Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 9. Landing Gear Pressure Gage

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 IX. - LANDING GEAR PRESSURE GAGE

36. Purpose and use.- Landing gear pressure gages measure and indicate the differential pressure in the hydraulic system that is used to raise and lower the main wheels, tail wheels, flaps, and bomb doors and to operate the other hydraulic units. The hydraulic system of the automatic pilot, however, is provided with its own pressure gage.

37. Description. a. The landing gear gage (fig. 13) consists of a bakelite case containing a Bourdon tube and a sector gear with pinion which amplifies the motion of the tube and transfers it to the pointer. A vent in the bottom of the case keeps the inside at atmospheric pressure and provides a drain for any accumulated moisture.

b. The scale of this gage ranges from 0 to 2,000 pounds per square inch and is graduated in increments of 200 pounds per square inch.

The pointer, numerals, and main graduations are painted with a luminous material and under average conditions of night flying will  give sufficient luminosity to be legible. Each gage is provided with a 3-volt light, the light receptacle being molded integral with the instrument case. A 1A-inch tube nipple is provided for connecting the gage into the hydraulic system.

FIGURE 13.-Landing gear pressure gage.

38. Operation.-Pressure for the operation of the various hydraulic units on the airplane is obtained from hydraulic pumps. These are driven either by the airplane engine or by a separate electric motor. With the exception of the installations employing a pressure tank or "accumulator," the operating pressure is built up only when needed; and since the landing gear gage is connected into some part of the main hydraulic system, the pressure will register on the gage only during these periods. When used in a hydraulic system employing a pressure tank, however, the gage will register continuously. While operating pressures of hydraulic systems vary for different airplanes, they generally fall in the range of 600 to 1,200 pounds per square inch.

39. Installation.-Due to the extremely high pressures employed in hydraulic systems, heavy walled tubing is used in the connecting lines. The general information on installation of aircraft instruments given in section III should be consulted for the installation of this gage.

40. Maintenance. -See section II.