. Leak detection system basically consists of sensing loops and BMC (Bleed Monitoring Computer). Sensing loops are nickel lines in an insulation made of aluminum oxide.
Two sensing loops are positioned along the bleed lines starting from APU (Auxiliary Power Unit), to inside the fuselage, the pylon and to the wings. These loops, which are named loop A and loop B, send senses to BMC with the impedance changing in case of an overheat condition on the bleed line and provide to monitor of faults such as ‘’APU AIR LEAK, R WING LOOP A, L WING LOOP B‘’ in ECAM (Electronic Centralised Aircraft Monitor) or MCDU (Multi-function Control and Display Unit). Having two loops is to confirm the failure.