with the electrical loads which can be imposed on the affected generator or alternator by installed equipment. When the comparison shows that the probable total connected electrical load can exceed cal hoe the output load limits of the generator (s) or alternator (s), the load should be reduced so that an overload cannot occur. When a storage battery is part of the electrical power system, ensure that the battery is continuously charged in fight, except when short, intermittent loads are connected such as a radio transmitter, a landing-gear motor, or other similar devices which may place short-time demand loads on the battery.
Controlling or Monitoring the Electrical Load Placards are recommended to inform crewmembers of an aircraft about the combination of electrical loads that can safely be connected to the power source.
In installations where the ammeter is in the battery lead, and the regulator system limits the maximum current that the generator or alternator can deliver, a voltmeter can be installed on the system bus. As long as the ammeter does not read idis.
charge" (except for short, intermittent loads such as operating the gear and flaps) and the voltmeter remains at "system voltage," the generator or alternator will not be overloaded.
In installations where the ammeter is in the generator or alternator lead, and the regulator system does not limit the maximum current that the generator or alternator can deliver, the ammeter can be redlined at 100% of the generator or alternator rating. If the ammeter reading is never allowed to exceed the red line, except for short, inermittent loads, the generator or alternator will not be overloaded.
Where the use of placards or monitoring devices is not practicable or desired, and where assurance is needed that the battery in a typical small aircraft generator/battery power source will be charged in flight, the total continuous connected electrical load may be held to approximately 80% of the total rated generator output capacity. (When more than one generator is used in parallel, the total rated output is the combined output of the installed generators.)
When two or more generators are operated in parallel and the total connected system load can exceed the rated output of one generator, means must be provided for quickly coping with the sudden overloads which can be caused by generator or engine failure. A quick load reduction system, or a specified procedure whereby the total load can be reduced to a quantity which is within the rated capacity of the remaining operable generator (s), can be employed.
Electrical loads should be connected to inverters, alternators, or similar aircraft electrical power sources in such a manner that the rated limits of the power source are not exceeded, unless some type of effective monitoring means is provided to keep the load within prescribed limits.
Circuit Protection Devices Conductors should be protected with circuit breakers or fuses located as close as possible to the electrical power source bus. Normally, the manufacturer of the electrical equipment specifies the fuse or circuit breaker to be used when installing equipment.
The circuit breaker or fuse should open the circuit before the conductor emits smoke. To accomplish this, the time current characteristic of the protection device must fall below that of the associated conductor. Circuit protector characteristics should be matched to obtain the maximum utilization of the connected equipment.
Figure 11-11 shows an example of the chart used in selecting the circuit breaker and fuse protection for copper conductors. This limited chart is applicable to a specific set of ambient temperatures and wire bundle sizes, and is presented as a typical example only. It is important to consult such guides before selecting a conductor for a specific purpose.
For example, a wire run individually in the open air may be protected by the circuit breaker of the next higher rating to that shown on the chart.
Wire AN Circuit Fuse amp.
gage copper breaker amperage 22 20 7.5 18 10 16 15 14 20 12 30 10 40 8 50 80 4 100 2 125 150 FIcURE 11-44. Wire and circuit protector chart.
All re-settable circuit breakers should open the Nominal system Type of load Derating circuit in which they are installed regardless of the voltage factor position of the operating control when an overload 24 v. d.c. Lamp 8 or circuit fault exists. Such circuit breakers are 24 v. d.c. Inductive referred to as "trip-free." Automatic re-set circuit (Relay-Solenoid) 4 breakers automatically re-set themselves. They 24 v. d.c. Resistive (Heater) 2 should not be used as circuit protection devices in 24 v. d.c. Motor 3 12 v. d.c. Lamp 5 12 v. d.c. Inductive (Relay-Solenoid) 2 A specifically designed switch should be used in 12 v. d.c. Resistive (Heater) 1 all circuits where a switch malfunction would be 12 v. d.c. Motor 2 hazardous. Such switches are of rugged construction and have sufficient contact capacity to break, FIGURE 11-45. Switch derating factors.
make, and carry continuously the connected load current. Snap-action design is generally preferred to obtain rapid opening and closing of contacts regardless of the speed of the operating toggle or plunger, thereby minimizing contact arcing.
The nominal current rating of the conventional aircraft switch is usually stamped on the switch housing. This rating represents the continuous current rating with the contacts closed. Switches should be derated from their nominal current rating for the following types of circuits:
(1) High rush-in circuits- Circuits containing incandescent lamps can draw an initial current which is 15 times greater than the continuous current. Contact burning or welding may occur when the switch is stored in solenoid coils or relays is re- circuits-Magnetic hen tho energy leased and appears as an arc when the
(3) Motors-Direct-current motors will draw several times their rated current during starting, and magnetic energy stored in their armature and field coils is released when the control switch is opened.
The chart in figure 11-45 is typical of those available for selecting the proper nominal switch rating when the continuous load current is known.
This selection is essentially a derating to obtain reasonable switch efficiency and service life.
Hazardous errors in switch operation can be avoided by logical and consistent installation. Twoposition "on-off" switches should be mounted so that the "on" position is reached by an upward or forward movement of the toggle. When the switch controls movable aircraft elements, such as landing gear or flaps, the toggle should move in the same direction as the desired motion. Inadvertent operation of a switch can be prevented by mounting a suitable guard over the switch.
Relays Relays are used as switching devices where a weight reduction can be achieved or electrical controls can be simplified. A relay is an electrically operated switch and is therefore subject to dropout under low system voltage conditions. The foregoing discussion of switch ratings is generally applicable to relay contact ratings.
AIRCRAFT LIGHTING SYSTEMS Aircraft lighting systems provide illumination for both exterior and interior use. Lights on the exterior provide illumination for such operations as landing at night, inspection of icing conditions, and safety from midair collision. Interior lighting provides illumination for instruments, cockpits, cabins, and other sections occupied by crewmembers and passengers. Certain special ights, such as indicator and warning lights, indicate the operational status of equipment.
Exterior Lights Position, anti-collision, landing, and taxi lights are common examples of aircraft exterior lights.
Some lights, such as position lights and anti-collision lights, are required for night operations. Other types of exterior lights, such as wing inspection lights, are of great benefit for specialized flying operations.
Position Lights Aircraft operating at night must be equipped with position lights that meet the minimum requirements specified by the Federal Aviation Regulations. A set of position lights consist of one red, one 459
A. Tail position light unit.
B. Wingtip position light unit.
FIGURE 11-46. Position lights.
green, and one white light. Position lights are sometimes referred to as "navigation" lights. On many aircraft each light unit contains a single lamp mounted on the surface of the aircraft (A of figure 11-16). Other types of position light units contain two lamps (B of figure 11-46), and are often streamlined into the surface of the aircraft structure The green light unit is always mounted at the extreme tip of the right wing The red unit is mounted in a similar position on the left wing The white unit is usually located on the vertical stabilizer in a position where it is clearly visible through a wide angle from the rear of the aircraft The wingtip lamps and the tail lamps are controlled by a double-pole, single-throw switch in the pilot's compartment. On "dim", the switch connects a resistor in series with the lamps Since the resistor decreases current flow, the light intensity is reduced. On "bright", the resistor is shorted out of the circuit, and the lamps glow at full brilliance On some types of installations a switch in the pilot's compartment provides for steady or flashing operation of the position lights. For flashing operation, a flasher mechanism is usually installed in the position light circuit. It consists essentially of a motor-driven camshaft on which two cams are mounted and a switching mechanism made up of two breaker arms and two contact screws. One breaker arm supplies d.c. current to the wingtip light circuit through one contact screw, and the other breaker arm supplies the tail light circuit through the other contact screw. When the motor rotates, it turns the camshaft through a set of reduction gears and causes the cams to operate the Right wing tip Н В light, green Dim. (sw. open) Clear Flasher mech 10 ^ Bright (sw. closed) tail light
• Кк
• Flash light, red
• Steady FIGURE 11-47. Position light circuitry.
28 v.d.c. main bus 10A Left wing tip light _B Navigation lights Tail light Right wing tip light B FIGURE 11-48. Single-circuit position light circuitry without flasher.
breaker which opens and closes the wing and tail light circuits alternately. Figure 11-47 is a simplified schematic diagram of a navigation light circuit which illustrates one type of position light circuitry.
The schematic diagram of another type of position light circuitry is shown in figure 11-48. Control of the position lights by a single on-off toggle switch provides only a steady illumination. There is no flasher and no dimming rheostat.
There are, of course, many variations in the position light circuits used on different aircraft. All circuits are protected by fuses or circuit breakers, and many circuits include flashing and dimming equipment. Still others are wired to energize a spesuch a location that the light will not affect the vision of the crewmember or detract from the conspicuousness of the position lights. In some cases one of the lights is mounted on the underside of the fuselage.
The simplest means of installing an anti-collision light is to secure it to a reinforced fuselage skin panel, as shown in figure 11-49.
Anti-collision light Fuselage skin cial warning light dimming relay, which causes all the cockpit warning lights to dim perceptibly when ~ Existing stringer the position lights are illuminated. Small aircraft are usually equipped with a simpli- Existing stringer fied control switch and circuitry. In some cases, one Approximately Reinforcing doubler control knob or switch is used to turn on several one inch spacing sets of lights; for example, one type utilizes a con- of %" min, dia. rivet trol knob, the first movement of which turns on the position lights and the instrument panel lights. Further rotation of the control knob increases the intensity of only the panel lights. A flasher unit is seldom included in the position light circuitry of very light aircraft, but is used in small twin-engine aircraft.
Anti-collision Lights An anti-collision light system may consist of one or more lights. They are rotating beam lights which FIcURE 11-49. Typical anti-collision light installation in are usually installed on top of the fuselage or tail in an unpressurized skin panel.
Anti-collision lights Oscillating mirrors
- Vertical stabilizer FIGURE: 11-51. Anti-collision light.
Light mounting fairing Mounting ring Vertical stabilizer FIcURE 11-50. Typical anti-collision light installation in a vertical stabilizer, An anti-collision light is often installed on top of the vertical stabilizer if the cross section of the stabilizer is large enough to accommodate the installation, and if aircraft flutter and vibration characteristics are not adversely affected. Such installations should be located near a spar, and formers should be added as required to stiffen the structure near the light. Figure 11-50 shows a typical anticollision light installation in a vertical stabilizer.
An anti-collision light unit usually consists of one or two rotating lights operated by an electric motor.
The light may be fixed, but mounted under rotating mirrors inside a protruding red glass housing. The mirrors rotate in an arc, and the resulting flash rate is between 40 and 100 cycles per minute. (See figure 11-51.) The anti-collision light is a safety light to warn other aircraft, especially in congested areas.
lined into the aircraft surface. Each light may be controlled by a relay, or it may be connected directly into the electric circuit.
Since icing of the lamp lenses reduces the illumination quality of a lamp, some installations use retractable landing lamps (figure 11-52). When the lamps are not in use, a motor retracts them into receptacles in the wing where the lenses are not exposed to the weather.
As shown in figure 11-53, one type of retractable landing light motor has a split-field winding. Two of the field winding terminals connect to the two outer terminals of the motor control switch through the points of contacts C and D, while the center terminal connects to one of two motor brushes. The brushes connect the motor and magnetic brake sole- Landing Lights Landing lights are installed in aircraft to illuminate runways during night landings. These lights are very powerful and are directed by a parabolic reflector at an angle providing a maximum range of illumination. Landing lights are usually located midway in the leading edge of each wing or stream- FICURE: 11-52. Retractable landing light.
noid into the electric circuit. The points of contact C are held open by the geared quadrant of the landing lamp mechanism. The points of contact D are held closed by the tension of the spring to the right of the contacts. This is a typical arrangement of a landing lamp circuit when the landing lamp is retracted and the control switch is in the "off" position. No current flows in the circuit, and neither the motor nor the lamp can be energized.
When the control switch is placed in the upper, or "extend," position (figure 11-53), current from the battery flows through the closed contacts of the switch, the closed contacts of contact D, the center terminal of the field winding, and the motor itself.
Current through the motor circuit energizes the brake solenoid, which withdraws the brake shoe from against the motor shaft, allowing the motor to turn and lower the lamp mechanism. After the lamp mechanism moves about 10°, contact A touches and rides along the copper bar B. In the meantime, relay F is energized, and its contacts close. This permits current to flow through the copper bar B, contact A, and the lamp. When the lamp mechanism F relay Extend Retrac Field coil B Magnetic brake Brake solenoid Mechanical linkage Brake spring is completely lowered, the projection at the top of the gear quadrant pushes the D contacts apart, opens the circuit to the motor, and causes the de-energized brake solenoid to release the brake. The brake is pushed against the motor shaft by the spring, stopping the motor and completing the lowering operation.
To retract the landing lamp, the control switch is placed in the "retract" position (figure 11-53). The motor and brake circuits are completed through the points of contact C, since these contacts are closed when the gear quadrant is lowered. This action completes the circuit, the brake releases, the motor turns (this time in the opposite direction) and the landing light mechanism is retracted. Since switching to "retract" breaks the circuit to relay F, the relay contacts open, disconnecting the copper bar and causing the landing lamp to go out. When the mechanism is completely retracted, contact points C open, and the circuit to the motor is again broken, the brake applied, and the motor stopped.
Retractable landing lights that can be extended to any position of their extension are employed on some aircraft. Landing lights used on high-speed aircraft are usually equipped with an airspeed pressure switch which prevents extension of landing lights at excessive airspeeds. Such switches also cause retraction of landing lights if the aircraft exceeds a predetermined speed.
Many large aircraft are equipped with four landing lights, two of which are fixed and two retractable. Fixed lights are usually located in either the wing root areas or just outboard of the fuselage in the leading edge of each wing. The two retractable lights are usually located in the lower outboard surface of each wing, and are normally controlled by separate switches. On some aircraft, the fixed Taxi light Inboard landing light FIcURE 11-53. Landing light mechanism and circuit. FIGURE 11-54. Fixed landing light and taxi light.
landing light is mounted in an area with a taxi light, as shown in figure 11-54.
Taxi Lights Taxi lights are designed to provide illumination on the ground while taxiing or towing the aircraft to or from a runway, taxi strip, or in the hangar area.
Taxi lights are not designed to provide the degree of illumination necessary for landing lights;
150. to 250-watt taxi lights are typical on many medium and heavy aircraft.
On aircraft with tricycle landing gear, either single or dual taxi lights are often mounted on the non-steerable part of the nose landing gear. As illustrated in figure 11-55, they are positioned at an oblique angle to the center line of the aircraft to provide illumination directly in front of the aircraft and also some illumination to the right and left of the aircraft's path. On some aircraft the dual taxi lights are supplemented by wingtip clearance lights controlled by the same circuitry.
Taxi lights are also mounted in the recessed areas of the wing leading edge, often in the same area with a fixed landing light.
Many small aircraft are not equipped with any type of taxi light, but rely on the intermittent use of a landing light to illuminate taxiing operations. Still other aircraft utilize a dimming resistor in the landing light circuit to provide reduced illumination for taxing. A typical circuit for dual taxi lights is shown in figure 11-56.
Some large aircraft are equipped with alternate taxi lights located on the lower surface of the aircraft, aft of the nose radome. These lights, operated by a separate switch from the main taxi lights, illuminate the area immediately in front of and below the aircraft nose.
Wing Inspection Lights Some aircraft are equipped with wing inspection lights to illuminate the leading edge of the wings to permit observation of icing and general condition of these areas in flight. On some aircraft, the wing inspection light system (also called wing ice lights) consists of a 100-watt light mounted flush on the outboard side of each nacelle forward of the wing.
These lights permit visual detection of ice formation on wing leading edges while flying at night. They are also often used as floodlights during ground servicing. They are usually controlled through a relay by an "on-off" toggle switch in the cockpit.
Some wing inspection light systems may include or be supplemented by additional lights, sometimes called nacelle lights, that illuminate adjacent areas such as cowl flaps or the landing gear. These are normally the same type of lights and can be controlled by the same circuits.
MAINTENANCE AND INSPECTION OF LIGHTING
SYSTEMS Inspection of an aircraft's lighting systems normally includes checking the condition and security of all visible wiring, connections, terminals, fuses, and switches. A continuity light or meter can be used in making these checks, since the cause of many troubles can often be located by systematically testing each circuit for continuity.
Switch D.C. bus FIGURE 11-55. Taxi lights mounted on non-steerable portion of nose landing gear. FIcURE 11-56. Typical taxi light circuit.
Indicator Indicator Indicat or Dry cells! lamp Dry cells lamp lamp Out Alligator Alligator Alligator clips alips clips Insulated wire disconnected Open Short Ground from terminals A. B. C. D.
FIcURE 11-57. Continuity testing with a continuity tester.
All light covers and reflectors should be kept clean and polished. Cloudy reflectors are often caused by an air leak around the lens.
The condition of the sealing compound around position light frames should be inspected regularly.
Leaks or cracks should be filled with an approved sealing compound.
Care should be exercised in installing a new bulb in a light assembly, since many bulbs fit into a socket in only one position and excessive force can cause an incomplete or open circuit in the socket area.
Circuit testing, commonly known as troubleshooting is a means of systematically locating faults in an electrical system. These faults are usually of three kinds:
(1) Open circuits in which leads or wires are broken.
(2) Shorted circuits in which grounded leads cause current to be returned by shortcuts to the source of power.
(3) Low power in circuits causing lights to burn dimly and relays to chatter. Electrical troubles may develop in the unit or in the wiring. If troubles such as these are carefully analyzed and systematic steps are taken to locate them, much time and energy not only can be saved, but damage to expensive testing equipent often can be avoided. (For a more extensive treatment of circuit testing than the summary provided here, refer to Chapter 8, Airframe and Powerplant Mechanics General Handbook, AC 05-9A.)
The equipment generally used in testing lighting circuits in an aircraft consists of a voltmeter, test light, continuity meter, and ohmmeter.
Although any standard d.c. voltmeter with flexible leads and test prods is satisfactory for testing circuits, portable voltmeters especially designed for test purposes are usually used.
The test lamp consists of a low wattage aircraft light. Two leads are used with this light.
Continuity testers vary somewhat. One type consists of a small lamp connected in series with two small batteries (flashlight batteries are very suitable) and two leads. (See A of figure 11-57.) Another type of continuity tester contains two batteries connected in series with a d.c. voltmeter and two test leads. A completed circuit will be registered by the voltmeter.
Whenever generator or battery voltage is available, the voltmeter and the test light can be used in circuit testing, since these sources of power will activate the test light and the voltmeter.
If no electrical power is available (the circuit is dead), then the continuity tester is used. The selfcontained batteries of the continuity tester force current through the circuit, causing the continuity meter to indicate when the circuit being tested is completed. When using the continuity meter, the circuit being tested should always be isolated from all other circuits by removing the fuse, by opening the switch, or by disconnecting the wires.
Figure 11-57 illustrates techniques which may be 465
used in checking circuits. The continuity tester contains a light to serve as an indicator. When the test leads are touched together, a complete circuit is established and the indicator light illuminates.
When the leads are brought into contact with a resistor or other circuit element, as shown in B of figure 11-57, and the light does not illuminate, then the circuit being tested is open.
For the open test to be conclusive, be sure the resistance of the unit tested is low enough to permit the lamp to light. In a test in which the resistance is too high, usually more than 10 ohms, connect a voltmeter in the circuit in place of the lamp. If the voltmeter pointer fails to deflect, an open circuit is indicated.
The test for shorts (C of figure 11-57) shows the continuity tester connected across the terminals of a switch in the "open" position. If the tester lamp lights, there is a short circuit in the switch.
To determine whether a length of wire is grounded at some point between its terminals, disconnect the wire at each end and hook one test clip to the wire at one end and ground the other test clip (D of figure 11-57). If the wire is grounded, the lamp will light. To locate the ground, check back at intervals toward the other end. The lighting of the lamp will indicate the section of the wire that is grounded.
The ohmmeter, although primarily designed to measure resistance, is useful for checking continuity. With an ohmmeter, the resistance of a lighting circuit can be determined directly by scale. Since an open circuit has infinite resistance, a zero reading on the ohmmeter indicates circuit continuity.
As illustrated in figure 11-58, the ohmmeter uses a battery as the source of voltage. There are fixed resistors, which are of such value that when the test prods are shorted together, the meter will read full scale. The variable resistor, in parallel with the meter, and the fixed resistors compensate for changes in voltage of the battery. The variable resistor provides for zero adjustment on the meter control panel.
On the meter there may be several scales, which are made possible by various values of resistance and battery voltage. The desired scale is selected by a selector switch on the face of the ohmmeter. Each scale reads low resistances at the upper end. Greater resistance in a circuit is indicated by less deflection of the indicator on the scale.
500 R R30 R 300 FIcURE 11-58. Typical ohmmeter internal circuitry.
When using an ohmmeter to check continuity, connect the leads across the circuit. A zero ohm reading indicates circuit continuity. For checking resistance, a scale should be chosen which will contain the resistance of the element being measured.
In general, a scale should be selected on which the reading will fall in the upper half of the scale.
Short the leads together and set the meter to read zero ohm by the zero adjustment. If a change in scales is made anytime, remember to re-adjust the meter to zero ohm.
When making circuit tests with the ohmmeter, never attempt to check continuity or measure the resistance in a circuit while it is connected to a source of voltage. Disconnect one end of an element when checking resistance, so that the ohmmeter will not read the resistance of parallel paths.
The following summary of continuity testing of lighting circuits is recommended, using either an ohmmeter or any other type of continuity tester.
(1) Check the fuse or circuit breaker. Be sure it is the correct one for the circuit being tested.
(2) Check the electrical unit (light).
(3) If fuse or circuit breaker and light are in good condition, check at the most accessible point for an open or short in the circuit.
(4) Never guess. Always locate the trouble in the positive lead of a circuit, the operating 466
24V 24V 24V 24V 24V Zero V Zero V Lamp on - normal readings Lamp out - open in fuse Open
• Open 24V 24V 24V 24V 24V Zero V • 24V 24V 24V| 24V| 24V 24 V Zero V Lamp out - open in lamp Lamp out - open in ground connection FIGURE 11-59. Continuity testing with a voltmeter.
unit, or the negative lead before removing any equipment or wires.
A voltmeter with long flexible leads provides a satisfactory, though different, method of checking the continuity of lighting system wiring in an aircraft. The voltage to be checked by the voltmeter is furnished by the storage battery in the aircraft.
The following procedure indicates the steps for continuity checking by a voltmeter in a circuit which consists of a 24-volt battery, a fuse, a switch, and a landing lamp.
(1) Draw a simple wiring diagram of the circuitry to be tested, as shown in figure
11-59.
(2) Check the fuse by touching the positive voltmeter lead to the load end of the fuse and the negative lead to ground. If the fuse is good, there will be an indication on the voltmeter. If it is burned out, it must be replaced. If it burns out again, 467 the circuit is grounded. Check for the ground at the lamp by removing the connector and replacing the fuse; if it burns out, the short is in the line. However, if the fuse does not burn out this time, the short is within the lamp.
(3) If the fuse tests good, the circuit has an open. Then with the negative clip of the voltmeter connected to ground, move the positive clip from point to point along the circuit, following the diagram as a guide.
Test each unit and length of wire. The first zero reading on the voltmeter indicates that there is an open circuit between the last point at which the voltage was normal and the point of the first zero reading. In the illustration of figure 11-59, open circuits are caused by an open fuse, an open lamp filament, and an open lamp-to-ground connection.
CHAPTER 12
AIRCRAFT INSTRUMENT SYSTEMS
GENERAL Safe, economical, and reliable operation of modern aircraft is dependent upon the use of instruments. The first aircraft instruments were fuel and oil pressure instruments to warn of engine trouble so that the aircraft could be landed before the engine failed. As aircraft that could fly over considerable distances were developed, weather became a problem. Instruments were developed that helped to fly through bad weather conditions.
Instrumentation is basically the science of meas urement. Speed, distance, altitude, attitude, direction, temperature, pressure, and r.p.m. are measured and these measurements are displayed on dials in the cockpit.
There are two ways of grouping aircraft instruments. One is according to the job they perform.
Within this grouping they can be classed as flight instruments, engine instruments, and navigation instruments. The other method of grouping aircraft instruments is according to the principle on which they work. Some operate in relation to changes in temperature or air pressure and some by fluid pressure. Others are activated by magnetism and electricity, and others depend on gyroscopic action.
The instruments that aid in controlling the inflight attitude of the aircraft are known as flight instruments. Since these instruments must provide information instantaneously, they are located on the main instrument panel within ready visual reference of the pilot. Basic flight instruments in an aircraft are the airspeed indicator, altimeter and the magnetic direction indicator. In addition, some aircraft may have a rate-of-turn indicator, a bank indicator, and an artificial horizon indicator. Flight instruments are operated by atmospheric, impact, differential, or static pressure or by a gyroscope.
Engine instruments are designed to measure the quantity and pressure of liquids (fuel and oil) and gases (manifold pressure), r.p.m., and temperature.
The engine instruments usually include a tachometer, fuel and oil pressure gages, oil temperature gage, and a fuel quantity gage. In addition some aircraft that are powered by reciprocating engines are equipped with manifold pressure gage (s), cylinder head temperature gage(s), and carburetor airtemperature gage(s). Gas turbine powered aircraft will have a turbine or tailpipe temperature gage (s), and may have an exhaust pressure ratio indica- Navigational instruments provide information that enables the pilot to guide the aircraft accurately along definite courses. This group of instruments includes a clock, compasses (magnetic compass and gyroscopic directional indicator), radios, and other instruments for presenting navigational information to the pilot.
INSTRUMENT CASES A typical instrument can be compared to a clock, in that the instrument has a mechanism, or works;
a dial, or face; pointers, or hands; and a cover glass. The instrument mechanism is protetced by a one; or two-piece case. Various materials, such as aluminum alloy, magnesium alloy, iron, steel, or plastic are used in the manufacture of instrument cases. Bakelite is the most commonly used plastic.
Cases for electrically operated instruments are made of iron or steel; these materials provide a path for stray magnetic force fields that would otherwise interfere with radio and electronic devices.
Some instrument mechanisms are housed in airtight cases, while other cases have a vent hole. The vent allows air pressure inside the instrument case to vary with the aircraft's change in altitude.
DIALS Numerals, dial markings, ard pointers of instruments are frequently coated with luminous paint.
Some instruments are coated with luminous calcium sulphide, a substance that glows for several hours atter exposure to light. Other instruments have a phosphor coating that glows only when excited by a small ultraviolet lamp in the cockpit. Some instruments are marked with a combination of radioactive 469
salts, zinc oxide, and shellac. In handling these instruments, care should be taken against radium poisoning. The effects of radium are cumulative and can appear after a long period of continued exposure to small amounts of radiation. Poisoning usually results from touching the mouth or nose after handling instrument dials or radioactive paint.
After handling either, the hands should be kept away from the mouth and nose, and washed thoroughly with hot water and soap as soon as possible.
RANGE MARKINGS Instrument range markings indicate, at a glance, whether a particular system or component is operating in a safe and desirable range of operation or in an unsafe range.
Instruments should be marked and graduated in accordance with the Aircraft Specifications or Type Certificate Data Sheets and the specific aircraft maintenance or flight manual. Instrument markings usually consist of colored decalcomanias or paint applied to the outer edges of the cover glass or over the calibrations on the dial face. The colors generally used as range markings are red, yellow, green, blue, or white. The markings are usually in the form of an arc or a radial line.
A red radial line may be used to indicate maximum and minimum ranges: operations beyond these markings are dangerous and should be avoided. A blue arc marking indicates that operation is permitted under certain conditions. A green arc indicates the normal operating range during continuous operation. Yellow is used to indicate caution.
A white index marker is placed near the bottom of all instruments that have range markings on the cover glass. The index marker is a line extending from the cover glass onto the instrument case. The marker shows if glass slippage has occurred. Glass slippage would cause the range markings to be in error.
INSTRUMENT PANELS With a few exceptions, instruments are mounted on a panel in the cockpit so that the dials are plainly visible to the pilot or copilot. Instrument panels are usually made of sheet aluminum alloy strong enough to resist flexing. The panels are nonmagnetic and are painted with a nonglare paint to eliminate glare or reflection.
In aircraft equipped with only a few instruments, only one panel is necessary; in some aircraft, additional panels are required. In such cases the forward instrument panel is usually referred to as the "main" instrument panel to distinguish it from ad ditional panels on the cockpit overhead or along the side of the flight compartment. On some aircraft the main instrument panel is also referred to as the pilot's or copilot's panel, since many of the pilot's instruments on the left side of the panel are duplicated on the right side.
The method of mounting instruments on their respective panels depends on the design of the instrument case. In one design, the bezel is flanged in such a manner that the instrument can be flushmounted in its cutout from the rear of the panel.
Integral self-locking nuts are provided at the rear faces of the flange corners to receive mounting screws from the front of the panel. The flanged type case can also be mounted from the front of the panel.
The mounting of instruments that have flangeless cases is a simpler process. The flangeless case is mounted from the front of the panel. A special expanding type of clamp, shaped and dimensioned to fit the instrument case, is secured to the rear face of the panel. As actuating screw is connected to the clamp and is accessible from the front of the panel.
The screw can be rotated to loosen the clamp, permitting the instrument to slide freely into the clamp.
After the instrument is positioned, the screw is rotated to tighten the clamp around the instrument case.
Instrument panels are usually shock-mounted to absorb low-frequency, high-amplitude shocks. Shock mounts are used in sets of two, each secured to Bolt Washer Rubber grommet Support Bonding Panel FIcURE 12-1. Section through instrument panel shock.