Monitoring of the aircraft structure

The ongoing monitoring of the technical condition of an aircraft in operation – known as health monitoring – has developed very unevenly. Whereas, for example, engine condition monitoring (ECM) is already largely integrated into flying operations, monitoring of the structure of an aircraft is as yet still in its infancy. The aim here is to significantly improve reliability and also the cost-effectiveness of flying operations. Hence the monitoring of aircraft structures is an important objective for engineers.

In 2011 Lufthansa Technik began trials for monitoring the aircraft structure in collaboration with research partners. Under the Aircraft Integrated Structural Health Assessment II (AISHA II) program sponsored by the European Union, the Department of Metallurgy and Materials Engineering, Leuven University, Belgium has developed a moisture detection sensor that Lufthansa Technik installed in the floor structure of a Lufthansa Boeing 737 underneath the galley. This area is very difficult to access, while at the same time the probability of spotting a problem is low. Yet if advanced stage corrosion is found here during an overhaul, the repair costs are substantial.

This application, also known as "hotspot monitoring", i.e. the retrospective installation of sensors in critical areas, had proved so useful after a short test phase that Lufthansa Technik decided to implement the system for real without delay. For this purpose twelve sensors were fitted in the critical floor structures of each of two Boeing 747 jumbo jets. The crucial factor behind this decision was the fact that the manufacturer requires an extensive extra inspection of those areas between overhauls. If the areas can be shown to be dry with the aid of moisture detection sensors, this inspection can be deferred. In this way the substantial cost of exposing the floor structure can be saved along with the time spent on the ground by the affected aircraft. As the sensor is built almost entirely from certified components, nothing stands in the way of a rapid introduction of the product – the engineers expect this to be a matter of one or two years.

With the new moisture detection sensor the structure of the aircraft can be monitored at critical points, an important objective that has been achieved in a highly efficient manner through a European cooperation program. But this is just a first step. Other issues are to be resolved in the planned future European research project AISHA +. One of the tasks to be undertaken during this program is to develop sensors for different liquids, for example, kerosene, hydraulic oil (Skydrol) and other mineral oils. However, the technical design and certification requirements that have to be satisfied in the case of an airworthy sensor are far more stringent than for the moisture detection sensor, making this an interesting application area for the AISHA + consortium.