Every rule, directive, regulation, instruction and procedure tells you to periodically inspect the underwing nozzle strainer and remove any collected contamination. Some directives tell you to inspect the contamination to see whether it indicates equipment failure. Rubber particles (black debris) indicate problems in the hose. Metallic particles indicate equipment failure. Sand, bits of wiping rags and other debris tell you that someone has been careless. So it is obvious that nozzle strainer inspections are important, but how you do it is the point we want to make in this GamGram. Recently a large quantity of contamination was found in a hose immediately after the aircraft had been fueled. Due to the fact that the nozzle was in a vertical position and the dirt was heavy, gravity allowed it to fall “down” when the flow stopped. Flow had carried the particles into the strainer, but when flow stopped, the particles fell back into the hose. This process can happen over and over again. This is what the title of the GamGram means — Hidden Contamination.