Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 4. Fuel Pressure 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 IV - FUEL PRESSURE GAGES

11. Purpose and use.-The fuel pressure gage measures and indicates the difference between the air and fuel pressures at their respective inlets to the carburetor. The specific uses of this measurement are to

a. Warn the pilot of impending engine failure due to failure of the fuel pump, a broken fuel line, or any other cause which prevents the fuel from reaching the carburetors under sufficient pressure.

b. Indicate that fuel is being supplied to the carburetors steadily under the proper pressure before the take-off. This is a check on the functioning of the fuel pump, the relief valve, and to a certain extent the entire fuel system.

c. Indicate uninterrupted flow of fuel to the carburetors while switching from one fuel tank to another.

d. Indicate proper fuel pressure relief valve adjustments for the various aircraft fuel systems.

12. Description.-a. The standard fuel pressure gage (fig. 5) is sealed airtight. It has a 17/8-inch case and dial with a 3-volt built-in light. Two 11/4-inch tube. nipples are provided in the back of the case. These nipples, marked "Fuel" and "Air," serve to connect the instrument to the engine fuel system. On internally supercharged engines only the "Fuel" connection is made. The air vent remains open and therefore the air pressure in the instrument case will be atmospheric or cockpit pressure. On externally supercharged engines the air vent connects to the air pressure chamber of the supercharger. In either case, the instrument measures and indicates the difference in pressure of the fuel and air at their intakes into the carburetor.

b. The fuel pressure gages used with pressure discharge carburetors have ranges from 0 to 25 pounds per square inch, while the gages used with all other types of carburetors have ranges from 0 to 10 pounds per square inch. In both cases, the dials are graduated in increments of 1 pound per square inch. The numerals and pointer are painted with radioactive luminous material for visibility at night under normal night conditions.

c. Figure 6 shows a cross section of a pressure gage of the Bourdon tube construction. A tube is led from the point where pressure is to be measured to the connection A at the back of the gage case. The pressure is transmitted through the opening X to the inside of the Bourdon tube T. The Bourdon tube is made of spring-tempered brass or bronze tubing which has an elliptical cross section and is sealed at the outer end M. This end of the tube is free to move. The opposite end N is fastened rigidly to the instrument case and is stationary at all times.

13. Operation.-a. The Bourdon tube, due to its spring qualities, will straighten out when internal pressure is applied. This straightening movement is resisted by the air pressure on the outside surface of the tube. Thus the gage is a differential pressure measuring device and any indication it gives is due to an internal pressure in the tube greater than the pressure on the outside. The tube. will always return to its normal position when the pressure is released. The movement of the tube, which is proportional to the amount of this difference in pressures, causes a movement of the links, levers, and pinions as shown in figure 6. This principle of operation applies to all Bourdon tube mechanisms, the chief difference being in the wall thickness and stiffness of the Bourdon tube. As the range of the instrument is increased, a heavier Bourdon tube is used.

b. On engines using pressure discharge carburetors, the normal pressure reading is 13 to 15 pounds per square inch. With all other types of carburetors, including fuel injection systems, the normal pressure will be 3 to 5 pounds per square inch. The instrument is sturdy and rugged enough for the purposes intended and will stand, without seriously disturbing the calibration, overpressure equivalent to one-half its calibrated range. However, if the hand pump is used too violently, there is a possibility that excess pressures beyond the limits of the relief valve will be built up resulting in damage to the fuel pressure gage. It is therefore advisable when using the hand pump to watch the gage and keep the pressure within its range limits.

(1) A-0 to 10-pound gage. (2) Rear view.    (3) C-0 to 25-pound gage.
FIGURE 5.-Fuel pressure gage.

FIGURE 6.-Bourdon tube mechanism.

14. Installation.-See section III.

15. Maintenance.-See section II.