Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 21. Fuel Level Gages

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXI - FUEL LEVEL GAGES

96. Purpose and use.-The purpose of these instruments is to indicate constantly to the pilot the amount of fuel in the fuel tank or tanks. This information enables him to judge the permissible distance of flight before it is necessary to refill the tanks or move the fuel cocks to the auxiliary or reserve tanks, and to determine the fuel consumption of an engine for a given time interval at certain speeds and throttle settings.

97. Description.-a. There is a large number of different types of these instruments which have various principles of operation. The use of any given type generally depends on the type and size of the airplane on which the instrument is to be used, the particular manufacturer of the airplane, and the number and location of the fuel tanks. The instruments are usually composed of a float arrangement in the fuel tank, an indicator in the cockpit, and a means of transmitting the fuel level from the point of measurement to the point of indication. In general, the various types may be classified by the means used to accomplish this transmission, that is, liquid pressure, mechanical, and electrical.

FIGURE 48.-Sight glass fuel level gage.

b. The simplest form of fuel level gage is a graduated sight glass mounted directly on the fuel tank. A diagram of this installation is shown in figure 48 with the drain cock and the cut-off valve. Another form of the fuel level type, the hydraulically operated "liquidometer," is shown in figure 49. The instrument consists of a closed hydraulic system, including an indicator unit, a tank unit, and the connecting flexible tubing. The indicator unit is made up of two multiple-ribbed bellows connected by a linkage to which is attached the dial pointer. The tank unit consists of a similar set, of bellows connected by a fixed arm to which is attached the tank float. The movements of the float in either direction are transmitted to the set of bellows in the tank unit, causing a corresponding displacement of the fluid in the connecting tubing which in turn operates the set of bellows in the indicator unit to control the position of the pointer. Any increase in pressure due to temperature expansion of the liquid in the closed system does not affect the position of the pointer since an equal force is created on both sides of the linkage. Thus the bellows are free to expand and contract without disturbing the position of the pointer, as it is supported by a bearing which is only affected when both bellows move in the same direction as a result of the float movement.

FIGURE 49.-Diagram of operating mechanism of hydraulically operated liquidometer.

c. Three forms of fuel level gages in the second group are now in general use, the universal gage, the float and lever gage, and the direct reading float gage. The universal gasoline gage (fig. 50) is generally used in large tanks. It consists of a float moving in a well in a gasoline tank. The movement of the float is communicated by means of a cord to an indicator placed on top of the tank or at some other point having good visibility to the pilot. The float is installed in the vertical well built for it in the fuel tank. The gage is mounted either directly on top of the tank or at some other location easily visible to the pilot. In the latter cases the cord is carried inside brass tubing, connections being accomplished by use of angle adapters (which contain rollers) at the necessary bends. The whole system is sealed against pressure or gasoline leakage, the glass of the gage being sealed in its case with a cork gasket and all connection joints by lead gaskets and shellac. The scale on the dial as furnished by the manufacturer is marked only at the point "empty". Other divisions representing the full and intermediate points usually represent fractions of the total full and are located by filling the tank with fuel in the desired amounts and marking each point on the dial. The dial is reversible and by drilling special holes in it to fit the dial screws, the instrument may be set at any angle. The pointer pinion can be meshed with the transmission gearing in such a position that the pointer will indicate correctly and the glass cover prevents the pointer pinion from becoming unmeshed. The float and lever fuel level gage and the direct reading float fuel gage are shown in figures 51 and 52.

FIGURE 50.-Diagram of the universaI fuel level gage.

FIGURE 51.-Float and lever type fuel level gage.

FIGURE 52.-Direct reading float type level gage

Both depend on direct mechanical linkage for their operation. In the first the float movement is transferred through two lever arms and a worm gear to the pointer, while in the second the float is attached directly to the indicator by a fixed arm.

FIGUTE 53.- Indicator unit of the ratio meter fuel level gage

d. Of the numerous forms of electrically operated fuel level gages, several employ a D'Arsonval mechanism in the indicator. The various wiring circuits that are used in conjunction with these indicators and the specific details of any given installation may be found in pertinent technical publications or installations drawings. One of the later types of electrically operated instruments, the ratio meter, includes the dial change indicator and selector switch combination unit shown in figure 53 and the tank unit shown in figure 54. The indicator unit has a revolving drum attached to which is a separate dial for each tank, a built-in selector switch, and a moving coil type ratio meter mechanism having two coils arranged to move in a nonlinear air gap of a permanent magnet. The coils carry the pointer and the control depends on the ratio of the currents in the coils. This indicator has a pointer travel of 70° and is arranged for indirect lighting. The tank unit consists of a circular housing to which is attached a supporting member on which a float arm pivots. Contained in this housing are a resistance strip (with provisions for adjusting stroke and end position) and a movable contact arm, this arm being connected by leverage to the float arm. A metal bellows mounted on the housing prevents fuel leakage.

FIGURE 54.- 'I'ank unit of  the ratio meter fuel level gage

98. Operation.-a. The operation of most types of fuel tank gages is quite simple and can be understood from the preceding description and inspection of the figures.

b. A wiring diagram of the ratio meter type with the "potentiometer unit" is shown in figure 55. A movable contact arm rotates on the variable resistance or "potentiometer unit" in the tank unit and controls the ratio of the current flowing through the two coils in the indicator. This in turn positions the coils in the nonuniform air gap and regulates the position of the pointer on the dial. A table is shown in the above figure giving the potentiometer resistance values for three conditions of pointer indication. When the selector switch knob is turned to a certain tank position, a dial calibrated for that tank automatically comes into view. The ratio meter indicator is not greatly affected by small changes in voltage and consequently a voltage compensation is not necessary.

99. Installation.-The fuel level gage installation varies for each type of instrument as well as each type of aircraft, therefore it is necessary to consult the Air Corps Technical Orders on the airplane concerned for specific installation instructions. For general instructions, see section III.

FIGURE 55.- Wiring diagram of the ratio meter fuel level gage.

100. Maintenance.-a. Direct reading sight glass types seldom require attention other than periodical draining to remove any sediment which may accumulate at the lower end. In case of breakage of the tube, the entire unit is removed and replaced with a new one. Periodic inspections should include a check of the tube throughout its length for cracks. The connections in which the tube is seated should be inspected for tightness, decomposed rubber fittings, loose clamps, or any other defect that might result in a leak. In the case of the "liquidometer" type, adjustment of the stroke of the float may be accomplished by means of two separate adjustments as shown in figure 56. The square block on the square shaft B controls the amount of pointer travel and the pointer position is controlled by the threaded adjustment C. Both these adjustments are so sensitive that only a small move is required in either direction. When the float is at the bottom of the tank, the adjustment screw C is moved until the pointer comes to the empty mark. The float is then moved slowly to the top of tank. If the pointer comes short of the full mark, the square block is moved in toward the transmission mechanism or outward if the pointer passes the full mark. Each time the square block is moved, it will be necessary to reset the adjustment controlling the pointer position. The float should be moved slowly when making these adjustments. This should be repeated until the pointer moves from the empty to the full mark, when the float reaches the top and bottom of its swing. The jam nuts D are then locked, making sure that the screw A in the square block is tightened and the adjustment cover replaced.

FIGURE 56.-Stroke adjustment mechanism of the hydraulically operated liquidometer.

b. Gages of the second group mechanical connection often fail to function from various causes, the principal ones being

(1) Sticking or bending of the float in the float well.

(2) The float losing its buoyancy, which in the case of wood floats may be caused by the fuel soaking into the wood, and by leaks in the case of metal floats.

(3) Sticking, binding, or breaking of the mechanism connecting the float to the gage. Any of these troubles would necessitate removal of the gage from the tank in order to remedy the malfunction. Extreme care should be exercised in case of removal not to damage the gaskets at the tank openings. The cords that connect the float with the indicator sometimes break in the case of the universal type of gage. To remedy this, the indicator is replaced with one on which the dial scale is graduated for that particular tank.

c. General information on maintenance of instruments given in section II apply to the electrically operated fuel level gages, and the specific maintenance instructions for any particular type may be found in the technical publications on that instrument.