This research, performed at Airbus Defence and Space Ground System Test Engineering department in Seville, Spain, is a part of ELDAS (Engineering for Leak Detection in Aircraft Systems) project. It deals with carrying out an investigation on the leak behaviour of the sealing systems used in an aircraft particularly A400M. The sealing integrity of a fuel system is, inarguably, a critical aspect in an aircraft sealing technology. The need for NO fluid leakage and consequently preventing any structural damage to the internal components of an aircraft drives the aircraft designers towards carrying out a detailed research on the sealing systems. The analysis is divided into two parts: firstly, to conduct a structural analysis of the current installation and to computationally study the rubber behaviour; secondly, the characterization of the leakage. This combined analysis is to investigate the current issues associated with the sealing system and propose methods for leak estimation by characterization of leak applicable to the fuel system of an aircraft. Several parameters e.g. fluid properties, material properties, surface roughness, flow conditions to name a few, are thought to be correlated with fluid leakage. This document along with the literature study report will summarize the investigation performed to study the effects of these parameters on the seal leak rate. The structural analysis is performed using Ansys Mechanical Parametric Design Language 14.5 to verify if the current design parameters are well within the prescribed limits. The non-linear behaviour of rubber seal is studied and verified with different available analytical models. A grid independence test is performed via error analysis to further strengthen the simulation obtained results. Various time dependent rubber viscoelastic behaviour are studied to understand the seal deterioration over time. This deterioration in stress within a rubber seal (Fluorosilicone) might increase leakage potential over a period of time. Investigation is also performed to understand the importance of lubrication on sealing performance along with the adverse effects of reusing an o-ring seal multiple times and the associated leak potential. Furthermore, analysis is done for different dimensional cases considering the manufacturing tolerance values. Lastly, the complex extrusion behaviour of the seal at high fluid pressure is studied and simulated to understand the seal failure at high pressure. During the literature review, it is observed that of all the leak estimation models available, no formulation appears to have general validity, so a more general model based on different flow regimes appears to be necessary taking into account the different fluid parameters, leak geometry and operating conditions. The study on the leak behaviour of static seals of an aircraft is performed to establish a correlation between gas and liquid leaks. This study can then be used to facilitate industrial leak testing by providing a correlation between leak rates for liquids and gases which can be used to replace liquid based testing with gas based testing. Obviously, the study can be used for planning further experimentation and validation purpose.