LOADING GRAPH CO-PILOT GALLONS * :./Gal.)
LOAD WEICHT IN POUNDS ‹ PILOT FUEL PASSENGERS 43 40 REAR 30 Add weight of items to be carried to the Licensed Empty Weight of the airplane. Add moment in thousands of pound - inches of these BAGGAGE items to the total airplane moment L in thousands of pound-inches found on the Center of Gravity Envelope.
40 2 4 10 12 14 16 18 20 22 24 26 28
MOMENT IN THOUSANDS OF POUND-INCHES FIGURE 3-13. Typical loading graph.
The c.g. is well within the c.g. range when fully loaded; however, the maximum allowable gross weight is exceeded by 10.5 pounds. In this case a number of alternatives are available to remedy this overloaded condition without appreciably reducing the aircraft payload or flight range, as follows:
Alternative No. 1-reduce baggage by 10.5 Alternative No. 2-reduce fuel by 10.5 Ibs., or 1.75 gals.
Alternative No. 3-reduce passenger load by one passenger.
Each alternative listed will require a placard stating the loading arrangement by which the gross weight and c.g. will be retained within their designated limits. Compute a new c.g. position for each alternate loading arrangement.
LOADING GRAPHS AND C.G. ENVELOPES The weight and balance computation system, commonly called the loading graph and c.g. envelope system, is an excellent and rapid method for determining the c.g. location for various loading arrangements. This method can be applied to any make and model of aircraft.
Aircraft manufacturers using this method of weight and balance computation prepare graphs similar to those shown in figure 3-13 and 3-14 for each make and model aircraft at the time of original certification. The graphs become a permanent part of the aircraft records. Along with the graphs are the data for the empty weight arm and moment (index number) for that particular make and model aircraft.
The loading graph illustrated in figure 3-13 is used to determine the index number of any item or weight that may be involved in loading the aircraft. To use this graph, find the point on the vertical scale that represents the known weight.
Project a horizontal line to the point where it intersects the proper diagonal weight line (i.e., pilot, copilot, baggage, etc.). From the point of intersection, read straight downward to the horizontal scale to find the moment or index number.
After the moment for each item of weight has been determined, all weights are added and all moments are added. With knowledge of the tota.
weight and moment, project a line from the respective point on the c.g. envelope shown in figure 3-14, and place a point at the intersection of the two lines. If the point is within the diagonal lines, the loading arrangement meets all balance requirements.
The following is an actual weight and balance computation using the graphs in figure 3-13 and
3-14. For this example, assume that the aircraft has an empty weight of 1,386.0 pounds and a moment of 52,772.0 pound-inches. The index 69
2400 AIRPLANE WEIGHT IN POUNDS 2300 2200 CENTER OF GRAVITY ENVELOPE 21 00 2000 1900 1800 1700 1600 Any point falling with- 1500 in the envelope meets all Balance require- 1400 ments.
40 50 60 70 80 90 100 110
MOMENT IN THOUSANDS OF POUND-INCHES FIGURE 3-14. Center of gravity envelope.
number for the empty weight of the aircraft is ELECTRONIC WEIGHING EQUIPMENT developed by dividing the empty-weight moment by 1,000. This gives an index number of 52.8 for the airplane's empty-weight moment. Load the aircraft to determine whether the c.g. will fall within the diagonal lines of figure 3-14. Arrange item weights and index numbers in an orderly form to facilitate adding.
Moment Electronic weighing equipment greatly simplified the mechanics of weighing large, heavy aircraft. Figure 3-15 shows one type of electronic scales. The complete weighing kit is contained in a portable carrier. Included in the kit are a steel tape, plumb bobs, spirit level, straightedge, hydrometer (for determining the fuel specific gravity) and the load cells. The load cells are actually strain gages that reflect the load imposed upon them by the aircraft in terms of voltage change.
Weight (thousands This change is indicated on a scale that is cali- Item (Ibs.) of Ib.-in.) brated to read in pounds.
One load cell is placed between the jack-pad Acft. EW 1,386.0 52.8 and the jack at each weighing point. Each load Oil 19.0 - 0.4 cell must be balanced or "zeroed" before applying Pilot & copilot 340.0 12.2 any load to the cell. After completing the weighing Rear passengers (two) 340.0 24.1 operation, remove all load from the cells and check Baggage 20.0 1.9 to see if the cell reading is still zero. Any deviation Fuel 245.0 11.8 from zero is referred to as the "zero scale shift" and constitutes the tare when using electronic Total 2,350.0 102.4 weighing scales. The direction of shift is the factor that determines whether the tare is added to or The total airplane weight in pounds is 2,350.0, and the moment is 102.4. Locate this point (2,350 @ 102.4) on the c.g. envelope illustrated in figure 3-14. Since the point falls within the diagonal lines, the loading arrangement meets all weight-and-balance requirements.
subtracted from the scale reading. Always follow the instructions of the manufacture whose scales you are using.
HELICOPTER WEIGHT AND BALANCE The weight and balance principles and procedures that have been described apply generally to 70
FIGURE 3-15. Aircraft electronic weighing kit.
helicopters. Each model helicopter is certificated for a specific maximum gross weight. However, it cannot be operated at this maximum weight under all conditions. Combinations of high altitude, high temperature, and high humidity determine the density altitude at a particular location. This, in turn, critically affects the hovering, takeoff, climb, autorotation, and landing performance of a helicopter. A heavily loaded helicopter has less ability to withstand shocks and additional loads caused by turbulent air. The heavier the load, the less the margin of safety for the supporting structures, such as the main rotor, fuselage, landing gear, etc.
Most helicopters have a much more restricted
c.g. range than do airplanes. In some cases this range is less than 3 inches. The exact location and length of the c.g. range is specified for each helicopter and usually extends a short distance fore and aft of the main rotor mast or the centroid of a dual rotor system. Ideally, the helicopter should have such perfect balance that the fuselage remains horizontal while in a hover, and the only cyclic adjustment required should be that made necessary by the wind. The fuselage acts as a pendulum suspended from the rotor. Any change in the center of gravity changes the angle at which it hangs from this point of support. More recently designed helicopters have loading compartments and fuel tanks located at or near the balance point. If the helicopter is not loaded properly and the c.g. is not very near the balance point, the fuselage does not hang horizontally in a hover. If the c.g. is too far aft, the nose tilts up, and excessive forward cyclic control is required to maintain a stationary hover. Conversely, if the
c.g. is too far forward, the nose tilts down and excessive aft cyclic control is required. In extreme out-of-balance conditions, full fore or aft cyclic control may be insufficient to maintain control.
Similar lateral balance problems may be encountered if external loads are carried.
Upon delivery by the manufacturer, the empty weight, empty weight c.g., and the useful load are noted on the weight and balance data sheet in the helicopter flight manual. If, after delivery, additional fixed equipment is added or removed, or if a major repair or alteration is made, which may affect the empty weight, empty weight c.g., or useful load, the weight and balance data must be revised. All weight and balance changes should be entered in the appropriate aircraft record.
GENERAL Fuel is a substance that, when combined with oxygen, will burn and produce heat. Fuels may be classified according to their physical state as solid, gaseous, or liquid.
Solid Fuels Solid fuels are used extensively for externalcombustion engines, such as a steam engine, where the burning takes place under boilers or in furnaces. They include such fuels as wood and coal. Solid fuels are not used in reciprocating engines, where the burning takes place inside the cylinder, because of their slow rate of burning, low heat value, and numerous other disadvantages.
Gaseous Fuels Gaseous fuels are used to some extent for internal-combustion engines, where a large supply of combustible gas is readily available. Natural gas and liquefied petroleum gas are two of the more common types. Gaseous fuels can be disregarded for use in aircraft engines. The large space they occupy limits the supply of fuel that can be carried.
Liquid Fuels Liquid fuels, in many respects, are the ideal fuel for use in internal-combustion engines.
Liquid fuels are classified as either nonvolatile or volatile. The nonvolatile fuels are the heavy oils used in diesel engines. The volatile class includes those fuels that are commonly used with a fuel metering device and are carried into the engine cylinder or combustion chamber in a vaporized or partially vaporized condition. Among these are alcohol, benzol, kerosene, and gasoline.
Aviation fuel is a liquid containing chemical energy that, through combustion, is released as heat energy and then converted to mechanical CHAPTER 4
FUELS AND FUEL SYSTEMS energy by the engine. This mechanical energy is used to produce thrust, which propels the aircraft.
Gasoline and kerosene are the two most widely used aviation fuels:
CHARACTERISTICS AND PROPERTIES OF
AVIATION GASOLINE Aviation gasoline consists almost entirely of hydrocarbons, namely, compounds consisting of hydrogen and carbon. Some impurities in the form of sulphur and dissolved water will be present. The water cannot be avoided, since the gasoline is exposed to moisture in the atmosphere. A small amount of sulphur, always present in crude petroleum, is left in the process of manufacture.
Tetraethyl lead (TEL) is added to the gasoline to improve its performance in the engine. Organic bromides and chlorides are mixed with TEL so that during combustion volatile lead halides will be formed. These then are exhausted with the combustion products. TEL, if added alone, would burn to a solid lead oxide and remain in the engine cylinder. Inhibitors are added to gasoline to suppress the formation of substances that would be left as solids when the gasoline evaporates.
Certain properties of the fuel affect engine performance. These properties are volatility, the manner in which the fuel burns during the combustion process, and the heating value of the fuel.
Also important is the corrosiveness of the gasoline as well as its tendency to form deposits in the engine during use. These latter two factors are important because of their effect on general cleanliness, which has a bearing on the time between engine overhauls.
Volatility Volatility is a measure of the tendency of a liquid substance to vaporize under given conditions. Gasoline is a complex blend of volatile 73
- Gage Bomb Sample container FIGURE 4-1. Vapor pressure test apparatus.
hydrocarbon compounds that have a wide range of boiling points and vapor pressures. It is blended in such a way that a straight chain of boiling points is obtained. This is necessary to obtain the required starting, acceleration, power, and fuel mixture characteristics for the engine.
If the gasoline vaporizes too readily, fuel lines may become filled with vapor and cause decreased fuel flow. If the fuel does not vaporize readily enough, it can result in hard starting, slow warmup, poor acceleration, uneven fuel distribution to cylinders, and excessive crankcase dilution.
The lower grades of automobile fuel are not held within the tolerances required for aviation gasoline and usually contain a considerable amount of cracked gasoline, which may form excessive gum deposits. For these reasons, automobile fuels should not be used in aircraft engines, especially air-cooled engines operating at high cylinder temperatures.
Vapor Lock Vaporization of gasoline in fuel lines results in a reduced supply of gasoline to the engine. In severe cases, it may result in engine stoppage.
This phenomenon is referred to as vapor locking A measure of a gasoline's tendency to vapor lock is obtained from the Reid vapor pressure test. In this test a sample of the fuel is sealed in a "bomb" equipped with a pressure gage. The apparatus (see figure 4-1) is then immersed in a constanttemperature bath and the indicated pressure is noted. The higher the corrected vapor pressure of the sample under test, the more susceptible it is to vapor locking. Aviation gasolines are limited to a maximum of 7 p.s.i. because of their increased tendency to vapor lock at high altitudes.
Carburetor Icing Carburetor icing is also related to volatility.
When the fuel changes from a liquid to a vapor state, it extracts heat from its surroundings to make this change. The more volatile the fuel, the more rapid the heat extraction will be. As the gasoline leaving the carburetor discharge nozzle vaporizes, it can freeze water vapor contained in the incoming air. The moisture freezes on the walls of the induction system, the venturi throat, and the throttle valves. This type of ice formation restricts the fuel and air passages of the carburetor. It causes loss of power and, if not eliminated, eventual engine stoppage. Extreme icing conditions can make operation of the throttle controls impossible. This icing condition is most severe in the temperature range of 30° to 40° F. outside air temperature.
Aromatic Fuels Some fuels may contain considerable quantities of aromatic hydrocarbons, which are added to increase the rich mixture performance rating of the fuel. Such fuels, known as aromatic fuels, have a strong solvent and swelling action on some types of hose and other rubber parts of the fuel system. For this reason, aromatic-resistant hose and rubber parts have been developed for use with aromatic fuels.
Detonation In an engine that is operating in a normal manner, the flame front traverses the charge at a steady velocity of about 100 feet per second until the charge is consumed. When detonation occurs, the first portion of the charge burns in a normal manner but the last portion burns almost instantaneously, creating an excessive momentary pres- 74
sure unbalance in the combustion chamber. This abnormal type of combustion is called detonation.
This tremendous increase in the speed of burning causes the cylinder head temperature to rise. In severe cases, the increase in burning speed will decrease engine efficiency and may cause structural damage to the cylinder head or piston.
During normal combustion, the expansion of the burning gases presses the head of the piston down firmly and smoothly without excessive shock. The increased pressure of detonation exerted in a short period of time produces a heavy shock load to the walls of the combustion chamber and the piston head. It is this shock to the combustion chamber that is heard as an audible knock in an automobile engine. If other sounds could be filtered out, the knock would be equally audible in an aircraft engine. Generally, it is necessary to depend upon instruments to detect detonation in an aircraft engine.
Surface Ignition Ignition of the fuel/air mixture by hot spots or surfaces in the combustion chamber is called surface ignition. If this occurs before the normal ignition event, the phenomenon is referred to as preignition. When it is prevalent, the result is power loss and engine roughness. Preignition is generally attributed to overheating of such parts as spark plug electrodes, exhaust valves, carbon deposits, etc. Where preignition is present, an engine may continue to operate even though the ignition has been turned off.
Present information indicates that gasoline high in aromatic hydrocarbon content is much more likely to cause surface ignition than fuels with a low content.
Octane and Performance Number Rating Octane and performance numbers designate the antiknock value of the fuel mixture in an engine cylinder. Aircraft engines of high power output have been made possible principally as a result of blending to produce fuels of high octane ratings.
The use of such fuels has permitted increases in compression ratio and manifold pressure, resulting in improved engine power and efficiency. However, even the high-octane fuels will detonate under severe operating conditions and when certain engine controls are improperly operated.
Antiknock qualities of aviation fuel are designated by grades. The higher the grade, the more compression the fuel can stand without detonating.
For fuels that have two numbers, the first number indicates the lean-mixture rating and the second the rich-mixture rating. Thus, grade 100/130 fuel has a lean-mixture rating of 100 and a richmixture rating of 130. Two different scales are used to designate fuel grade. For fuels below grade 100, octane numbers are used to designate grade. The octane number system is based on a comparison of any fuel with mixtures of iso-octane and normal heptane. The octane number of a fuel is the percentage of iso-octane in the mixture that duplicates the knock characteristics of the particular fuel being rated. Thus, grade 91 fuel has the same knock characteristics as a blend of 91 percent iso-octane and 9 percent normal heptane.
With the advent of fuels having antiknock characteristics superior to iso-octane, another scale was adopted to designate the grade of fuels above the 100-octane number. This scale represents the performance rating of the fuel-its knock-free power available as compared with that available with pure iso-octane. It is arbitrarily assumed that 100 percent power is obtained from iso-octane alone. An engine that has a knocklimited horsepower of 1,000 with 100-octane fuel will have a knock-limited horsepower of 1.3 times as much (1,300 horsepower) with 130 performance The grade of an aviation gasoline is no indication of its fire hazard. Grade 91/96 gasoline is as easy to ignite as grade 115/145 and explodes with as much force. The grade indicates only the gasoline's performance in the aircraft's engine.
A convenient means of improving the antiknock characteristics of a fuel is to add a knock inhibitor.
Such a fluid must have a minimum of corrosive or other undesirable qualities, and probably the best available inhibitor in general use at present is TEL (tetraethyl lead). The few difficulties encountered because of the corrosion tendencies of ethylized gasoline are insignificant when compared with the results obtained from the high antiknock value of the fuel. For most aviation fuels the addition of more than 6 ml. per gallon is not permitted. Amounts in excess of this have little effect on the antiknock value, but increase cor- There are two distinct types of corrosion caused by the use of ethyl gasoline. The first is caused by the reaction of the lead bromide with hot metallic surfaces, and occurs when the engine is 75
in operation; the second is caused by the condensed products of combustion, chiefly hydrobromic acid, when the engine is not running.
Purity Aviation fuels must be free of impurities that would interfere with the operation of the engine or the units in the fuel and induction system.
Even though all precautions are observed in storing and handling gasoline, it is not uncommon to find a small amount of water and sediment in an aircraft fuel system. A small amount of such contamination is usually retained in the strainers in the fuel system. Generally, this is not considered a source of great danger, provided the strainers are drained and cleaned at frequent intervals. However, the water can present a serious problem because it settles to the bottom of the fuel tank and can then be circulated through the fuel system. A small quantity of water will flow with the gasoline through the carburetor metering jets and will not be especially harmful. An excessive amount of water will displace the fuel passing through the jets and restrict the flow of fuel; it will cause loss of power and can result in engine stoppage.
Under certain conditions of temperature and humidity, condensation of moisture (from the air) occurs on the inner surfaces of the fuel tanks.
Since this condensation occurs on the portion of the tank above the fuel level, it is obvious that the practice of servicing an airplane immediately after flight will do much to minimize this hazard.
Fuel Identification Gasolines containing TEL must be colored to conform with the law. In addition, gasoline may be colored for purposes of identification. For example, grade 100 low lead aviation gasoline is blue, grade 100 is green and grade 80 is red. See figure 4-2.
100/130 gasoline is manufactured (1975) in two grades- high-lead, up to 4.6 milliliters of lead per gallon and low-lead, not over 2.0 milliliters per gallon. The purpose being to eliminate two grades of lower octane fuel (80/87) and 91/96). The high-lead will continue to be colored green whereas the low-lead will be blue.
The low-lead will replace the 80/87 and 91/96 octane fuels as they are phased out. Engine manufacturers have prepared instructions to be followed in making adjustments necessary for changeover to the 100 octane fuel.
A change in color of an aviation gasoline usually indicates contamination with another product or a loss of fuel quality. A color change can also be caused by a chemical reaction that has weakened the lighter dye component. This color change in itself may not affect the quality of the fuel.
A color change can also be caused by the preservative in a new hose. Grade 115/145 gasoline that has been trapped for a short period of time in new hose may appear green. Flushing a small amount of gasoline through the hose usually removes all traces of color change.
AVGAS 115 Military or Special Use Civilian AVGAS 100 Civilian
HIGH LEAD AVGAS 100 Civilian
LOW LEAD
• AVGAS 80 Civilian FIGURE 4-2. Identification of avgas.
JET A JET A Civiliun (Kerosene—4Q°F) (Kerosene-58° F) JET A-1 JET A-1 Military JET B JET B Military (Naptha-Kerosene-76° F) FIGURE 4-3. Identification of jet fuels.
ruel Identification Markings The most positive method of identifying the type and grade of fuel includes the following:
1. Marking of Hose. A color band not less than one foot wide painted adjacent to the fitting on each end of hose used to dispense fuel. The bands completely encircle the hose, and the name and grade of the product is stenciled longitudinally in one-inch letters of a contrasting color over the color band.
2. Marking of Fuel Carriers, Pits and Fill Stands. Tags identifying the name and grade of the product permanently affixed to each discharge meter and fill pipe. Porcelain tags (4"X6") carrying the same information permanently bolted to the outside of the rear compartment of fuel servicing equipment. The delivery pipes of truck fill stands are banded with colors corresponding to that used on the dispensing hose.
TURBINE ENGINE FUELS The aircraft gas turbine is designed to operate on a distillate fuel, commonly called jet fuel. Jet fuels are also composed of hydrocarbons with a little more carbon and usually a higher sulphur content than gasoline. Inhibitors may be added to reduce corrosion and oxidation. Anti-icing additives are also being blended to prevent fuel icing.
Two types of jet fuel in common use today are:
(1) Kerosene grade turbine fuel, now named Jet A;
and (2) a blend of gasoline and kerosene fractions, designated Jet B. There is a third type, called Jet A-1, which is made for operation at extremely low temperatures. See figure 4-3.
There is very little physical difference between Jet A (JP-5) fuel and commercial kerosene. Jet A was developed as a heavy kerosene having a higher flash point and lower freezing point than most kerosenes. It has a very low vapor pressure, so there is little loss of fuel from evaporation or boiloff at higher altitudes. It contains more heat energy per gallon than does Jet B (JP-4).
Jet B is similar to Jet A. It is a blend of gasoline and kerosene fractions. Most commercial turbine engines will operate on either Jet A or Jet B fuel.
However, the difference in the specific gravity of the fuels may require fuel control adjustments.
Therefore, the fuels cannot always be considered interchangeable.
Both Jet A and Jet B fuels are blends of heavy distillates and tend to absorb water. The specific gravity of jet fuels, especially kerosene, is closer to water than is aviation gasoline; thus, any water introduced into the fuel, either through refueling or condensation, will take an appreciable time to settle out. At high altitudes, where low temperatures are encountered, water droplets combine with the fuel to form a frozen substance referred to as "gel." The mass of "gel" or "icing" that may be generated from moisture held in suspension in jet fuel can be much greater than in gasoline.
Volatility One of the most important characteristics of a jet fuel is its volatility. It must, of necessity, be a compromise between several opposing factors.
A highly volatile fuel is desirable to aid in starting in cold weather and to make aerial restarts easier and surer. Low volatility is desirable to reduce 77
20 Percent of gas by volume Avgas Jet B Too rich Flammable range OF 50F 100F 150F 200F Temperature FIGURE 4-4. Vaporization of aviation fuels at atmospheric pressure.
the possibility of vapor lock and to reduce fuel losses by evaporation.
At normal temperatures, gasoline in a closed container or tank can give off so much vapor that the fuel/air mixture may be too rich to burn.
Under the same conditions, the vapor given off by Jet B fuel can be in the flammable or explosive range. Jet A fuel has such a low volatility that at normal temperatures it gives off very little vapor and does not form flammable or explosive fuel/air mixtures. Figure 4-4 shows the vaporization of aviation fuels at atmospheric pressure.
Identification Because jet fuels are not dyed, there is no onsight identification for them. They range in color from a colorless liquid to a straw-colored (amber) liquid, depending on age or the crude petroleum source.
Jet fuel numbers are type numbers and have no relation to the fuel's performance in the aircraft engine.
FUEL SYSTEM CONTAMINATION There are several forms of contamination in aviation fuel. The higher the viscosity of the fuel, the greater is its ability to hold contaminants in suspension. For this reason, jet fuels having a high viscosity are more susceptible to contamination than aviation gasoline. The principal contaminants that reduce the quality of both gasoline and turbine fuels are other petroleum products, water, rust or scale, and dirt.
Water Water can be present in the fuel in two forms:
(1) Dissolved in the fuel or (2) entrained or suspended in the fuel. Entrained water can be detected with the naked eye. The finely divided droplets reflect light and in high concentrations give the fuel a dull, hazy, or cloudy appearance.
Particles of entrained water may unite to form droplets of free water.
Fuel can be cloudy for a number of reasons.
If the fuel is cloudy and the cloud disappears at the bottom, air is present. If the cloud disappears at the top, water is present. A cloud usually indicates a water-in-fuel suspension. Free water can cause icing of the aircraft fuel system, usually in the aircraft boost-pump screens and low-pressure filters. Fuel gage readings may become erratic because the water short-circuits the aircrafts electrical fuel cell quantity probe. Large amounts of water can cause engine stoppage. If the free water is saline, it can cause corrosion of the fuel system components.
Foreign Particles Most foreign particles are found as sediment in the fuel. They are composed of almost any material with which the fuel comes into contact.
The most common types are rust, sand, aluminum and magnesium compounds, brass shavings, and rubber.
Rust is found in two forms: (1) Red rust, which is nonmagnetic and (2) black rust, which is magnetic. They appear in the fuel as red or black powder (which may resemble a dye), rouge, or grains. Sand or dust appears in the fuel in a crystalline, granular, or glasslike form.
Aluminum or magnesium compounds appear in the fuel as a form of white or gray powder or paste. This powder or paste becomes very sticky or gelatinous when water is present. Brass is found in the fuel as bright gold-colored chips or dust. Rubber appears in the fuel as fairly large irregular bits. All of these forms of contamination can cause sticking or malfunctions of fuel metering devices, flow dividers, pumps, and nozzles.
Contamination with Other Types or Grades of Fuel The unintentional mixing of petroleum products can result in fuels that give unacceptable per- 78
formance in the aircraft. An aircraft engine is designed to operate most efficiently on fuel of definite specifications. The use of fuels that differ from these specifications reduces operating efficiency and can lead to complete engine failure.
Operators of turbine-powered aircraft are sometimes forced by circumstances to mix fuels. Such mixing, however, has very definite disadvantages.
When aviation gasoline is mixed with jet fuel, the TEL in the gasoline forms deposits on the turbine blades and vanes. Continuous use of mixed fuels may cause a loss in engine efficiency. However, on a limited usage basis, they will have no detrimental effects on the engine.
Aviation gasoline containing by volume more than 0.5 percent of jet fuel may be reduced below the allowable limits in knock rating. Gasoline contaminated with turbine fuel is unsafe for use in reciprocating engines.
Microbial Growth Microbial growth is produced by various forms of micro-organisms that live and multiply in the water interfaces of jet fuels. These organisms may form a slime similar in appearance to the deposits found in stagnent water. The color of this slime growth may be red, brown, gray, or black. If not properly controlled by frequent removal of free water, the growth of these organisms can become extensive. The organisms feed on the hydrocarbons that are found in fuels, but they need free water in order to multiply.
Micro-organisms have a tendency to mat, generally appearing as a brown blanket which acts as a blotter to absorb more moisture. This mixture or mat accelerates the growth of micro-organisms.
The buildup of micro-organisms not only can interfere with fuel flow and quantity indication, but, more important, it can start electrolytic corrosive action.
Sediment Sediment appears as dust, powder, fibrous material, grains, flakes, or stain. Specks or granules of sediment indicate particles in the visible size range, i.e., approximately 40 microns or larger in size. (See figure 4-5.) The presence of any appreciable number of such particles indicates either a malfunction of the filter/separators or a source of contamination downstream of the filter/ separator, or else an improperly cleaned sample container. Even with the most efficient filter/ separators and careful fuel handling, an occasional TIME. I HOUR _ CLEAN FUEL CLEAN FUEL 5-MICRON S-MICRON
PARTICLES PARTICLES AVGAS JP-4 JP-5 FIGURE 4-5. Comparison of particle's rate of settling in three types of fuel.
visible particle will be encountered. These strays are usually due to particle migration through the filter media and may represent no particular problem to the engine or fuel control. The sediment ordinarily encountered is an extremely fine powder, rouge, or silt. The two principle components of this fine sediment are normally sand and rust.
Sediment includes both organic and inorganic matter. The presence of appreciable quantities of fibrous materials (close to naked eye visibility) is usually indicative of filter element breakdown, either because of a ruptured element or mechanical disintegration of a component in the system.
Usually, high metal content of relatively large particles suggest a mechanical failure somewhere in the system which is necessarily not limited to a metallic filter failure.
In a clean sample of fuel, sediment should not be visible except upon the most meticulous inspection. Persistent presence of sediment is suspect and requires that appropriate surveillance tests and corrective measures be applied to the fuel handling system.
Sediment or solid contamination can be separated into two categories: (1) coarse sediment and (2) fine sediment.
Course Sediment Sediment that can be seen and that easily settles out of fuel or can be removed by adequate filteration is coarse sediment. Ordinarily, particles 10 microns in size and larger are regarded as coarse sediment. (See figure 4-6.)
"HUMAN HAIR IS ABOUT I00 MICRONS
DIAMETER 40- MICRON IO-MICRON 5- MICRON
PARTICLE PARTICLE PARTICLE 25,400 MICRONS • | INCH FIGURE 4-6. Enlargement of small particles and comparison to human hair.
Coarse particles clog orifices and wedge in sliding valve clearances and shoulders, causing malfunctions and excessive wear of fuel controls and metering equipment. They are also effective in clogging nozzle screens and other fine screens throughout the aircraft fuel system.
Fine Sediment Particles smaller than 10 microns may be defined as fine sediment. (See figure 4-6.) Ninety eight percent of the fine sediment in fuel can be removed by proper settling, filtration, and centrifuging. Particles in this range accumulate throughout fuel controls, appearing as a dark shellac-like surface on sliding valves, and may also be centrifuged out in rotating chambers as sludge-like matter, causing sluggish operation of fuel metering equipment. Fine particles are not visible to the naked eye as distinct or separate particles; they will, however, scatter light and may appear as point flashes of light or a slight haze in fuel.
Maximum possible settling time should be allowed in fuel tanks after filling to allow reasonable settlement of water and sediment.
Contamination Detection Coarse contamination can be detected visually.
The major criterion for contamination detection is that the fuel be clean, bright, and contain no perceptible free water. Clean means the absence of any readily visible sediment or entrained water.
Bright refers to the shiny appearance of clean, dry fuels. Free water is indicated by a cloud, haze, or a water slug. A cloud may or may not be present when the fuel is saturated with water.
Perfectly clear fuel can contain as much as three times the volume of water considered to tolerable.
Several field methods for checking water content have been devised. One is the adding of a food color that is soluble in water, but not in fuel.
Colorless fuel samples acquire a definite tint if water is present. Another method uses a gray chemical powder that changes color to pink through purple, if 30 or more p.p.m. (parts per million) of water are present in a fuel sample.
In a third method a hypodermic needle is used to draw a fuel sample through a chemically treated filter. If the sample changes the color of the filter from yellow to blue, the fuel contains at least 30 p.p.m. of water.
Since fuel drained from tank sumps may have been cold-soaked, it should be realized that no method of water detection can be accurate while the fuel entrained water is frozen into ice crystals.
There is a good chance that water will not be drained or detected if the sumps are drained while the fuel is below 32° F. after being cooled in flight. The reason for this is that the sump drains may not be at the lowest point in the fuel tank while the airplane is in a flight attitude, and water may accumulate and freeze on other areas of the tank where it will remain undetected until it thaws.
Draining will be more effective if it is done after the fuel has been undisturbed for a period of time during which the free water can precipitate and settle to the drain point. The benefits of a settling period will be lost, however, unless the accumulated water is removed from the drains before the fuel is disturbed by internal pumps.
Contamination Control The aircraft fuel system can be considered as being divided into three parts when discussing clean fuel. The manufacturer produces clean fuel.
Contamination can occur at any time after the fuel is produced. The first part of the fuel system is the delivery and storage system between the refinery and the airport fuel service truck. Although this system is not physically a part of the aircraft, it is of equal importance in controlling contamination.
Anytime fuel is transferred it is susceptible to contamination. Therefore, all aviation maintenance personnel should be familiar with the following means of contamination control.
Fundamental in the control of contamination of turbine fuels are the methods followed by the industry in receiving and storing any bulk shipment of a petroleum product. These methods have long been established as sound, and they are too well known to need repetition here. The refueling facilities used by operators of turbine powered aircraft should incorporate the following features:
1. Fuel being pumped into airport storage should pass through a filter-separator.
should meet the requirements of U.S. Government Specification MIL-F-8508A.
2. Turbine fuels should be allowed to settle for a period of one hour per foot of depth of the fuel before being withdrawn for use. This means that ordinarily more than one storage tank must be primard provided for each grade of product.
3. Storage tanks should be checked with litmus paper after each new load of fuel is received and the fuel has settled. The litmus paper should remain submerged for a minimum of 15 seconds.
During periods of heavy rain underground tanks should be checked with litmus paper more frequently.
tank. Floating suction is the only logical way to take full advantage of gravity in removing water and particulate matter contamination. Its importance must not be minimized.
5. Fuel being withdrawn from storage should be passed through a filter-separator meeting the specification MIL-F-8508A.
6. Great care should be exercised in loading mobile fuelers to exclude airborne dust and dirt, rain or other foreign material.
7. To lessen the likelihood of rust and scale the tanks of mobile fuelers should be constructed of either stainless steel, nonferrous material or steel coated with a reliable, inert material.
8. As turbine fuel is being dispensed into the aircraft from truck or hydrant it should be filtered to a degree of 5 microns for solid particles and contain no more than 0.0015 per cent of free and entrained water. Bypass valves around the filter should not be permitted.
employed.
in use."
FUEL SYSTEM
4. Suction lines should be a minimum of 6 inches from the bottom of the tank. Kerosene storage tanks should be equipped with floating type suction lines. Floating suction does not remove the bottom product, which may not have settled sufficiently. It also prevents reintroduction into the fuel of any contamination at the bottom of the functioning.