Water run-off in aircraft fuel tanks

When warm humid air enters an aircraft fuel tank, condensation will occur on surfaces that are colder than the dew point. These surfaces can include the tank ceiling, vertical and sloping walls and the air/fuel free surface. The microscopic water droplets deposited by condensation will eventually make their way to the fuel tank sump under the competing influences of gravity and surface tension. Accumulated water in fuel tanks can lead to fuel contamination and must be drained from the sump on a regular basis. One proposed method to improve the efficiency of water drainage to the sump is to use hydrophobic coatings on the walls of the tank. Hydrophobic coatings should reduce the critical size of water droplets pinned to the surface and increase the runoff speed of drops once in motion. We discuss experiments to measure these critical sizes and runoff speeds on different surfaces and compare the results to predictions from simple theoretical models of drop run-off. In order to predict the overall effect of the hydrophobic surface coatings on water migration through the tank, the movement of water drops through the fuel layer should also be taken into account. Here, we discuss possible interactions between fuel and water drops. We show that very small water drops can float, hanging from the fuel free surface. Drops running down a rigid surface which meet the fuel free surface undergo a transition into the fuel layer that involves detachment from the rigid wall and sometimes the formation of satellite drops.