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.
separate brackets. The two mounts absorb most of the vertical and horizontal vibration, but permit the instruments to operate under conditions of minor vibration. A cross sectional view of a typical shock mount is shown in figure 12-1.
The type and number of shock mounts to be used for instrument panels are determined by the weight of the unit. The weight of the complete unit is divided by the number of suspension points. For example, an instrument panel weighing 16 Ibs.
which is supported at four points would require eight shock absorbers, each capable of supporting 4 ibs. When the panel is mounted, the weight should deflect the shock absorbers approximately /g in.
Shock-mounted instrument panels should be free to move in all directions and have sufficient clearance to avoid striking the supporting structure.
When a panel does not have adequate clearance, inspect the shock mounts for looseness, cracks, or deterioration.
REPAIR OF AIRCRAFT INSTRUMENTS The repair of aircraft instruments is highly specialized, requiring special tools and equipment. Instrument repairmen must have had specialized training or extensive on-the-job training in instrument repair. For these reasons, the repair of instruments must be performed by a properly certificated instrument repair facility. However, mechanics are responsible for the installation, connection, remova., servicing, and functional checking of the instruments.
AIRCRAFT PRESSURE GAGES Pressure gages are used to indicate the pressure at which engine oil is forced through the bearings, FIGURE 12-2. Engine gage unit.
Spring stop screw Set screw Bourdon tube Pinion Sector Socket assembly FICURE 12-3. Bourdon tube pressure gage.
oil passages, and moving parts of the engine and the pressure at which fuel is delivered to the carburetor or fuel control. They are also used to measure the pressure of air in de-icer systems and gyroscope drives, of fuel/air mixtures in the intake manifold, and of liquid or gases in several other systems.
Engine Gage Unit The engine gage unit is comprised of three separate instruments housed in a single case. A typical engine gage unit, containing gages for oil and fuel pressure and oil temperature, is shown in figure
12-2.
Two types of oil temperature gages are available for use in an engine gage unit. One type consists of an electrical resistance type oil thermometer, supplied electrical current by the aircraft d.c. power system. The other type, a capillary oil thermometer, is a vapor pressure type thermometer consisting of a bulb connected by a capillary tube to a Bourdon tube. A pointer, connected to the Bourdon tube through a multiplying mechanism, indicates on a dial the temperature of the oil.
The Bourdon tube is an aircraft instrument made of metal tubing, oval or somewhat flattened in cross section (figure 12-3). The metal tubing is closed at one end and mounted rigidly in the instrument case at its other end.
The fluid whose pressure is to be measured is introduced into the fixed end of the Bourdon tube by a small tube leading from the fluid system to the instrument. The greater the pressure of the fluid, the more the Bourdon tube tends to become straight. When the pressure is reduced or removed, the inherent springiness of the metal tube causes it to curve back to its normal shape.
If an indicator needle or pointer is attached to the free end of the Bourdon tube, its reactions to changes in the fluid pressure can be observed.
Hydraulic Pressure Gage The mechanisms used in raising and lowering the landing gear or flaps in most aircraft are operated by a hydraulic system. A pressure gage to measure the differential pressure in the hydraulic system indicates how this system is functioning. Hydraulic pressure gages are designed to indicate either the pressure of the complete system or the pressure of an individual unit in the system.
A typical hydraulic gage is shown in figure 12-4.
The case of this gage contains a Bourdon tube and a gear-and-pinion mechanism by which the Bourdon tube's motion is amplified and transferred to the pointer. The position of the pointer on the calibrated dial indicates the hydraulic pressure in p.s.l.
The pumps which supply pressure for the operation of an aircraft's hydraulic units are driven either by the aircraft's engine or by an electric motor, or both. Some installations use a pressure 1000' accumulator to maintain a reserve of fluid under pressure at all times. In such cases the pressure gage registers continuously. With other installations, operating pressure is built up only when needed, and pressure registers on the gage only during these periods.
De-icing Pressure Gage The rubber expansion boots, which de-ice the leading edges of wings and stabilizers on some aircraft, are operated by a compressed air system. The de-icing system pressure gage measures the difference between prevailing atmospheric pressure and the pressure inside the de-icing system, indicating whether there is sufficient pressure to operate the de-icer boots. The gage also provides a method of measurement when adjusting the relief-valve and the regulator of the de-icing system.
A typical de-icing pressure gage is shown in figure 12-5. The case is vented at the bottom to keep the interior at atmospheric pressure, as well as to provide a drain for any moisture which might accumulate.
10 20-
DE-ICING
PRESSURE 0200 PRESSURE FIcURE 12-5. De-icing pressure gage.
LB. SQ. IN.
The pressure-measuring mechanism of the de-icing pressure gage consists of a Bourdon tube and a sector gear, with a pinion for amplifying the motion of the tube and transferring it to the pointer. The de-icing system pressure enters the Bourdon tube FIGURE 12-4. Hydraulic pressure gage through a connection at the back of the case.
The range of the gage is typically from zero p.s.i.
to 20 p.s.i., with the scale marked in 2-p.s.i. graduations as shown in figure 12-5.
When installed and connected into an aircraft's de-icing pressure system, the gage reading always remains at zero unless the de-icing system is operating. The gage pointer will fluctuate from zero p.s.i.
to approximately 8 p.s.i. under normal conditions, because the de icer boots are periodically inflated and deflated. This normal fluctuation should not be confused with oscillation.
Diaphragm-Type Pressure Gages This type of pressure gage uses a diaphragm for measuring pressure. The pressure or suction to be measured is admitted to the pressure-sensitive diaphragm through an opening in the back of the instrument case (figure 12-6).
Diaphragm Pressure entrance Pinion gear Rocker arm Pointer Sector gear Vent J FICURE 12-6. Diaphragm-type pressure gage.
An opposing pressure, such as that of the atmosphere, is admitted through a vent in the case (figure 12-6). Since the walls of the diaphragm are very thin, an increase of pressure will cause it to expand, and a decrease in pressure will cause it to contract. Any movement of the diaphragm is transferred to the pointer by means of the rocker shaft, sector, and pinion, which are connected to the front side of the diaphragm. This gage is also a differential-pressure measuring device since it indicates the difference between the pressure applied at the vent of the case and the pressure or suction inside the diaphragm.
Suction Gages Suction gages are used on aircraft to indicate the amount of suction that actuates the air-driven gyroscopic instruments. The spinning rotors of gyroscopic instruments are kept in motion by streams of air directed against the rotor vanes. These airstreams are produced by pumping air out of the instrument cases by the vacuum pump. Atmospheric pressure then forces air into the cases through filters, and it is this air that is directed against the rotor vanes to turn them.
The suction gage indicates whether the vacuum system is working properly. The suction gage case is vented to the atmosphere or to the line of the air filter, and contains a pressure-sensitive diaphragm Plus the usual multiplying mechanism which amplifies the movement of the diaphragm and transfers it to the pointer. The reading of a suction gage indicates the difference between atmospheric pressure and the reducd pressure in the vacuum system.
Manifold Pressure Gage The manifold pressure gage is an important instrument in an aircraft powered by a reciprocating engine. The gage is designed to measure absolute pressure. This pressure is the sum of the air pressure and the added pressure created by the supercharger. The dial of the instrument is calibrated in inches of mercury (Hg).
When the engine is not running, the manifold pressure gage records the existing atmospheric pressure. When the engine is running, the reading obtained on the manifold pressure gage depends on the engine's r.p.m. The manifold pressure gage indicates the manifold pressure immediately before the cylinder intake ports.
The schematic of one type of manifold pressure gage is shown in figure 12-7. The outer shell of the gage protects and contains the mechanism. An opening at the back of the case provides for the connection to the manifold of the engine.
The gage contains an aneroid diaphragm and a linkage for transmitting the motion of the diaphragm to the pointer. The linkage is completely external to the pressure chamber, and thus is not exposed to the corrosive vapors of the manifold.
The pressure existing in the manifold enters the sealed chamber through a damping tube, which is a short length of capillary tubing at the rear of the 473
Altitude compensating aneroid Manifold pressure diaphragm Capillary assembly Frame Rocking shaft FIGURE 12-7. Manifold pressure gage.
case. This damping tube acts as a safety valve to prevent damage to the instrument by engine backfire. The sudden surge of pressure caused by backfire is considerably reduced by the restricted capillary tubing.
When installing a manifold pressure gage, care should be taken to ensure that the pointer is vertical when registering 30 in. Hg.
When an engine is not running, the manifold pressure gage reading should be the same as the local barometric pressure. It can be checked against a barometer known to be in proper operating condition. In most cases the altimeter in the aircraft can be used, since it is a barometric instrument. With the aircraft on the ground, the altimeter hands should be set to zero and the instrument panel tapped lightly a few times to remove any possible frictional errors. The barometer scale on the altimeter face will indicate local atmospheric pressure when the altimeter hands are at zero. The manifold pressure gage should agree with this pressure reading. If it does not, the gage should be replaced with a gage that is operating properly.
If the pointer fails to respond entirely, the mechanism is, in all probability, defective. The gage should be removed and replaced. If the pointer responds but indicates incorrectly, there may be moisture in the system, obstruction in the lines, a leak in the system, or a defective mechanism.
When doubt exists about which of these items is the cause of the malfunction, the engine should be operated at idle speed and the drain valve (usually located near the gage) opened for a few minutes.
This will usually clear the system of moisture. To clear an obstruction, the lines may be disconnected and blown clear with compressed air. The gage mechanism may be checked for leaks by disconnecting the line at the engine end and applying air pressure until the gage indicates 50 in. Hg. Then the line should be quickly closed. A leak is present if the gage pointer returns to atmospheric pressure.
If a leak is evident but cannot be found, the gage should be replaced.
PITOT-STATIC SYSTEM Three of the most important flight instruments are connected into a pitot-static system. These instruments are the airspeed indicator, the altimeter, and the rate-of-climb indicator. Figure 12-8 shows these three instruments connected to a pitot-static tube head.
Impact (pitot) pressure Rate of climb Airspeed Altimeter FIGURE 12-8. Pitot-static system.
The pitot-static system head, or pitot-static tube as it is sometimes called, consists of two sections. As shown in figure 12-9, the forward section is open at the front end to receive the full force of the impact air pressure. At the back of this section is a baffle plate to protect the pitot tube from moisture and dirt that might otherwise be blown into it. Moisture can escape through a small drain hole at the bottom of the forward section.
The pitot, or pressure, tube leads back to a chamber in the "shark-fin" projection near the rear of the assembly. A riser, or upright tube, leads the air from this chamber through tubing to the airspeed indicator.
Pressure chamber Static tube Pitot tube riser riser Pitot tabe Static hole . Drain hole Baffle plate Drain Hote Static hole . Static Electrical chamber contact Trater Heater FIGURE 12-9. Pitot-static system head.
The rear, or static, section of the pitot-static tube head is pierced by small openings on the top and bottom surfaces. These openings are designed and located so that this part of the system will provide accurate measurements of atmospheric pressure in a static, or still, condition. The static section contains a riser tube which is connected to the airspeed indicator, the altimeter, and the rate-of-climb indicator.
Many pitot-static tubes are provided with heating elements to prevent icing during flight (figure 12-9). During ice-forming conditions, the electrical heating elements can be turned on by means of a switch in the cockpit. The electrical circuit for the heater element may be connected through the ignition switch. Thus, in case the heater switch is inadvertently left in the "on" position, there will be no drain on the battery when the engine is not operating.
The pitot-static tube head is mounted on the outside of the aircraft at a point where the air is least likely to be turbulent. It is pointed in a forward direction parallel to the aircraft's line of flight. One general type of tube head is designed for mounting on a streamlined mast extending below the nose of the aircraft fuselage. Another type is designed for installation on a boom extending forward of the leading edge of the wing. Both types are shown in figure 12-10. Although there is a slight difference in their construction, they operate identically.
Most pitot-static tubes are manufactured with a union connection in both lines from the head, near Wing leading edge mounting Fuselage nose mounting FIcURE 12-10. Pitot-static tube heads.
In many aircraft equipped with a pitot-static tube, an alternate source of static pressure is provided for emergency use. A schematic diagram of a typical system is shown in figure 12-11. As shown in the diagram, the alternate source of static pressure may be vented to the interior of the aircraft.
Another type of pitot-static system provides for the location of the pitot and static sources at sepa- Pitot-static tube Pitot line Static linethe point at which the tube head is attached to the mounting boom or mast (figure 12-10). These connections simplify removal and replacement, and Alternate static source are usually reached through an inspection door in (cockpit air) the wing or fuselage. When a pitot-static tube head is to be removed, these connections should be disconnected before any mounting screws and lock- FICURE 12-11. Pitot-static system with alternate source washers are removed. of static pressure.
Sources of Alt. V/S A/S static pressure Pitot line (Schematic diagram of pitot head) Static vent Static line Heating line Pitot-pressure chamber Drain opening Pitot-heat element FIcURE 12-12. Pitot-static system with separate sources of pressure.
rate positions on the aircraft. This type of system is illustrated in figure 12-12.
Impact pressure is taken from the pitot-head (figure 12-12) which is mounted parallel to the longitudinal axis of the aircraft and generally in line with the relative wind. The leading edge of the wing, nose section, or vertical stabilizer are the usual mounting positions, since at those points there is usually a minimum disturbance of air due to motion of the aircraft.
Static pressure in this type of pitot-static system is taken from the static line attached to a vent or vents mounted flush with the fuselage or nose section. On aircraft using a flush-mounted static source, there may be two vents, one on each side of the aircraft. This compensates for any possible variation in static pressure on the vents due to erratic changes in aircraft attitude. The two vents are usually connected by a Y-type fitting. In this type of system, clogging of the pitot opening by ice or dirt (or failure to remove the pitot cover) affects the airspeed indicator only.
A pitot-static system used on a pressurized, mülti-engine aircraft is shown in figure 12-13. Three additional units, the cabin pressure controller, the cabin differential pressure gage, and the autopilot system are integrated into the static system. Both heated and unheated flushmounted static ports are used.
Altimeters There are many kinds of altimeters in general use today. However, they are all constructed on the same basic principle as an aneroid barometer. They all have pressure-responsive elements (aneroids) which expand or contract with the pressure change of different flight levels. The heart of an altimeter is its aneroid mechanism (figure 12-14). The expansion or contraction of the aneroid with pressure changes actuates the linkage, and the indicating hands show altitude. Around the aneroid mechanism of most altimeters is a device called the bimetal yoke. As the name implies, this device is composed of two metals and performs the function of compensating for the effect that temperature has on the metals of the aneroid mechanism.
The presentation of altitude by altimeters in current use varies from the multi-pointer type to the drum and single pointer, and the digital counter and single pointer types.
The dial face of the typical altimeter is graduated with numerals from zero to 9 inclusive, as shown in figure 12-15. Movement of the aneroid element is transmitted through a gear train to the three hands on the instrument face. These hands sweep the calibrated dial to indicate the altitude of the aircraft.
The shortest hand indicates altitude in tens of thousands of feet; the intermediate hand, in thousands of feet; and the longest hand, in hundreds of feet in 20-ft. increments. A barometric scale, located at the right of the instrument face, can be set by a knob located at the lower left of the instrument case. The barometric scale indicates barometric pressure in inches of mercury.
Since atmospheric pressure continually changes, the barometric scale must be re-set to the local station altimeter setting before the altimeter will indicate the correct altitude of the aircraft above 476
Pilot's instrument Copilot's instrument pitot tube Flush-mounted pitot tube ' Flush-mounted unheated unheated static tube Static vent -Static vent drain static tube drain valve in valve in nose nose gear well gear well Copilot's equalizer manifold Pilot's equali manifold Sumps and drains Flush-mounted Flush-mounted heated static tubes heated static tubes Auto- Pilot's flight instrument panel (2) instrument panel Copilot's flight pilot static drain valve Static manifold Static drain tee
- Static manifold
• Static selector Autopilot Static valve amplifier Cabin pressure control panel valve selector
1. Altimeter indicator Airspeed indicator Static system Drain valves 3. 4. Rate-of-climb indicator Cabin pressure controller mum Pitot pressure system
5. Cabin differential pressure gage To alternate static source FicURE 12-13. Schematic of typical pitot-static system on pressurized multi-engine aircraft.
sea level. When the setting knob is turned, the barometric scale, the hands, and the aneroid element move to align the instrument mechanism with the new altimeter setting.
Two setting marks, inner and outer, indicate barometric pressure in feet of altitude. They operate in conjunction with the barometric scale, and indications are read on the altimeter dial. The outer mark indicates hundreds of feet, and the inner mark thousands of feet. Since there is a limit to the graduations which can be placed on the barometric scale, the setting marks are used when the barometric pressure to be read is outside the limits of the scale.
In addition to the errors in the altimeter mechanism, another error called installation error affects Barometric the accuracy of indications. The error is caused by dial the change of alignment of the static pressure port with the relative wind. The change of alignment is caused by changes in the speed of the aircraft and in the angle of attack, or by the location of the Handstatts static port in a disturbed pressure field. Improper Balance / 100-ft. scale • Adjustment knob ass y- 1,000 ft. scale Out: Sets barometrical dial installation or damage to the pitot-static tube will 10,000-ft. scale In: Sets dial hands also result in improper indications of altitude.
FIGURE 12-14. Mechanism of a sensitive altimeter. Rate-of-Climb Indicators The rate-of-climb, or vertical velocity, indicator (figure 12-16) is a sensitive differential pressure gage that indicates the rate at which an aircraft is climbing or descending. The rate-of-climb indicator is connected to the static system and senses the 100 FEET rate of change of static pressure.
9 1000 FEET indicator dial, is positive in a climb and negative The rate of altitude change, as shown on the 10 000 FEET 8 29€ 4 Restricted passage FICURE 12-15. Sensitive altimeter.
Altimeter Errors rors. A common one is that the scale is not correctly Altimeters are subject to various mechanical er- 2 oriented to standard pressure conditions. Altimeters 4 should be checked periodically for scale errors in IMB altitude chambers where standard conditions exist.
Another mechanical error is the hysteresis error.
This error is induced by the aircraft maintaining a DOWN given altitude for an extended period of time, then suddenly making a large altitude change. The resulting lag or drift in the altimeter is caused by the elastic properties of the materials which comprise the instrument. This error will eliminate itself with slow climbs and descents or after maintaining a new altitude for a reasonable period of time. FIcURE 12-16. Typical rate-of-climb indicator.