Types of Aircraft contaminants

Types of contaminants:

a.   Solid particles: Sand blown into the storage tanks or into the aircraft tank during the fuelling operation or rust from unclean storage tanks are solid particles which can clog strainers and restrict the flow of fuel.

b.                                    Surficant:                These are partially soluble compounds which are by-products of the fuel processing, or they may come from fuel additives. They have the tendency to adhere to other contaminants and cause them to drop out of the fuel and settle to the bottom of the fuel tank as sludge.

c. Water:                                 Although it has always been present in avia­tion fuel, water is now considered to be a major source of contamination since aircraft fly at alti­tudes; where the temperature is low enough to cause water entrained, or “dissolved,” in the fuel to precipitate out and form free water. This free, water can freeze and clog the fuel screens.

d.                                    Micro organisms: Tiny micro organisms grow from airborne bacteria and gather in the fuel. They lie dormant until they can come into contact with free water, but then they grow at a prodigious rate as they live in the water and feed on the hydrocarbon fuel, and on some of the surficant contaminants. The scum which they form holds water against the walls of the fuel tanks and causes corrosion.

Detection of contaminants:

Draining a sample of fuel from the main strainers of an aircraft has in the past been con­sidered an acceptable method of assuring that the fuel in the system is clean. But the fuel cleanli­ness requirements for modern aircraft engines re­quire more extensive testing than this.

One test recommended by the FAA for check­ing for fuel contaminants is to drain about a quart of fuel into a spotlessly clean ten-quart white enamel bucket. Stir the fuel with a clean paddle and swirl it in the form of a vortex cone. All of the contaminants will gather at the centre of the vortex and can be easily seen. A few drops of food colouring will dye any water that is in the fuel so it will be readily visible, but the food colouring will not colour the fuel

.

A commercial water test kit is used far more often than the white-bucket test. This kit con­tains a small glass jar and a supply of capsules containing a grayish-white powder, and a 100-cc sample of fuel is taken from the truck and put in­to the jar. Then one of the test capsules is dumped into this fuel, the lid is screwed on, and the contents are shaken for about ten seconds. If the powder changes its colour from gray-white to pink or purple, the fuel has more than 30 parts per million of water, and it is not considered safe for use. It may be pumped through the water traps and filters in the truck and another sample taken. This test is fail-safe, because any error in performing the test will cause it to give an unsafe indication.

If the fuel sample is taken from the aircraft, the fuel should be drawn from the main strainer and some fuel should be allowed to flow from every one of the tanks. Drawing fuel from the main strainer when the tank selector is in the both position will not necessarily get fuel from all of the tanks.

Fire Hazards: when Fuelling and De fuelling Aviation fuels are both highly flammable and volatile, and special care must be exercised when transferring them into or out of an aircraft. Be sure that the proper type of fire extinguisher is available at the aircraft and that it has been pro­perly serviced and has not been used, even par­tially, since it was last serviced.

Never service an aircraft with fuel inside a hangar or in any other closed area. If fuel is stored in containers other than the fuel service truck or the aircraft fuel tanks, be sure that the containers are closed, not only to prevent the en­try of contaminants, but also to prevent the re­lease of fuel vapours.

If any fuel is spilled, wipe it up immediately; or, if too much has been spilled to wipe up, wash it away with water or cover it with sand.

Be sure that there are no open fires, gasoline motor exhausts, or electrical equipment opera­ting in the vicinity of fuelling or defuelling opera­tions, and be sure that all electrical power on the aircraft except that required for the fuelling opera­tion is turned off.

Wear only a type of shoes that will not cause sparks and clothing that will not produce sparks from static electricity. And, be sure that the air­craft and fuel truck or hydrant are properly bond­ed electrically.

Static electricity builds up when fuel flows through the fuel lines and its discharge must be guarded against.) About fifteen minutes is needed for a tank full of fuel, especially turbine fuel, to relax itself of the charges of static electricity after the tank has been filled.

When fuelling or refuelling an aircraft, first connect the fuelling vehicle or fuel cabinet to the aircraft with a static ground wire, and then con­nect the nozzle to the grounding point at the tank filler opening. Only after this has been done should the filler cap be removed.