Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 32. C-1 Portable Instrument Field Test Set

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 XXXII -C-1 Portable Instrument Field Test Set

149. General.-a. Maintenance testing of aircraft instruments may be divided into two distinct classes: field tests and bench tests. Field tests refer to those performed without removal of the instrument from the aircraft. Bench tests refer to those wherein the instrument is removed from the aircraft and tested elsewhere.

b. Apparatus used for testing aircraft instruments consists of two types : C and A. Type C equipment is portable and used for line maintenance and to some extent for base functions; type A is stationary or shop equipment and is used for base maintenance. Type C-1 field test set and the application thereof are discussed in this section.

c. Only properly trained and authorized personnel are permitted to use test equipment for the reason that such equipment (as well as the instrument being tested) is delicate, expensive, and easily damaged with improper use.

150. Purpose and use.-a. Type C-1 test set is a portable, light-weight, compact unit mounted on a three-wheel chassis to permit it to be moved conveniently to a place adjacent to aircraft in the hangar or in the field. The unit (fig. 100) may be transported by aircraft or truck to a remote location for field operation; the test set should be towed only by hand.

b. The test set consists of various master indicators, gages, and devices to actuate instruments to be tested, and provides a means of performing a majority of inspections and tests without removal of instruments from the instrument panel. Thus by connecting the instruments to the various operating devices and the respective master indicators or gages, comparative readings may be taken and the accuracy of the instruments determined within suitable limits. The test set is used to perform scale error tests, friction error tests, and tests to insure correct and accurate operation of instruments during flight.

151. Description.-a. General.-The test set consists of fixed units and others which are considered auxiliary equipment. The auxiliary equipment may be removed from the test set and carried into the aircraft. The control panel of the test set is shown in figure 101. The location of the fixed units may be observed by reference to figures 102 and 103.

FIGURE 100.-Type C-1 portable instrument field test set.

b. Control panel.-The control panel serves as a mounting base for eight control knobs, three master instruments, and a master thermometer. The control knobs are numbered and marked in accordance with their respective purposes. All reference to control knobs in operating instructions herein will be made by number. The panel is installed in an inclined position for convenient operation of control knobs and reduction of parallax error when reading the master instruments and thermometer. The master instruments consist of a type C chronometric tachometer, a type D air speed indicator, and a differential pressure gage. The tachometer and the air speed indicator have standard scales and graduations. The differential pressure gage has a full scale range of 10 inches of mercury, or approximately 5 pounds per square inch. During vacuum or pressure tests, the air speed pointer will move simultaneously with that of the pressure gage as they are attached to a common connection. The direct reading master thermometer is graduated in degrees centigrade.

FIGURE 101.-Type 1-C test set control panel.

c. Power unit.-This unit consists of an a-c and a d-c motor built integrally. It is located in and attached to the floor panel of the chassis. A reel with 100 feet of extension wire is attached to the floor panel for connection to remote electrical outlet. A fuse is installed in each circuit to prevent damage to the power unit in the event of overloading. The fuses are of the plug type and are accessible through a door in the rear panel. Control knob (5) operates a ring type brush holder which holds four carbon composition brushes on the commutator. This knob controls the speed of the a-c motor. The control mechanism consists of a screw thread attachment to a cable which in turn is led through the tubing to the a-c motor. The brush adjustment to which the cable is attached embodies a joint designed to insure flexibility of movement. The speed of the d-c motor is con-trolled by knob (6) which is connected to a rheostat by means of link-age. The rheostat is located on the under side of the top panel. The power unit is connected to the bevel gear assembly of a transmission unit by means of a shaft equipped with a flexible coupling. The transmission unit is provided with take-offs for driving the air-vacuum pump and the tachometer drive. A special clutch, remotely controlled by knob (3), engages either the air-vacuum pump or the tachometer drive to the transmission, but only one of the units may be driven at one time. Before changing the position of control knob (3), it is necessary to stop the power unit.

FIGURE 102.-Type C-1 test set, rear panel removed.

FIGURE 103.-Type C-1 test set, front panel removed.

d. Air-vacuum unit.-(1) This unit consists of a standard airplane vacuum pump connected to a tank. Both the inlet and outlet ports of the pump are connected to a combination oil-air separator, through which the selector valve, operated by control knob (1), connects the pump to a vacuum-pressure supply tank. Therefore, the position of control knob (1) determines whether vacuum or pressure will be obtained, but not both at the same time. Pressure and vacuum relief valves are connected to the supply tank and are adjustable to the maximum output of the pump. A schematic diagram of the vacuum and pressure systems is shown in figure 104. Tube outlets, 1/4 inch and 3/8 inch in size, used respectively for the pressure and vacuum connections, are accessible through a door located in the rear panel of the test set. The 3/8-inch outlet (A) is used when connection is made to gyro instruments; suction is obtained by setting control knobs (1) and (2) to their correct positions. The 1/4-inch outlet (B) is used for testing air speed pitot static tubes, fuel pressure gages, manifold pressure gages, or suction gages. Pressure or vacuum may be obtained by correct setting of knobs (1) and (2). A filter is located adjacent to the supply tank in the line leading to the pressure-vacuum (1/4-inch) outlet. A needle type valve is located in the same line and affords a means of delicate adjustment of pressure or vacuum; the valve is operated by means of control knob (8).

(2) A combination, two-compartment oil supply and oil-air separator is installed between the vacuum pump and supply tank. A line connects the oil supply tank or lower compartment with the inlet port of the vacuum pump, and the outlet port of the pump is connected to the oil-air separator or top compartment of the unit. Oil is drawn into the pump as a means of lubrication and as a seal for efficient operation. A sight glass connector is inserted in the line connecting the oil supply tank to the inlet port of the pump. This sight glass serves to indicate the quantity of oil vapor entering the vacuum pump during operation. Oil is expelled from the pump through the outlet port into the separator unit. The separator unit contains a series of baffles whereby a quantity of oil is removed and returned to the oil supply compartment. The remainder of the oil and exhaust air goes into the supply tank or is bypassed overboard through a screened vent. Under normal operation when the pump is receiving proper lubrication, a slight amount of oil vapor is exhausted through the vent to the atmosphere.

FIGURE 104.-Diagram of vacuun abd pressure systems, type C-1 test set.

e. Electrical outlets.-Two standard. 110-volt outlets for two-prong plugs are mounted on the same panel with the pressure and vacuum connections. These electrical outlets provide current for miscellaneous equipment such as extension light, electric soldering iron, etc.

f. Master pressure gage unit (hydraulic).-This unit (fig. 105) consists of a cylinder equipped with a manually operated piston for obtaining desired pressure. An oil reservoir, which is located on the top of the cylinder, is connected through a control valve to the inside of the cylinder. A flexible rubber connection is also made to the control valve and is used for attachment to the gage under test. The master gage is connected to the right side of the cylinder and is scaled with unit graduations of 1 pound per square inch and has a scale range of from 0 to 300 pounds per square inch. This unit has wide application in performing tests. In addition to testing an engine oil pressure gage, it may be used for testing a fuel pressure gage, an automatic pilot oil pressure gage, and to a limited extent the landing gear pressure gage. Instructions for performing pressure gage tests are outlined in paragraph 153.

FIGURE 105.-Master pressure gage unit.

g. Master altimeter unit.-This unit (fig. 106) consists of a standard Air Corps type C sensitive altimeter mounted in a carrying case. A ratchet is installed in the panel of the case for manually vibrating the altimeter to remove any friction error which may be present. A calibration correction card showing the accurate calibration of the unit is placed in a holder inside the top cover of the carrying case. The entire assembly may be removed from the test set and carried into the aircraft.

FIGURE 106.-Master altimeter unit.

h. Thermometer tester unit.-The thermometer tester unit (fig. 107) is designed for testing a cylinder temperature indicator. It consists of a millivoltmeter and two rheostats mounted on a panel in a hard-wood carrying case. The case is designed with a compartment in the top cover for accessories which include a direct reading master mercurial thermometer, a small screw driver, and a set of leads provided with alligator terminal grips for attachment of the test unit to the circuit of the aircraft thermocouple. The millivoltmeter is scaled in millivolts and degrees centigrade. It is manually controlled with the two rheostats, one being a combination master switch with adjustment for zero pointer position, and the other for selected scale range positions. A plunger type safety switch is installed in the panel and is controlled by the opening or closing of the top cover. In the event the master switch is left in the "on" position, the current flow is automatically interrupted when the top cover is closed.

FIGURE 107.-Thermometer tester unit.

i. Electric continuity meter tester.-This unit (fig. 108) consists of a combination milliammeter, voltmeter, and ohmmeter mounted in a composition case. The instrument is equipped with receptors which are marked for scale and range values. Two toggle switches are provided to select either a-c or d-c volt, ohm, ampere, or milliampere scales. A set of leads complete with plugs and rods is furnished with the tester. The unit is used to test voltage and resistance in aircraft electric circuits.

FIGURE 108.-Electric continuity meter tester.

j. Accessories and attachments.-In addition to the specified units described herein, the accessories and attachments shown in figure 109 are used with the type C-1 test set.

FIGURE 109.-Accessories and attachments, type C-1 test set.

152. Operation.-a. The test set is equipped to operate on 110 volts alternating or direct current. Recommended practice is to use alternating current when operating at permanent or base stations. When operating in the field where a source of alternating current is not available, type C-1 electric portable power plant unit should be used to supply direct current (see Air Corps Technical Order 19-30-2 for instructions), in which case it may be necessary to increase the voltage slightly above 110 volts to compensate for voltage drop through the line and to obtain sufficient motor speed for checking tachometer.

b. Before connecting the test set to the source of power, the following operations should be performed :

(1) Set control knob (4) to the "off" position.

(2) Set control knob (7) to the appropriate position, that is, either "d-c volts" or "a-c volts," depending upon the source of supply control knob (4) can then be adjusted only to the source indicated by control knob (7).

(3) Set control valve (8) closed. This is a needle type valve and should not be tightened too firmly.

(4) Set control knob (5) or (6) (depending upon the source of supply) to "fast" position.

(5) If the vacuum-pressure system is to be used, set control knob (3) to "pump drive," otherwise set it to "tach drive." (The operator should never attempt to change the position of control knob (3) while the motor is operating.)

c. The current may be applied to the test set following the performance of the above-mentioned operations. As all instruments are designed to operate under vibration, at any time an instrument is tested without engine operation the panel should be lightly tapped to over-come friction and sticking of the pointer. Connections between the various operating units of the test set and the aircraft instruments should be made as close to the engine as possible. The tests should be conducted as outlined in paragraph 153.

153. Application.-a. Testing tachometers.-The scale error test may be performed on either the chronometric or electric tachometer. A scale error test is merely a check of simultaneous readings on the air-craft instrument against a master indicator.

(1) Chronometric tachometer (shaft-driven type).-(a) Disconnect the aircraft tachometer shaft at the engine and connect to the auxiliary flexible drive shaft which should be attached to the tachometer drive take-off in the rear of the test set as shown in figure 110. Avoid making any bends of less than 6-inch radius in the flexible, shafts.

(b) Set control knob (3) to "tech drive" and knob (5) or (6) (depending on whether alternating current or direct current is used) to "fast" position : then set control knob (7) for the type of current to be used and control knob (4) to desired position. Adjust the motor speed so that the master tachometer reads even hundreds, starting at either slow or fast speed (the slow speed of the motor should not be below 500 rpm). Progress up or down the scale and compare readings of the airplane instrument with those on the master indicator. If the readings differ by more than the amount specified in the Air Corps Technical Order, the aircraft tachometer should be replaced.

(2) Electric tachometer (generator-voltmeter type).-Disconnect the tachometer generator from the drive on the engine and connect the flexible drive shaft from the test set to the drive end of the tachometer generator (fig. 111). Avoid making a bend of less than 6-inch radius in the flexible shaft. Operate the test set and note whether the polarity of the generator is correct. If the polarity is reversed, disconnect and reverse the leads at the tachometer generator. After all connections are completed and correct polarity obtained, the test procedure is the same as outlined above for the chronometric (shaft-driven) tachometer. Make comparative readings of the master tachometer and the aircraft tachometer at each 100 rpm graduation. If the readings differ by more than the amount specified in the Air Corps Technical Order, the airplane tachometer generator and indicator should be replaced. Slight adjustments may be made by means of the "zero adjuster" on the tachometer indicator in the air-craft. Do not retard or advance the pointer by means of the adjuster more than one-half graduation.

FIGURE  110.- Connection for chronometric tachometer (shaft driven type) test.

(3) Synchronisation (tachometer generators).-(a) Disconnect the two tachometer generators from the engines and attach them to the tachometer drive adapter (two-way) as shown in figure 112. With the use of test leads, connect the tachometer generators to the disconnected terminal leads at the engine.

(b) Set control knob (3) to "tach drive" and control knob (5) or (6) (depending on whether alternating current or direct current is used) to "fast" position; then set control knob (7) for the type of current to be used. and set control knob (4) to desired position. With the speed adjusted to 1,800 rpm (adjustment made by control knob (5) and (6)) check the electric tachometers and engine synchronizer by using the switch in the aircraft. If the generators are not synchronized small adjustments may be performed, using a nonmagnetic screw driver.

FIGURE 111.-Connection for electric tachometer (generator-voltmeter type) test.

FIGURE 112.-Connection for synchronization of tachometer generators.

b. Tests requiring air pressure.-(1) The master differential pressure gage on the test set is used to indicate either suction or pressure. These tests are limited because of the pressure supply. The pressure (approximately 5 pounds per square inch maximum) supplied by the test set unit is sufficient to test aircraft instruments through cruising and critical ranges. Tests are performed on the following instruments:

(a) Fuel pressure gage.

(b) Manifold pressure gage.

(c) Air speed indicator.

(d) Air speed tube (leak test, pitot line).

FIGURE: 113.-Conncetion for suction or pressure gage test.

(2) Connect the smaller tubing to the 3/4-inch (B) outlet in the rear of the test set. Break the line connection of the instrument to be tested at the engine (fig. 113), or in case of air speed tube test use the pitot fixture as shown in figure 114.

(3) Set control knob (1) on "press" (pressure), knob (2) on "air speed test," knob (3) on "pump drive," and set control knob (8) closed. Next apply current to the motor (control knobs (4) and (7) ) and adjust the pump speed with control knob (5) or (6). Slowly open control knob (8) and check the readings of the aircraft instrument with that of the master gage.

FIGURE 114.-Connection for air speed tube test (pitot fixture)

c. Tests requiring suction (except gyroscopic instruments).-(1) These tests are also limited because of the pump supply; the tests include case leak and system tests. Tests are performed on the following instruments :

(a) Manifold pressure gage.

(b) Suction gage.

(c) Air speed tube (leak test-static line).

(d) Altimeter, rate of climb indicator, and air speed indicator.

(e) Fuel pressure gage.

(2) Connect the smaller tubing to the 1/4-inch (B) outlet in the rear of the test set. Break the line connection of the instrument to be tested at the engine (fig. 113), or in case of air speed tube test use the static fixture as shown in figure 115.

FIGURE 115.-Connection for air speed tube test (static fixture).

(3) Set control knob (1) on "vacuum," knob (2) on "air speed test," knob (3) on "pump drive," and set control knob (8) closed. Next apply current to the motor and adjust the speed with control knob (5) or (6). Slowly open control knob (8) and check the readings of the aircraft instrument with that of the master gage.

d. Testing gyroscopic instruments.-(1) Tests are performed on the bank and turn indicator, turn indicator (directional gyro), flight indicator, and gyroscopic type driftmeter, and include

(a) Dynamic.

(b) Coast.

(c) Cardinal.

(d) Starting.

(e) Alinement.

(2) Connect the larger tubing to the 3/8-inch (A) outlet in the rear of the test set. Break the vacuum line on the aircraft installation between the relief valve and the pump, if possible, and connect the larger tubing from the test set to this line (fig. 116). If, however, the aircraft is equipped with a venturi tube, connect the tubing to the aircraft with venturi seals as shown in figure 117.

FIGURE 116.-Connection for testing gyroscopic instruments.

(3) Set control knob (1) on "vacuum," knob (2) on "gyro test," and knob (3) on "pump drive." Operate the motor on the set at a speed sufficient to obtain specified suction for the instruments as indicated by the suction gage in the aircraft. The procedures for the individual tests are outlined in Air Corps Technical Orders relevant thereto.

e. Testing pressure gages (master pressure gage unit) .-Scale error tests may be performed on the fuel pressure gage, de-icer pressure gage, and oil pressure gage (engine and automatic pilot). The landing gear pressure gage may be checked for operation up to 300 pounds per square inch.

(1) Break the pressure gage line as close as possible to the engine and make connection between the tester line extension and the aircraft gage in the manner shown in figure 118.

FIGURE  117.-Connection for venturi seals.

(2) With the piston all the way in, set the control valve on the tester to "reservoir" and screw the piston all the way out in order to fill the cylinder with oil. Then set the control valve to "line" and slowly turn the pressure screw, checking readings of the instrument against the master gage at a sufficient number of successive graduations. On excessively long lines it may be necessary to fill the cylinder from the reservoir several times before a sufficient supply of oil is present in the line to build up a working pressure. After the gage has been checked throughout its range and with the control valve remaining in "line" position, turn pressure screw all the way out, then set valve to "reservoir" and turn pressure screw all the way in. If additional oil has been used, precaution should be taken when draining the oil from the line back into the reservoir to permit all of the excess oil to run out through the filler plug in the reservoir.

FIGURE 118.- Gage connection to master pressure gage unit.

f. Testing altimeters (for zero setting error) .-Remove the master altimeter (fig. 106) from the test set and take it into the aircraft wherein the altimeter to be checked is installed. Both altimeters should be at approximately the same height. Set the reference markers on both altimeters to zero and compare the readings of the pointers. The two readings should correspond within the tolerance specified by the Air Corps Technical Order. If the difference is greater, the aircraft altimeter should be corrected as follows:

(1) Set pointers on the aircraft altimeter to correspond with those of the master altimeter, making corrections from scale correction card.

(2) Sufficiently loosen (do not remove) the small screw located to the left of the setting knob to permit the screw head to clear the case, and then pull it all the way to the left.

(3) Pull out the setting knob and turn it until the reference markers read zero. Push the knob back into place, reset, and tighten the screw.

(4) If a scale error test or any of the special tests are required to be performed, it is necessary to remove the altimeter from the aircraft and place it in a vacuum chamber. The master altimeter from the test set may be used as a standard for comparison if placed in the same vacuum chamber with the altimeter which is being tested. Corrections for the master altimeter should be made from the correction card located in the top of the case cover.

g. Testing cylinder temperature indicator (scale error test).-(1) Disconnect the positive thermocouple lead from the aircraft cylinder temperature indicator. Adjust the pointer by means of the zeroadjusting screw to give an indication of 20° C. Lightly tap the indicator to prevent the pointer from sticking. As shown in figure 119, connect the positive (+ ) lead of the thermometer tester to the positive (+) terminal of the cylinder temperature indicator in the air craft, and connect the negative (-) lead of the thermometer tester to the disconnected positive (+) thermocouple lead. The tester should be kept as nearly level as possible.

FIGURE 119.-Connection for testing cylinder tempearture indicator.

(2) With the battery switch in the "off" position, set or check zero of the thermometer tester at 20° C. mark. Then turn the battery switch of the thermometer tester to the "on" position and set the selector switch at 350° C. Adjust the rheostat until tester indicates 350° C. and after this has been accomplished do not change the rheostat adjustment. Next set the selector switch to 300° C., lightly tap the indicator to remove all friction error, and compare the readings of the tester and the aircraft cylinder temperature indicator. Repeat the above for increments of 50° C. The thermometer tester and the cylinder temperature indicator should agree within the tolerances as specified in the Air Corps Technical Order.

(3) After the tester is disconnected, set the aircraft indicator pointer to correspond with the reading on the portable glass thermometer furnished with the set, then connect the positive thermocouple lead with the indicator.

h. Lead resistance test.-As the reading of the cylinder temperature indicator is affected by a change in resistance of the lends, it is very important that the resistance remains constant. The leads should be checked at any time there is doubt concerning the accurate operation of this instrument on an aircraft installation. The thermometer tester is used for this purpose.

(1) With the battery switch in the "off" position on the thermometer tester, set or check the pointer at 20° C. mark. The flexible test lead clips should be free and not touch each other.

(2) Turn the battery switch to the "on" position, set the selector switch at 350° C., and adjust the rheostat until the meter reads 350° C. Precaution should be taken not to move the rheostat during the remainder of the test. Then carefully attach the flexible lead clips, one to each of the disconnected thermocouple leads (fig. 120), and note the reading on the thermometer tester. The indication should be within the limits as specified in the Air Corps Technical Order.

i. Testing voltage drop (pitot static tube heating element).-The electrical circuit of the pitot static tube is tested to determine whether the heating element is functioning properly and whether a sufficient source of supply is available at the head. The following procedure may he used to check the source of supply:

(1) Remove the inspection plate from the airfoil or fuselage of the aircraft at the rear of the pitot static boom to obtain access to the tube unions and electrical "snap on" connector.

(2) Set the volt-ohm milliammeter test unit (fig. 108) with the right-hand toggle switch to the "V-MA" (up) position and the left-hand toggle switch to the "DCV-MA" (down) position. Insert the plug of one lead into the receptor marked "common" and the plug of the other lead into the receptor marked "15V." With the current supplied to the pitot static tube. check the heating element for operation, and check the electrical circuit of the pitot static tube with the prods of the test instrument leads (fig. 121). The voltmeter readings should be within the tolerance as specified in the Air Corps Technical Order.

FIGURE 120.-Connection for thermocouple lead resistance test.

FIGURE 121.-Connection for voltage drop test.

154. Inspection and maintenance.-In order to insure efficient operation of the test set, various parts of the set should be regularly inspected and necessary maintenance performed as follows:

a. Weekly.-The tires should be checked and a pressure of 20 pounds per square inch maintained therein.

b. Monthly.-(1) Motor.-(a) The commutator should be cleaned and polished with a clean cloth. No oil or emery cloth should ever be used on the commutator. Roughness of the commutator may be removed by application of fine grade sandpaper.

(b) When brushes are to be replaced, a sheet of fine sandpaper should be folded around the commutator and the rotor revolved by hand in the correct direction until proper brush fit is obtained. Emery cloth should never be used to clean brushes.

(c) The motor bearings should be lubricated with specified soft cup grease.

(2) Transmission unit.-The transmission should be lubricated with specified oil.

(3) Pressure-vacuum system.-(a) The lines and valves of the pressure-vacuum system should be be inspected for leaks. If valves are found which are difficult to operate, they should be lubricated with specified lubricating grease.

(b) To prevent any oil from reaching the instruments on the aircraft, the brass or bronze wool of the air filter should be re-moved and washed in gasoline to remove all trace of dirt and oil. The presence of oil vapor should be checked by placing the test set in operation for an air speed test and holding a sheet of clean white paper over the 1/4-inch (B) outlet; any oil vapor present will immediately appear on the paper.

(4) General.-All bolts, screws, electrical connections, etc., should be checked for security; miscellaneous moving parts, such as reel assembly, motor and valve controls, etc., should be lubricated with specified light grade engine oil.

c. Yearly.-The wheel bearings should be repacked with specified aircraft grease.

d. Overhaul.-When maintenance becomes necessary which is beyond the scope of the organization or station personnel, the test set should be forwarded to a repair depot for general reconditioning. Minor maintenance of the test set such as replacement of the air-vacuum pump, master instruments, etc., may be accomplished by the activity concerned.

e. Master instruments.-Care should be observed to avoid subjecting the master instruments to misuse in order that their accuracy is not impaired. Calibration cards for each instrument should be kept for ready reference in the holder provided in the top of the test set. Master instruments should be checked frequently for ac-curacy by stations or depots. When necessary, additional master instruments together with calibration data should be requisitioned from depots.

f. Tachometer drive shaft.-Care should be exercised to avoid undue friction created in the tachometer drive shaft as a result of sharp bends or undue strain on the casing.

g. Air-vacuum pump lubrication.-The oil in the combination oil-air separator should be maintained at the proper level. The sight glass should be observed to note if pump is being lubricated. Specified oils should be used respectively for ranges of temperature from -45° to 0° C. and from 0° to 45° C.

h. Records.-A complete record should he kept on suitable inspection, lubrication, and maintenance form.