Float FIGURE 4-17. Float-and-lever type fuel level gage.
own level. The tube is calibrated in gallons or has a metal scale near it. The sight glass may have a shutoff valve so that the fuel can be shut off for cleaning and for preventing loss of fuel if the tube is broken.
The mechanical-type fuel quantity gage is usually located in the tank and is known as a direct reading gage. It has an indicator connected to a float resting on the surface of the fuel. As the fuel level changes, the float mechanically operates the indicator, thus showing the level of fuel in the tank. One type of mechanical fuel gage is illustrated in figure 4-17.
The electrical-type quantity gage consists of an indicator in the cockpit and a float-operated transmitter installed in the tank. As the fuel level changes, the transmitter sends an electric signal to the indicator, which shows the changing fuel level. Two important advantages of this fuel quantity gage (and the electronic type discussed in the next paragraph) are that the indicator can be located any distance from the tank and the fuel levels of several tanks can be read on one indicator.
The electronic-type (capacitance) fuel quantity gage differs from the other types in that it has no movable devices in the fuel tank. Instead of floats and their attendant mechanical units, the dielectric qualities of fuel and air furnish a measurement of fuel quantity. Essentially, the tank transmitter is a simple electric condenser. The dielectric (or nonconducting material) of the condenser is fuel and air (vapor) above the fuel.
The capacitance of the tank unit at any one time will depend on the existing proportion of fuel and vapors in the tank. The capacitance of the transmitter is compared to a reference capacitor in a rebalance-type bridge circuit. The unbalanced signal is amplified by the voltage amplifiers that drive a phase discriminating power stage. The output stage supplies power to one phase of a two-phase a.c. motor that mechanically drives a rebalancing potentiometer and indicator pointer.
The electronic type system of measuring fuel quantity is more accurate in measuring fuel level, as it measures the fuel by weight instead of in gallons. Fuel volume will vary with temperature (a gallon of gasoline weighs more when it is cold than when it is hot); thus, if it is measured in pounds instead of gallons, the measurement will be more accurate.
In addition to the cockpit fuel quantity indicating system, some aircraft are provided with a means to determine the fuel quantity in each tank when the aircraft is on the ground. This is accomplished in several different ways. Some manufacturers use float-operated, direct-reading fuel gages mounted in the lower surface of the wing. Another means is to use under-wing bayonet gages. There are two types in use, the drip gage and the sight gage.
When using the drip gage it is necessary to proceed slowly, using the trial-and-error method to find the exact fuel level. In large area tanks a proportionately large amount of fuel is represented by a fraction of an inch variation in fuel level.
The long, hollow drip tubes require some time to drain once they are filled with fuel, and a substantial error in reading will be made if the diminishing drainage drip is mistaken for the steady drip that signifies that the tube is properly po- When the cap and hollow drip tube are drawn out from the lower wing surface, the fuel enters the open top of the tube when it reaches the level of the fuel. As stated previously steady drip from a drip hole signifies that the tube is properly positioned with a tiny head of fuel above the opening. The drip gage tube may be calibrated in pounds or inches. When calibrated in inches, the reading is compared with a special chart to give a reading of fuel quantity in gallons.
The sight gage is somewhat simpler in construction than the drip gage, and offers unmistakable visual evidence when it is properly positioned for reading. As shown in figure 4-18, the sight gage is basically a long lucite rod, protected by a calibrated tube, which terminates at the top in an exposed quartz tip. When the tip is above the fuel it acts as a reflector. Light rays traveling up the lucite rod are deflected at right angles by 89
Fuel Flowmeter Quartz tip (May be either conical The fuel flowmeter is normally used only in or chisel point) multiengine aircraft. The system consists of a transmitter and an indicator. The transmitter is Lucite rod installed in the fuel inlet line to the engine, where Permanently it measures the rate of fuel flow. The transmitter mounted is electrically connected to the indicator located Protective tube housing in the cockpit. This gage shows the rate of fuel (Has embossed consumption in pounds per hour.
calibration marks) The transmitter signal may be developed by a Removable guide movable vane mounted in the fuel flow path.
housing The impact of fuel causes the vane to swing and move against the restraining force of a calibrated spring. The final position assumed by the vane represents a measure of the rate at which fuel is Latch spring passing through the flowmeter and the corresponding signal to be sent to the indicator. A vane-type fuel flowmeter system is illustrated in figure 4-19.
The transmitter used with turbine engines is the mass-flow type having a range of 500 to 2,500 Read tank quantity at this point Latching lug and pin, Lower surface of wing typical three places (Retains bayonet Sight in stowed window position) FIGURE 1-18. Under-wing sight gage.
the 45° surface at one side of the tip and deflected 90° again by the 45° surface at the opposite side and returned down the lucite rod.
Any portion of the tip submerged in fuel will not act as a reflector. Consequently, when the fuel level is part way up the taper, a light pattern is created that is visible at the lower end of the lucite rod and that has the dimension and shape described by the intersection of the tip and the fuel. When the reflected light is reduced to the smallest perceptible point in the case of conetipped gages, or hairline in the case of chisel-tipped gages, the rod is properly positioned. The fuel tank quantity can be read on the tube where it emerges from the recessed guide housing. Drip gage readings are taken at this location also.
pounds per hour. It consists of two cylinders placed in the fuel stream so that the direction of fuel flow is parallel to the axes of the cylinders.
(See figure 4-20.) The cylinders have small vanes in the outer periphery. The upstream cylinder, called the impeller, is driven at a constant angular velocity by the power supply. This velocity imparts an angular momentum to the fuel. The fuel then transmits this angular velocity to the turbine (the downstream cylinder), causing the turbine to rotate until a restraining spring force balances the force due to the angular momentum of the fuel. The deflection of the turbine positions a magnet in the second harmonic transmitter to a position corresponding to the fuel flow. The turbine position is transmitted to the flight station indicator by means of a selsyn system.
Fuel Pressure Gage The fuel pressure gage indicates the pressure of the fuel entering the carburetor. This gage may be included with the oil pressure gage and the oil temperature gage in one casing, called the engine gage unit. Most aircraft today have separate gages for these functions. An engine gage unit is shown in figure 4-21.
The fuel pressure gage is a differential pressure indicator with two connections on the back of the indicator housing. The air connection (see figure 4-22) is vented to the carburetor air inlet, and the fuel connection is attached to the fuel 90
26 V 400 - AC Transmitter Fuel flow — Restraining spring FIGURE 4-19. Vane-type fuel flowmeter system.
inlet chamber of the carburetor. In this way the gage indicates the difference between the fuel pressure entering the carburetor and the air pressure at the carburetor air inlet. In some installations, the air fitting on the gage is left open to the air pressure of the cockpit, which is generally the same as the pressure of the atmosphere. When this venting arrangement is used, the relief valve of the engine-driven fuel pump is also vented to the atmosphere, and the gage indicates the fuel pressure resulting from the adjusted spring pressure only. In order to dampen pressure pulsations that cause pointer fluctuation, a restrictor fitting (A) is installed at the carburetor end of the fuel gage line. (See the Y connection shown in figure
4-22.) The second restrictor (B) meters fuel to the oil system during oil dilution. The arrangement of these restrictors provides an indicated drop in fuel pressure when the oil dilution system is used. The oil-dilution system will be discussed Decoupling disk Fuel flow Second harmonic transmitter Impeller motor Calibrated restraining Fluid passage Fluid passage springs | 115VAC Timing contacts B 00 28VDC Indicator Power supply FIGURE 4-20. Schematic of a turbine engine fuel flow indicating system.
200 200 20 FUEL / 11 FIGURE 4-21. Engine gage unit.
thoroughly in the Powerplant Handbook, and is mentioned at this time only because the fuel pressure indicator provides a means for a check on the operation of other fuel system units.
In small aircraft the fuel pressure gage may be actuated by a Bourdon tube (an instrument that converts changes in pressure to mechanical motion), or an aneroid and bellows type, installed with a pressure line leading directly from the carburetor to the indicator. On larger aircraft, where the fuel pressure gage is located some distance from the carburetor, a transmitter is usually installed. The pressure transmitter may be a simple cast metal cell that is divided into two chambers by a flexible diaphragm. Pressure applied by the fuel source to the transmitter inlet pushes against the diaphragm and builds up an equal pressure to a thin fluid (highly refined kerosene), which transfers the pressure to the indicator mechanism. Some installations, however, use electrical transmitters to register fuel pressure on the gage. In this electrical arrangement, the pressure-indicating unit is contained in the transmitter. Fuel pressure, acting upon the aneroid and bellows portion of the unit, causes motion of one part of an electrical unit (the synchro transmitter). As the unit turns, it causes a similar movement of a corresponding unit (the synchro motor). This receiving unit actuates the indicator on the instrument panel. These pressure and electrical arrangements make it unnecessary for combustible fuel to enter the cockpit or flight deck, thereby reducing fire risk.
A fuel pressure gage often used with fuel injection systems on light aircraft engines is illustrated in figure 4-23. A gage of this type registers metered fuel pressure at the fuel injection unit distributor valve and is a direct indication of engine power output when installed in a fuel injection system for light aircraft engines. The dial of the gage is marked to indicate percent of power. The gage does not indicate either the engine-driven pump or the boost pump pressure.
Pressure Warning Signal In an aircraft with several tanks, there is always the possible danger of allowing the fuel supply in one tank to become exhausted before the selector valve is switched to another. To prevent this, pressure-warning signals are installed in some aircraft. The complete installation, shown in figure 4-22, consists of a pressure-sensitive mechanism and a warning light. The warning mechanism has both a fuel and an air connection.
The connection marked "fuel is connected to the fuel pressure line of the carburetor. The air connection is vented to either atmospheric or carburetor air inlet pressure. This arrangement prevents the warning mechanism from acting in response to changes in the absolute pressure of the fuel. If, for example, the absolute pressure of the fuel decreases because of a change in atmospheric or carburetor air inlet pressure, the change is also reflected at the warning mechanism, which then cancels the effects of the change. Normal fuel pressure against the power surface of the diaphragm holds the electrical contacts apart. When the fuel pressure drops below specifed limits, the contacts close and the warning light is turned on.
This alerts the operator to take whatever action is necessary to boost the fuel pressure.
Valve-In-Transit Indicator Lights On large multiengine aircraft, each of the fuel crossfeed and line valves may be provided with a valve-in-transit indicator light. This light is on only during the time the valve is in motion and is off when movement is complete.
Fuel Temperature Indicator A means for checking the temperature of the fuel in the tanks and at the engine is provided on some turbine-powered aircraft. During extreme cold, especially at altitude, the gage can 92
Air scoop › Warning light Electrical connections to warning light Pressure gage Warning mechanism Contacts Adjustment screw To oil Y connection Carburetor dilution Diaphragm Inlet from valve Air Fuel tuel pump Location of units in the system Pressure waming mechanism FIGURE 4-22. Fuel pressure-indicating system.
be checked to determine when fuel temperatures are approaching those at which there may be danger of ice crystals forming in the fuel.
MULTIENGINE FUEL SYSTEMS The design of the fuel system for an aucraft having two or more engines presents problems not normally encountered in single-engine fuel systems. A large number of tanks are often required to carry the necessary fuel. These tanks may be located in widely separated parts of the aircraft, such as the fuselage and the inboard and outboard sections of the wings. The individual engine fuel systems must be interconnected so that fuel can be fed from the various tanks to any engine. In case of engine failure, the fuel normally supplied to the inoperative engine must be made available to the others.
Crossfeed System The twin-engine fuel system illustrated in figure 4-24 is the simple crossfeed type. As shown, the tank selector valves are set to supply fuel from the main tanks to the engines. These valves can also be positioned to supply fuel from the auxiliary tanks. The crossfeed valve is shown in the off position. It can also be set to supply fuel from the fuselage tank to either or both engines and to crossfeed. A few of the numerous combinations in which the three valves can be set are also illustrated.
Manifold System The main feature of the four-engine system % 45 55 shown in figure 4-25 is the fuel manifold. This FUEL CRUISE POWER 65 fuel manifold system is actually a variation of the crossfeed. As shown, fuel is being supplied from PRESSURE the main tanks directly to the engines. The mani- PSE fold valves can also be set so that all tanks feed 7000' into the manifold and each engine receives its fuel 5000 supply from this line. The auxiliary fuel supply SEA 3000' can be delivered to the engines only through the LEVEL manifold. The main advantage of this system is its flexibility. Should an engine fail, its fuel is 16 immediately available to the other engines. If a tank is damaged, the corresponding engine can be supplied with fuel from the manifold.
Another advantage of this system is that all FIGURE 4-23. Fuel pressure gage for fuel-injection fuel tanks can be serviced at the same time system. through a single line manifold connection. This 93
Left engine Right engine Fuselage tank Engine-driven pump Filter Crossfeed valve Main tank Auxiliary Auxiliary Main tank tank tank Boost pump Tank selector valve Main Aux. Main Aux. Main Aux sam Aux. Main Auxiliary tanks Right main tank Fuselage tank Fuselage tank to engines to both engines to left engine to both engines FIGURE 4-24. Twin-engine crossfeed system schematic.
method of fuel servicing has greatly reduced servicing time on large aircraft because fuel can be introduced into the fueling manifold under high pressure.
FUEL JETTISON SYSTEMS A fuel jettison system is required for transport category and general aviation aircraft if the maximum take-off weight exceeds the maximum landing weight. The maximum take-off and landing weights are design specifications and may be found in the Aircraft Type Certificate data sheets.
A fuel jettison system must be able to jettison enough fuel within 10 minutes for general aviation, or 15 minutes for transport category aircraft, to meet the requirements of the specifications and Federal Air Regulations. It must be operable under the conditions encountered during all operations of the aircraft.
Design requirements are that fuel jettisoning must be stopped with a minimum of fuel for 45 minutes of cruise at maximum continuous power for reciprocating engines. Turbine powered aircraft require enough fuel for take-off and landing and 45 minutes cruising time.
The fuel jettisoning system is usually divided into two separate, independent systems, one for each wing, so that lateral stability can be maintained by jettisoning fuel from the "heavy" wing if it is necessary to do so. Normally, if an unbalanced fuel load exists, fuel will be used from the "heavy" wing by supplying fuel to engines on the opposite wing.
The system consists of lines, valves, dump chutes and chute-operating mechanisms. Each wing contains either a fixed or an extendable dump chute depending upon system design. In either case the fuel must discharge clear of the airplane.
TROUBLESHOOTING THE FUEL SYSTEM In order to become proficient at the art of troubleshooting, one must be familiar with the complete system. To do this, one can become familiar with the schematics of various portions of the system, the nomenclature of the units, and their particular function within the system by studying aircraft and engine maintenance manuals.
Engine Engine Engine Engine No. 1 No. 2 No. 3 No. 4
• Filter Auxiliary tank Engine-driven Firewall Manifold Refueling pump shut-off valve • valves receptacle Tank Fuel manifold shut-off valve No. 1 tank No. 2 tank Auxiliary tank No. 3 tank No. 4 tank Boost pump Flow to engine Flow blocked FIGURE 4-25. A typical manifold crossfeed system.
Location of Leaks and Defects The location of leaks and defects within the internal portions of the fuel system is usually a matter of observation of the pressure gage and operation of the selector valves to determine where the trouble lies. Troubleshooting of the internal fuel system can be aided by visualizing the path of flow of the fuel from the fuel tank to the fuel-metering device, noting the location of the pump(s), selector valves, emergency shutoff valves, etc.
The location of leaks or defects in the external portions of the fuel system involves very little time in comparison to locating leaks within the internal system. Usually, fuel leaks are evidenced by stains or wet spots, if they are newly developed, and by the presence of fuel odor. The plumbing, clamps, gaskets, supports, etc., are to be examined carefully at each inspection period. Any defect or leak in the internal or external fuel system is a potential hazard.
Replacement of Gaskets, Seals, and Packings In order to prevent leakage of fuel, it is of utmost importance that all gaskets, seals, and packings be properly installed. Listed below are some of the general precautions that should always be observed.
When replacing units of the fuel system, it is necessary to check each part for cleanliness, ensure that all of the old gasket material is removed, and ensure that none of the old seal remains in the groove seat. Always replace old gaskets and seals with new ones, check the new gaskets and seals for cleanliness and integrity, and ensure that it is the right part for the job. Mating surfaces should be perfectly flat so that the gasket can do the job for which it is designed. Screws, nuts, and bolts that hold units together should be evenly tightened or torqued to prevent leakage past the gasket or seal.
FUEL TANK REPAIRS 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 95
local instructions and the manufacturer's recommendations. Inerting the tank may be accomplished by slowly discharging a CO, 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 ½ 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, ¾ to ½ psi air presure is adequate. All fuel cell maintenance must be accomplished in accordance with the manufacturer's specifications.
3/4 DIA 1-1/2 DI/
A - SLOW SEEP
B - SEEP 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 cake The four classes of heavy seep, or running leak.
leaks are shown in figure 4-26. A slow seep is a leak in which the fuel wets an area around the
- AREA WHERE FUEL
APPEARS TO FLOW
OR RUN, FOLLOWING
CONTOUR OF SKIN
WHEN THIS AREA IS WIPED DRY.
C - HEAVY SEEP
D - RUNNING LEAK FIGURE 4-26. Fuel leak classification.
leak source not over ¾ of an inch in diameter.
A seep is a leak that wets an area from ¾ inches to 1½ inches in diameter. A heavy seep is a fuel leak that wets an area around the leak source from 1½ 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 safei 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 spils 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.
CHAPTER 5
FLUID LINES AND FITTINGS
GENERAL The term "aireraft plumbing" refers not only to the hose, tubing, fittings, and connectors used in the aircraft, but also to the processes of forming Occasionally it may be necessary to repair or replace damaged aircraft plumbing lines. Very often the repair can be made simply by replacing the tubing. However, if replacements are not available, the needed parts may have to be fabricated. Replacement tubing should be of the same size and material as the original line. All tubing is pressure tested prior to initial installation, and is designed to withstand several times the normal operating pressure to which it will be subjected.
If a tube bursts or cracks, it is generally the result of excessive vibration, improper installation, or damage caused by collision with an object.
All tubing failures should be carefully studied and the cause of the failure determined.
PLUMBING LINES Aircraft plumbing lines usually are made of metal tubing and fittings or of flexible hose. Metal tubing is widely used in aircraft for fuel, oil, cooloxygen, instrument, and hydraulic lines.
Flexible hose is generally used with moving parts or where the hose is subject to considerable vibra- Generally, aluminum alloy or corrosion-resistant steel tubing have replaced copper tubing: The high fatigue factor of copper tubing is the chief reason for its replacement. It becomes hard and brittle from vibration and finally breaks, however it may be restored to its soft annealed state by heating it red hot and quenching it in cold water.
Cooling in air will result in a degree of softness but not equal to that obtained with the cold water quench: This annealing process must be accomplished if copper tubing is removed for any reason.
Inspection of copper tubing for cracks, hardness, brittleness and general condition should be accomplished at regular intervals to preclude failure. The workability, resistance to corrosion, and lightweight of aluminum alloy are major factors in its adoption for aircraft plumbing.
In some special high-pressure (3,000 p.s.i.)
hydraulic installations, corrosion-resistant steel tubing, either annealed or ¼-hard, is used. Corrosionresistant steel tubing does not have to be annealed for flaring or forming; in fact, the flared section is somewhat strengthened by the cold working and strain hardening during the flaring process.
higher tensile strength permits the use of tubing with thinner walls; consequently the final installation weight is not much greater than that of the thicker-wall aluminum alloy tubing.
IDENTIFICATION OF MATERIALS Before making repairs to any aircraft plumbing, it is important to make accurate identification of plumbing materials. Aluminum alloy or steel tubing can be identified readily by sight where it is used as the basic plumbing material. However, it is difficult to determine whether a material is carbon steel or stainless steel, or whether it is 1100, 3003, 5052-0, or 2024-T aluminum alloy.
It may be necessary to test samples of the material for hardness by filing or scratching with a scriber. The magnet test is the simplest method for distinguishing between the annealed austenitic and the ferritic stainless steels. The austenitic types are nonmagnetic unless heavily cold worked, whereas the straight chromium carbon and low alloy steels are strongly magnetic. Figure 5-1 gives the methods for identifying five common Magnet Nitric acid Material test test Carbon steel- - Strongly Slow chemical magnetic. action, brown.
18-8_. Nonmagnetic. No action.
Pure nickel. Strongly Slow action, magnetic. pale green.
Monel-- Slightly Rapid action, magnetic. greenish blue.
Nickel steel__. Nonmagnetic. Rapid action, greenish bluę.
FIGURE 5-1. Identification of metallic materials.
metallic materials by using the magnet and concentrated nitric acid tests.
By comparing code markings of the replacement tubing with the original markings on the tubing being replaced, it is possible to identify definitely the material used in the original installation.
The alloy designation is stamped on the surface of large aluminum alloy tubing. On small aluminum alloy tubing, the designation may be stamped on the surface, but more often it is shown by a color code. Bands of the color code, not more than 4 inches in width, are painted at the two ends and approximately midway between the ends of some tubing. When the band consists of two colors, one-half the width is used for each color.
Painted color codes used to identify aluminum alloy tubing are:
Aluminum aloy number 1100 3003 Green 2014 Gray 2024 Red 5052 Purple 6053 Black 6061 7075 Aluminum alloy tubing, 1100 (/½-hard) or 3003 (½-hard), is used for general purpose lines of low or negligible fluid pressures, such as instrument lines and ventilating conduits. The 2024-T and 5052-0 aluminum alloy materials are used in general purpose systems of low and medium pressures, such as hydraulic and pneumatic 1,000 to 1,500
p.s.i. systems and fuel and oil lines. Occasionally, these materials are used in high-pressure (3,000
p.s.i.) systems.
Tubing made from 2024-T and 5052-0 materials will withstand a fairly high pressure before bursting. These materials are easily flared and are soft enough to be formed with handtools. They must be handled with care to prevent scratches, dents, Corrosion-resistant steel tubing, either annealed or ¼-hard, is used extensively in high-pressure hydraulic systems for the operation of landing gear, flaps, brakes, and the like. External brake lines should always be made of corrosion-resistant steel to minimize damage from rocks thrown by the tires during takeoff and landing, and from careless ground handling. Although identification markings for steel tubing differ, each usually includes the manufacturer's name or trademark, the SAE number, and the physical condition of the metal.
Metal tubing is sized by outside diameter, which is measured fractionally in sixteenths of an inch.
Thus Number 6 tubing is 16 (or ⅜ inch) and Number 8 tubing is 16 (or ½ inch), etc.
In addition to other classification or means of identification, tubing is manufactured in various wall thicknesses. Thus, it is important when installing tubing to know not only the material and outside diameter, but also the thickness of the wall.
FLEXIBLE HOSE Flexible hose is used in aircraft plumbing to connect moving parts with stationary parts in locations subject to vibration or where a great amount of flexibility is needed. It can also serve as a connector in metal tubing systems.
Synthetics Synthetic materials most commonly used in the manufacture of flexible hose are: Buna-N, Neoprene, Butyl and Teflon (trademark of DuPont Corp.). Buna-N is a synthetic rubber compound which has excellent resistance to petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester base hydraulic fluid (Skyrol ®).
Neoprene is a synthetic rubber compound which has an acetylene base. Its resistance to petroleum products is not as good as Buna-N but has better Do not use for phosphate ester base hydraulic fluid (Skydrol ®). Butyl is a synthetic rubber compound made from petroleum raw materials. It is an excellent material to use with phosphate ester based hydraulic fluid (Skydrol ® ).
Do not use with petroleum products. Teflon is the DuPont trade name for tetrafluoroethylene resin.
It has a broad operating temperature range (-65° F. to +450° F.). It is compatible with nearly every substance or agent used. It offers little resistance to flow; sticky viscous materials will not adhere to it. It has less volumetric expansion than rubber and the shelf and service life is practically limitless.
Rubber Hose Flexible rubber hose consists of a seamless synthetic rubber inner tube covered with layers of cotton braid and wire braid, and an outer layer of rubber-impregnated cotton braid. This type of hose is suitable for use in fuel, oil, coolant, and hydraulic systems. The types of hose are normally classified by the amount of pressure they are designed to withstand under normal operating conditions.
1. Low pressure, any pressure below 250 p.s.i.
Fabric braid reinforcement.
Two cotton braids - impregnated with synthetic compound MIL H-8794 Size 6-2/68 Mfg Symbol
- Yellow Single wire braid - Synthetic inner tube A. Flame- and aromatic-resistant hose White numerals, letters and stripe - RED 7 MIL-H-6000-Size- 3-4/68 to Sanbole as (Views showing opposite sides of hose) B. Nonself-sealing, Aromatic and Heat-resistant hose White T Mfg Symbol:
Red numerals and letters C. Flame-, Aromatic-, and Oil-resistant hose Yellow numerals, letters and stripe Red numerals, letters and stripe IL H-5593- D. Nonself-sealing, Aromatic-resistant hose.
E. Self-sealing, Aromatic-resistant hose FIGURE 5-2. Hose identification markings.