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

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 I  - GENERAL

1. General. a. Safe, economical, and reliable operation of modern aircraft and their power plants is absolutely dependent upon the use of instruments. Due to the inability of the human senses to cope with the many and variable climatic conditions and the complicated mechanical devices, it is essential that certain physical quantities be measured and indicated. These measured indications must be extremely accurate and readily accessible to the flight crew of the aircraft.

b. Figure 1 shows schematically the pilot's cockpit of a conventional two-engine bomber or transport airplane. There are about 105 instruments and controls for fuel, radio, landing gear, flaps. and engines. Approximately one-half of these are instruments. They must do for the airplane something of what the brain and nerves do for the human body. The crew chief is not only responsible for the proper inspection and maintenance of these instruments, but he must learn to use them for intelligent diagnosis of troubles.

2. Design characteristics.-The following general points are considered in the design of modern aircraft instruments :

a. Temperature compensation.-Aircraft instruments must operate and function satisfactorily through temperatures ranging from -35° C. to +60° C. Normal temperature is considered +15° C.

b. Vibration.-Aircraft instruments must function satisfactorily under vibration. Since this may be excessive at certain times, all instruments are mounted on shockproof panels which minimize the amplitude and frequencies of vibration to which the instrument is subjected. However, standard methods of shockproofing in use do not eliminate all vibration; consequently the instruments are designed to function accurately only when subjected to some vibration.

c. Sealing.-All aircraft instruments are sealed in either of two ways. The more rugged of the differential pressure-operated instruments have raintight seals. This kind of seal prevents dust and moisture from entering the instrument case. Instruments of this type are easily recognized by the presence of a small hole located in the bottom of the instrument case. Sensitive differential pressure gages, absolute pressure gages, and gyro actuated instruments require airtight seals. It is very important that the seal he maintained on each instrument after its installation in an airplane.

d. Position.-Mechanisms of all aircraft instruments must be either balanced or restrained in such a manner that the indication will not be affected by any degree of incline from normal position up to 180° in any direction.

e. Damping.-All aircraft instruments must operate correctly or within very close limits regardless of surging acceleration and centrifugal forces. Where necessary, suitable restrainers, restrictions, or other damping devices are used.

f. Range.-All aircraft instruments have a range suitable for the function to be measured; normally the calibrated range is from 50 to 100 percent more than that ordinarily required.

q. Luminous markings.-Numerals, dial graduations, and pointers of aircraft instruments are coated with luminous paint. This paint is a radium-treated composition which makes reading possible in the dark without the aid of artificial light. This composition is highly poisonous and its application or reapplication is accomplished only at authorized repair depots.

h. Lighting.-Some standard instruments are provided with a built-in receptacle, molded integral with the instrument case, to receive a 3-volt lamp. A reflector ring is located under the cover glass so that the light is distributed evenly around the dial surface. Intensity of the light can be varied by rheostats located on the instrument panel. To use the standard 12- to 24-volt battery-generator system of the airplane as a source of current, it is necessary to use resistors placed in series with the lamps. Provision is made in all airplanes for carrying spare lamps and resistors. For each ten instruments or fraction thereof installed on the airplane, there is one spare lamp and resistor. Some later type instruments still under development do not have the built-in individual lamp; instead, the dial markings and pointers are designed and treated for reactance to fluorescent lighting which is obtained from one or two cockpit flood lamps of this kind.

i. Instrument cases.-(1) Most instrument cases are made of molded phenolic composition, the newer ones being of two-piece construction consisting of the main body and the cover glass hold-down ring. Gaskets are used to make the seal between the main body and the cover glass. Machine screws are used to fasten the cover glass hold-down ring to the case body. Threaded brass inserts are molded integral with the instrument case for receiving pipe and tube nipples which are used to attach the instrument to its operating units. The lamp receptacle is located in the upper right-hand corner at the front of the instrument case. On the other three corners are molded lugs fitted with threaded brass inserts for receiving the mounting screws which secure the instrument on the panel. The lug inserts are all provided with self-locking devices of one kind or another.

(2) Instruments which have a total weight of more than 5 pounds use metal cases made of aluminum. Lighting and mounting features are practically the same and are comparable with those made of the phenolic composition.

FIGURE 1.- Typical control cockpit in a two-engine transport airplane.

j. Sizes.-With the exception of the gyro instruments and the automatic pilot control units, the mechanisms of which are too large, the mounting and dial diameters of all aircraft instruments are of two standard sizes, 17/8 inches and 23/1 inches. Most pressure gages, thermometers, and clocks are placed in the smaller cases while the remainder require the larger size.

k. Operation markings.-Due to the wide divergence in the operating temperatures, pressures, and speeds, it is necessary to paint operation markings of various colors on the cover glass of some of the instruments.

(1) Engine instruments installed in aircraft are marked to indicate the operating limits prescribed in the various operating instructions for airplanes and engines. Air speed indicators installed in aircraft are marked with a red line extending from the center of the dial and passing directly over the point corresponding to the maximum permissible indicated air speed. The maximum permissible indicated air speed for each type airplane is that specified in the latest Air Corps Technical Orders or Operation Instructions.

(2) Lines are painted on the cover glass as shown in figure 2. Before applying markings, care should be taken to insure that the cover glass is tight in its case. Short radial lines are used to indicate limits. Arcs of circles are used to indicate the range in which a condition governs. A short radial index line is placed on the glass and case at the bottom of the instrument to make apparent any movement of the glass. The use of finely pointed brushes for the enamel and drafting pens and compasses for the lacquer should facilitate the application of these markings. To prevent a slippage of the compass on the cover glasses, the use of masking tape at the center of arc is suggested.

(3) A certain amount of parallax error is unavoidable with this system of marking unless the instruments are viewed directly from the front.. In some of the later models of instruments now in use in service activities, provisions are made for adjustable colored markers to indicate the operating limits. The cover glasses of these instruments are not to be painted, as movable sections are provided to indicate the operating limits as required.

FIGURE 2.- Examples of operation markings on aircraft instruments