Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 3. Installation

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 III - INSTALLATION

8. General.-Because of wide variation in construction, dependent units, and function of aircraft instruments, there are necessarily different methods and procedures for the installation of the various types. There are, however, some instructions common to all installations which are discussed in this section. The detailed installation instructions for each instrument are presented in succeeding sections with the general discussion of that particular instrument.

9. Mounting panels and connections.-a. Instrument panels are made of 0.125-inch sheet aluminum or aluminum alloy. On panels of excessive width or depth, it is necessary to add stiffeners to provide the required strength and rigidity. All surfaces of the panel and the faces of all instruments have a dull or black satin finish which causes the luminous markings on the dials and pointers to stand out more clearly. If crash pads are used to surround the instruments the same color of finish is used.

b. Dimensions of instrument panels vary with the different sizes of airplanes. Normally all of the frontal space available is required to mount the instruments. As the size of the airplane increases, the number of instruments also increases because of the additional number of engines used. On some of the smaller airplanes the width of the fuselage in the pilot's cockpit does not permit enough frontal space on the instrument panel and it is necessary to add auxiliary panels for switches, control handles, etc., at the sides of the fuselage. Height of panels may be two-, three-, or four-tier depending upon the space available. As nearly as possible, flight instruments are located so they are at eye level with the pilot when he is sitting in a normal position in the cockpit. All other instruments are located around this group as conveniently as possible.

FIGURE 3.--Method of instrument panel shockproofing

c. All instrument panels are shockproofed. The standard method is to mount the panel on rubber shock-absorbing mounts. These absorbers are used in pairs; one is attached to a bracket fastened to the panel and the other to a bracket attached to a solid member of the fuselage as shown in figure 3. The proper size and location of each absorbing unit are determined by the airplane manufacturer when installed at the factory. In the service, periodic inspections are required of each of the units to locate any defects or breaks in the rubber cups. If a replacement is necessary, care should be taken to replace the defective one with the proper size. The load rating of each absorber is stamped on the inner ring of the cup.

d. (1) The majority of all aircraft instruments are dependent upon proper connection either to the engine or some of its accessories or to, some other independent operating unit. Since the instruments are, mounted on shockproof panels, all connecting lines leading to or away from the instruments must be provided with a length of flexible line immediately back of the instrument. Where lines made of metal tubing are used, this is accomplished in either of two ways. In most cases, a standard flexible connection is used. This consists of a 10 to 14-inch length of pressure resistant synthetic rubber tubing with suitable fittings at each end. Where the standard flexible connection cannot be used, it is necessary to place a coil in the tubing close to the instrument to provide the required flexibility.

(2) Electrically operated instruments are connected by either the common eyelet type of terminal soldered to the lead and secured on a binding post with a nut, or by special multiple line plug connectors.

(3) All instruments requiring tubing connections are provided with union nipples. These nipples have a pipe thread on one end and a straight thread on the other. The pipe thread end must always be inserted into the instrument. This standard rule is applicable to all instruments which require tubing connections. Connection of the tubing to the other end of the nipple requires a silver-soldered cone and cone union nut. Variation from this standard practice where it is desirable to use either two- or three-piece solderless connections is possible by the use of special adapters.

10. Installation instructions.-a. Standard hardware is used in all instrument installations. The tubing diameter and wall thickness and the specific items of connection or fittings are specified for each installation by Air Corps Technical Orders and installation drawings. All tubing should have a minimum number of bends. In vacuum lines particularly these bends should have radii as large as possible. Connections may be of the two-piece solderless, three-piece solderless, or silver-soldered cone types, care being taken, if the connections are threaded, that the correct thread is used in each case. Figure 4 shows a diagram of a typical instrument installation.

b. All connecting lines, tubes, leads, capillaries, etc., should be securely anchored at not less than 18-inch intervals and properly bonded. Unless otherwise directed, all replacements are duplicates of the original installation.

e. Care is taken that the connections of units sealed during manufacture such as thermometer capillaries, liquidometer capillaries, and other calibrating units are not broken, as this damages them beyond economical repair.

d. Standard aircraft power and lighting cable of either the shielded type or unshielded in aluminum conduit is used to connect all electrical instruments to their dependent units. The sizes, type of shielding, and terminals, etc., required for each installation are specified in Air Corps Technical Orders and installation drawings.

FIGURE: 4.-Vacuum-operated instruments on an airplane installation.