There are three basic type fuel cells used in aircraft. Welded sheet metal integral and fuel cell. No fuel system is airworthy if it will not contain fuel. Inspection of the fuel tank bays or aircraft structure for evidence of fuel leaks is a very important part of the preflight inspection.

Welded Steel Tanks

Welded tanks are most common in the smaller single and twin engine aircraft. If the access plates to the fuel tank compartment are discolored the tank should be inspected for leaks. When leaks are found, the tank must be drained and inerted. Fuel will be drained in accordance with local instructions and the manufacturer's recommendations.

Inerting the tank may be accomplished by slowly discharging a CO2 fire extinguisher (5 lb minimum size) into the tank. Dry nitrogen may be used if it is available. If the tank is to be welded, removal is necessary.

Before welding, the tank must be steamed for a minimum of 8 hours. This is to remove all traces of fuel. Air pressure not over 1/2 psi may be used to detect the leaking area. Liquid soap or bubble solution brushed in the suspected area may identify the leak. Aluminum tanks are fabricated from weldable alloys. After riveting patches in place, the rivets may be welded to insure no leaks from that area. Pressure checks should be performed after repairs are completed to assure that all leaks were corrected.

Fuel Cells

Fuel cell leaks will usually appear on the lower skin of the aircraft. A fuel stain in any area should be investigated immediately. Fuel cells suspected of leaking should be drained, removed from the aircraft and pressure checked. When performing a pressure check, 1/4 to 1/2 psi air pressure is adequate. All fuel cell maintenance must be accomplished in accordance with the manufacturer's specifications.

Integral Fuel Tanks

The integral tank is a nonremovable part of the aircraft. Because of the nature of an integral tank, some leaks allow fuel to escape directly to the atmosphere. This makes it completely feasible to disregard certain minute leaks that do not represent a fire hazard or too great a loss of fuel. In order to standardize the procedures for integral tank fuel storage maintenance, the various rates of fuel leakage are classified.

Fuel Leak Classification

The size of the surface area that a fuel leak moistens in a 30 minute period is used as the classification standard. Wipe the leak area completely dry with clean cotton cloths. Compressed air may also be used to dry the leak areas that are difficult to wipe. Wear goggles when using compressed air to dry the leak area. Dust the leak area with dyed red talcum powder. The talcum powder turns red as the fuel wets it, making the wet area easier to see.
At the end of 30 minutes, each leak is classified into one of four classes of leaks: slow seep, seep, heavy seep, or running leak. The four classes of leaks are shown in figure 4-26. A slow seep is a leak in which the fuel wets an area around the leak source not over 3/4 of an inch in diameter.

A seep is a leak that wets an area from 3/4 inches to 1 1/2 inches in diameter. A heavy seep is a fuel leak that wets an area around the leak source from 1 1/2 inches to 3 inches in diameter. In none of these three leak classifications does the fuel run, flow, drip, or resemble any of these conditions at the end of the 30 minute time period.

The last classification, a running leak, is the most severe and the most dangerous. It may drip from the aircraft surface, it may run

down vertical surfaces, or it may even run down your finger when you touch the wet area. The aircraft is unsafe for flight and must be grounded for repair. When possible, the fuel from the leaking tank should be removed after you mark the leak location. If it is impossible to defuel the tank immediately, the aircraft should be isolated in an approved area. Place appropriate warning signs around the aircraft until qualified personnel can defuel the leaking tank.

Grounding of the aircraft for slow seeps, seeps, and heavy seeps, is determined by the applicable aircraft handbook. This determination may depend on the location of the fuel leak. For example, can the leakage progress to a potential fire source? The number of fuel leaks in a given area is also a contributing factor. There is no rule of thumb for determining if the aircraft is to be grounded. Running leaks ground the aircraft regardless of location.

You may only have to make appropriate entries on the aircraft forms and periodically observe the progress of the fuel leak if it is determined that the aircraft is airworthy and no repair is required. When repair is required, you must find the cause of the fuel leak and make an effective repair.

Leak Repairs

Repair of leaks in integral fuel tanks must be accomplished in accordance with the aircraft manufacturer's specifications. No attempt will be made in this handbook to discuss integral tank repairs further.

Fire Safety

The first and most difficult step in the achievement of fire safety is to correct the misconceptions about the "safety" of turbine fuels. At the time these fuels were first introduced many people said, "fire problems in aircraft are over, turbine fuel is completely safe." This is obviously nonsense but it has been persistent nonsense.

Flight line personnel have agreed that aviation gasoline will burn, and therefore they have exercised reasonable care and caution in handling it. However, it has been difficult to convince them that under some circumstances turbine fuels are just as dangerous from the fire standpoint.

The characteristics of turbine fuel do vary from those of gasoline. Kerosene, for example, has a slow flame propagation and burning rate, which makes it less hazardous in the event of spill or a ground accident. However, it does ignite readily when vaporized or when misted, as when sprayed through a small leak in a service hose.

One disadvantage of the low volatility fuels is that they will not evaporate readily and completely if spilled on the ramp, so special treatment of the spill area is required. Small spills of kerosene should be removed with a commercial absorbent cleaning agent. On large spills it is better to apply an approved emulsifier and then flush away the resulting mixture with large volumes of water. This will prevent or appreciably lessen any oily residue.

Just as with gasoline, an electrostatic charge may be built up in pumping turbine fuel through a service hose. In fact, the amount of the charge is higher in kerosene because of the higher specific gravity and wider boiling range. Also, the amount of the charge increases with high linear rate of fuel flow, such as is required for servicing turbine powered aircraft.

In consequence, all of the fire safety precautions observed in the handling of gasoline must be followed with equal care in the handling of turbine fuels. These precautions are well known and have been detailed by the National Fire Protection Association in its bulletin No. 407. It is recommended that this bulletin be made required reading for all personnel handling turbine fuel.