Basic Aircraft Instruments PDF

BASIC AIRCRAFT INSTRUMENTS Pitot/Static Systems Aircraft that are flown in instrument Pitot pressure, or impact air pressure, is sensed meteorological conditions (IMC) are equipped through an open-end tube pointed directly into with instruments that provide attitude a...

BASIC AIRCRAFT INSTRUMENTS Pitot/Static Systems Aircraft that are flown in instrument Pitot pressure, or impact air pressure, is sensed meteorological conditions (IMC) are equipped through an open-end tube pointed directly into with instruments that provide attitude and the relative wind flowing around the aircraft. direction reference, as well as navigation The pitot tube connects to pressure operated instruments that allow precision flight from flight instruments such as the ASI. takeoff to landing with limited or no outside visual reference. The instruments discussed in Static Pressure this chapter are those required by Title 14 of Other instruments depend upon accurate the Code of Federal Regulations (14 CFR) part sampling of the ambient still air atmospheric 91, and are organized into three groups: pitot- pressure to determine the height and speed of static instruments, compass systems, and movement of the aircraft through the air, both gyroscopic instruments. horizontally and vertically. This pressure, called static pressure, is sampled at one or more The chapter concludes with a discussion of how locations outside the aircraft. The pressure of to preflight these systems for IFR flight. This the static air is sensed at a flush port where the chapter addresses additional avionics systems air is not disturbed. On some aircraft, air is such as Electronic Flight Information Systems sampled by static ports on the side of the (EFIS), Ground Proximity Warning System electrically heated pitot-static head. [Figure 3- (GPWS), Terrain Awareness and Warning System 1] Other aircraft pick up the static (TAWS), Traffic Alert and Collision Avoidance pressure through flush ports on the side of the System (TCAS), Head Up Display (HUD), etc., fuselage or the vertical fin. that are increasingly being incorporated into general aviation aircraft. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 1  BASIC AIRCRAFT INSTRUMENTS These ports are in locations proven by flight tests to be in undisturbed air, and they are normally paired, one on either side of the aircraft. This dual location prevents lateral movement of the aircraft from giving erroneous static pressure indications. The areas around the static ports may be heated with electric heater elements to prevent ice forming over the port and blocking the entry of the static air. Three basic pressure-operated instruments are found in most aircraft instrument panels. These are the sensitive altimeter, ASI, and vertical speed indicator (VSI). All three receive pressures sensed by the aircraft pitot-static system. The static ports supply pressure to the ASI, Figure 3-1. A Typical Electrically altimeter, and VSI. Heated Pitot-Static Head Blockage Considerations The pitot tube is particularly sensitive to blockage especially by icing. Even light icing can block the entry hole of the pitot tube where ram air enters the system. This affects the ASI and is the reason most airplanes are equipped with a pitot heating system. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 2  BASIC AIRCRAFT INSTRUMENTS Indications of Pitot Tube Blockage If the pitot tube becomes blocked, the ASI At the altitude where the blockage occurs, displays inaccurate speeds. At the altitude airspeed indications would be normal. where the pitot tube becomes blocked, the ASI At altitudes above which the static ports remains at the existing airspeed and doesn’t became blocked, the ASI displays a lower-than- reflect actual changes in speed. actual airspeed continually decreasing as altitude At altitudes above where the pitot tube is increased. became blocked, the ASI displays a higher-than- At lower altitudes, the ASI displays a higher- actual airspeed increasing steadily as altitude than-actual airspeed increasing steadily as increases. altitude decreases. At lower altitudes, the ASI displays a lower- than-actual airspeed decreasing steadily as altitude decreases. Indications from Static Port Blockage Many aircraft also have a heating system to protect the static ports to ensure the entire pitot-static system is clear of ice. If the static ports become blocked, the ASI would still function but could produce inaccurate indications. Figure 3-2. A Typical Pitot-Static System. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 3  BASIC AIRCRAFT INSTRUMENTS The trapped pressure in the static system particularly at a high angle of attack with the causes the altimeter to remain at the altitude landing gear and flaps down, the air around the where the blockage occurred. The VSI remains static port may be disturbed to the extent that at zero. On some aircraft, an alternate static air it can cause an error in the indication of the source valve is used for emergencies. [Figure 3- altimeter and ASI. Because of the importance of 2] If the alternate source is vented inside the accuracy in these instruments, part of the airplane, where static pressure is usually lower certification tests for an aircraft is a check of than outside static pressure, selection of the position error in the static system. The alternate source may result in the following POH/AFM contains any corrections that must erroneous instrument indications: be applied to the airspeed for the various configurations of flaps and landing gear. 1. Altimeter reads higher than normal, 2. Indicated airspeed (IAS) reads greater than Pitot/Static Instruments normal, and Sensitive Altimeter 3. VSI momentarily shows a climb. Consult A sensitive altimeter is an aneroid barometer the Pilot’s Operating Handbook/Airplane that measures the absolute pressure of the Flight Manual (POH/ AFM) to determine the ambient air and displays it in terms of feet or amount of error. meters above a selected pressure level. Effects of Flight Conditions Principle of Operation The sensitive element in a sensitive altimeter is The static ports are located in a position where a stack of evacuated, corrugated bronze aneroid the air at their surface is as undisturbed as capsules. [Figure 3-3] The air pressure acting on possible. But under some flight conditions, these aneroids tries to compress them against their natural springiness, MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 4  BASIC AIRCRAFT INSTRUMENTS which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument. Below 10,000 feet, a striped segment is visible. Above this altitude, a mask begins to cover it, and above 15,000 feet, all of the stripes are covered. [Figure 3-4] Another configuration of the altimeter is the drum-type. [Figure 3-5] These instruments have only one pointer that makes one revolution for every 1,000 feet. Each number represents 100 feet and each mark represents 20 feet. A drum, marked in thousands of feet, is geared to the mechanism that drives the pointer. To read this type of altimeter, first look at the drum to get the thousands of feet, and then at the pointer Figure 3-3. Sensitive Altimeter to get the feet and hundreds of feet. Components. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 5  BASIC AIRCRAFT INSTRUMENTS A sensitive altimeter is one with an adjustable Altimeter Errors barometric scale allowing the pilot to set the A sensitive altimeter is designed to indicate standard reference pressure from which the altitude is changes from standard conditions, but most flying measured. This scale is visible in a small involves errors caused by nonstandard conditions window called the Kollsman window. A knob and the pilot must be able to modify the indications on the instrument adjusts the scale. The range to correct for these errors. There are two types of of the scale is from 28.00" to 31.00" inches of errors: mechanical and inherent. mercury (Hg), or 948 to 1,050 millibars. Mechanical Rotating the knob changes both the barometric A preflight check to determine the condition of an scale and the altimeter pointers in such a way altimeter consists of setting the barometric scale to that a change in the barometric scale of 1" Hg the local altimeter setting. The altimeter should changes the pointer indication by 1,000 feet. indicate the surveyed elevation of the airport. If This is the standard pressure lapse rate below the indication is off by more than 75 feet from the 5,000 feet. When the barometric scale is surveyed elevation, the instrument should be adjusted to 29.92" Hg or 1,013.2 millibars, the referred to a certificated instrument repair station pointers indicate the pressure altitude. The for recalibration. Differences between ambient pilot displays indicate altitude by adjusting the temperature and/or pressure causes an erroneous barometric scale to the local altimeter setting. indication on the altimeter. The altimeter then indicates the height above the existing sea level pressure. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 6  BASIC AIRCRAFT INSTRUMENTS Figure 3-4. ThrFeeig-Puorein 3te-r4.A Figure 3-5. Drum-Type Altimeter. Tlthimreeet-epro. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 7  BASIC AIRCRAFT INSTRUMENTS Inherent Altimeter Error When the aircraft is flying in air that is warmer Temperature errors from ISA result in true than standard, the air is less dense and the altitude being higher than indicated altitude pressure levels are farther apart. When the whenever the temperature is warmer than ISA aircraft is flying at an indicated altitude of 5,000 and true altitude being lower than indicated feet, the pressure level for that altitude is altitude whenever the temperature is colder than higher than it would be in air at standard ISA. True altitude variance under conditions of temperature, and the aircraft is higher than it colder than ISA temperatures poses the risk of would be if the air were cooler. If the air is inadequate obstacle clearance. Under extremely colder than standard, it is denser and the cold conditions, pilots may need to add an pressure levels are closer together. When the appropriate temperature correction determined aircraft is flying at an indicated altitude of 5,000 from the chart in Figure 3-7 to charted IFR feet, its true altitude is lower than it would be if altitudes to ensure terrain and obstacle clearance the air were warmer. [Figure 3-6] with the following restrictions: Cold Weather Atimeter Altitudes specifically assigned by Air Traffic Control Errors (ATC), such as “maintain 5,000 feet” shall not be A correctly calibrated pressure altimeter corrected. Assigned altitudes may be rejected if indicates true altitude above mean sea level the (MSL) when operating within the International pilot decides that low temperatures pose a risk of Standard Atmosphere (ISA) parameters of inadequate terrain or obstacle clearance. pressure and temperature. Nonstandard pressure conditions are corrected by applying the correct local area altimeter setting. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 8  BASIC AIRCRAFT INSTRUMENTS If temperature corrections are applied to If compensation charted IFR altitudes (such as procedure turn is applied by the FMS or manually, ATC must be altitudes, final approach fix crossing altitudes, informed that the aircraft is not flying the etc.), the pilot must advise ATC of the applied assigned altitude. Otherwise, vertical correction. separation from other aircraft may be reduced creating a potentially hazardous situation. The ICAO Cold Temperature table in Figure 3-7, derived from International Error Table Civil Aviation The cold temperature induced altimeter error may be significant when considering obstacle clearances when temperatures are well below standard. Pilots may wish to increase their minimum terrain clearance altitudes with a corresponding increase in ceiling from the normal minimum when flying in extreme cold temperature conditions. Higher altitudes may need to be selected when flying at low terrain clearances. Most flight management systems (FMS) with air data computers implement a capability to compensate for cold temperature Figure 3-6. The loss of altitude errors. Pilots flying with these systems should experienced when flying into an area where the air is colder (more dense) ensure they are aware of the conditions under than standard. which the system will automatically compensate. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 9  BASIC AIRCRAFT INSTRUMENTS When using the cold temperature error table, the altitude error is proportional to both the height above the reporting station elevation and the temperature at the reporting station. For IFR approach procedures, the reporting station elevation is assumed to be airport elevation. It is important to understand that corrections are based upon the temperature at the reporting station, not the temperature observed at the Figure 3-7. ICAO Cold Temperature aircraft’s current altitude and height above the Error Table. reporting station and not the charted IFR Organization (ICAO) standard formulas, shows how altitude. much error can exist when the temperature is To see how corrections are applied, note the extremely cold. To use the table, find the reported following example: temperature in the left column, and then read Airport Elevation 496 feet across the top row to the height above the Airport Temperature - 50° C airport/reporting station. Subtract the airport elevation from the altitude of the final approach A charted IFR approach to the airport provides fix (FAF). The intersection of the column and row is the following data: Minimum Procedure Turn the amount of possible error. Example: The Altitude 1,800 feet Minimum FAF Crossing reported temperature is -10° Celsius and the FAF is Altitude 1,200 feet Straight-in Minimum 500 feet above the airport elevation. The reported Descent Altitude 800 feet current altimeter setting may place the aircraft as Circling MDA 1,000 feet much as 50 feet below the altitude indicated by the altimeter. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 10  BASIC AIRCRAFT INSTRUMENTS The Minimum Procedure Turn Altitude of 1,800 correcting the charted value of 1,800 feet above feet will be used as an example to demonstrate MSL (equal to a height above the reporting determination of the appropriate temperature station of 1,300 feet) requires the addition of 400 correction. Typically, altitude values are feet. Thus, when flying at an indicated altitude of rounded up to the nearest 100-foot level. The 2,200 feet, the aircraft is actually flying a true charted procedure turn altitude of 1,800 feet altitude of 1,800 feet. minus the airport elevation of 500 feet equals Minimum Procedure Turn Altitude 1,300 feet. The altitude difference of 1,300 feet 1,800 feet charted = 2,200 feet corrected falls between the correction chart elevations of Minimum FAF Crossing Altitude 1,000 feet and 1,500 feet. At the station 1,200 feet charted = 1,500 feet corrected temperature of -50°C, the correction falls Straight-in MDA between 300 feet and 450 feet. Dividing the 800 feet charted = 900 feet corrected Circling difference in compensation values by the MDA 1,000 feet charted = 1,200 feet corrected difference in altitude above the airport gives Nonstandard Pressure on the error value per foot. an Altimeter In this case, 150 feet divided by 500 feet = 0.33 Maintaining a current altimeter setting is critical feet for each additional foot of altitude above because the atmosphere pressure is not 1,000 feet. This provides a correction of 300 constant. That is, in one location the pressure feet for the first 1,000 feet and an additional might be higher than the pressure just a short value of 0.33 times 300 feet, or 99 feet, which distance away. Take an aircraft whose altimeter is rounded to 100 feet. 300 feet + 100 feet = setting is set to 29.92" of local pressure. As the total temperature correction of 400 feet. For aircraft moves to an area of lower pressure (Point the given conditions, A to B in Figure 3-8) MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 11  BASIC AIRCRAFT INSTRUMENTS and the pilot fails to readjust the altimeter setting Figure 3-8. Effects of Nonstandard Pressure on an (essentially calibrating it to local pressure), then Altimeter of an Aircraft Flown into Air of Lower Than Standard Pressure as the pressure decreases, the true altitude will (Air is be lower. Adjusting the altimeter settings Less Dense). compensates for this. When the altimeter shows an indicated altitude of 5,000 feet, the true Altimeter Enhancements altitude at Point A (the height above mean sea (Encoding) level) is only 3,500 feet at Point B. The fact that It is not sufficient in the airspace system for only the altitude indication is not always true lends the pilot to have an indication of the aircraft’s itself to the memory aid, “When flying from hot altitude; the air traffic controller on the ground to cold or from a high to a low, look out below.” must also know the altitude of the aircraft. To [Figure 3-8] provide this information, the aircraft is typically equipped with an encoding altimeter. When the ATC transponder is set to Mode C, the encoding altimeter supplies the transponder with a series of pulses identifying the flight level (in increments of 100 feet) at which the aircraft is flying. This series of pulses is transmitted to the ground radar where they appear on the controller’s scope as an alphanumeric display around the return for the aircraft. The transponder allows the ground controller to identify the aircraft and determine the pressure altitude at which it is flying. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 12  BASIC AIRCRAFT INSTRUMENTS A computer inside the encoding altimeter All aircraft 18,000 feet and above use a measures the pressure referenced from 29.92" standard altimeter setting of 29.92" Hg, and Hg and delivers this data to the transponder. the altitudes are in reference to a standard When the pilot adjusts the barometric scale to hence termed FL. Between FL 180 and FL 290, the local altimeter setting, the data sent to the the minimum altitude separation is 1,000 feet transponder is not affected. This is to ensure between aircraft. However, for flight above FL that all Mode C aircraft are transmitting data 290 (primarily due to aircraft equipage and referenced to a common pressure level. ATC reporting capability; potential error) ATC equipment adjusts the displayed altitudes to applied the requirement of 2,000 feet of compensate for local pressure differences separation. FL 290, an altitude appropriate for allowing display of targets at correct altitudes. an eastbound aircraft, would be followed by FL 14 CFR part 91 requires the altitude transmitted 310 for a westbound aircraft, and so on to FL by the transponder to be within 125 feet of the 410, or seven FLs available for flight. With altitude indicated on the instrument used to 1,000-foot separation, or a reduction of the maintain flight altitude. vertical separation between FL 290 and FL 410, an additional six FLs become available. This results in normal flight level and direction Reduced Vertical Separation management being maintained from FL 180 Minimum (RVSM) through FL 410. Hence the name is Reduced Below 31,000 feet, a 1,000 foot separation is Vertical Separation Minimum (RVSM). Because the minimum required between usable flight it is applied domestically, it is called United levels. Flight levels (FLs) generally start at States Domestic Reduced Vertical Separation 18,000 feet where the local pressure is 29.92" Minimum, or DRVSM. Hg or greater. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 13  BASIC AIRCRAFT INSTRUMENTS However, there is a cost to participate in the that are equipped with an automatic altitude DRVSM program which relates to both aircraft control system with flight equipage and pilot training. For example, management/performance system inputs. altimetry error must be reduced significantly That aircraft must be equipped with an altitude and operators using RVSM must receive alert system that signals an alert when the authorization from the appropriate civil aviation altitude displayed to the flight crew deviates authority. RVSM aircraft must meet required from the selected altitude by more than (in most altitude-keeping performance standards. cases) 200 feet. For each condition in the full Additionally, operators must operate in RVSM flight envelope, the largest combined accordance with RVSM policies/procedures absolute value for residual static source error applicable to the airspace where plus the avionics error may not exceed 200 feet. they are flying. Aircraft with TCAS must have compatibility with The aircraft must be equipped with at RVSM Operations. Figure 3-9 illustrates the least one automatic increase in aircraft permitted between FL 180 and FL 410. Most noteworthy, however, is the altitude control— economization that aircraft can take advantage of Within a tolerance band of ±65 feet about an by the higher FLs being available to more aircraft. acquired altitude when the aircraft is operated in straight-andlevel flight. Within a tolerance band of ±130 feet under no turbulent, conditions for aircraft for which application for type certification occurred on or before April 9, 1997 MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 14  BASIC AIRCRAFT INSTRUMENTS Vertical Speed Indicator (VSI) The VSI in Figure 3-10 is also called a vertical velocity indicator (VVI), and was formerly known as a rate-ofclimb indicator. It is a rate-of- pressure change instrument that gives an indication of any deviation from a constant pressure level. Inside the instrument case is an aneroid very much like the one in an ASI. Both the inside of this aneroid and the inside of the instrument case are vented to the static system, but the case is vented through a calibrated orifice that causes the pressure inside the case to change more slowly than the pressure inside the aneroid. As the aircraft ascends, the static pressure becomes lower. The pressure inside the case compresses the aneroid, moving the pointer upward, showing a climb and indicating Figure 3-9. Increase in Aircraft Permitted the rate of ascent in number of feet per minute Between FL 180 and FL 410. (fpm). MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 15  BASIC AIRCRAFT INSTRUMENTS Dynamic Pressure Type Instruments When the aircraft levels off, the pressure no longer changes. The pressure inside the case Airspeed Indicator (ASI) becomes equal to that inside the aneroid, and An ASI is a differential pressure gauge that the pointer returns to its horizontal, or zero, measures the dynamic pressure of the air position. When the aircraft descends, the static through which the aircraft is flying. Dynamic pressure increases. The aneroid expands, pressure is the difference in the ambient static air moving the pointer downward, indicating a pressure and the total, or ram, pressure caused descent. by the motion of the aircraft through the air. These two pressures are taken from the pitot- The pointer indication in a VSI lags a few static system. seconds behind the actual change in pressure. However, it is more sensitive than an altimeter and is useful in alerting the pilot of an upward or downward trend, thereby helping maintain a constant altitude. Some of the more complex VSIs, called instantaneous vertical speed indicators (IVSI), have two accelerometer-actuated air pumps that sense an upward or downward pitch of the aircraft and instantaneously create a pressure differential. By the time the pressure caused by the pitch acceleration dissipates, the altitude Figure 3-10. Rate of Climb or Descent pressure change is effective. in Thousands of Feet Per Minute. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 16  BASIC AIRCRAFT INSTRUMENTS The mechanism of the ASI in Figure 3-11 consists Calibrated Airspeed (CAS) of a thin, corrugated phosphor bronze aneroid, or CAS is the speed at which the aircraft is moving diaphragm, that receives its pressure from the through the air, which is found by correcting IAS pitot tube. The instrument case is sealed and for instrument and position errors. The connected to the static ports. As the pitot POH/AFM has a chart or graph to correct IAS for pressure increases or the static pressure these errors and provide the correct CAS for the decreases, the diaphragm expands. This various flap and landing gear configurations. dimensional change is measured by a rocking Equivalent Airspeed (EAS) shaft and a set of gears that drives a pointer across the instrument dial. Most ASIs are EAS is CAS corrected for compression of the air calibrated in knots, or nautical miles per hour; inside the pitot tube. EAS is the same as CAS in some instruments show statute miles per hour, standard atmosphere at sea level. As the and some instruments show both. airspeed and pressure altitude increase, the CAS becomes higher than it should be, and a Types of Airspeed correction for compression must be subtracted Just as there are several types of altitude, there from the CAS. are multiple types of airspeed: Indicated Airspeed (IAS), Calibrated Airspeed (CAS), True Airspeed (TAS) Equivalent Airspeed (EAS), and True Airspeed TAS is CAS corrected for nonstandard pressure (TAS). and temperature. TAS and CAS are the same in standard atmosphere at sea level. Under Indicated Airspeed (IAS) nonstandard conditions, TAS is found by IAS is shown on the dial of the instrument, applying a correction for pressure altitude and uncorrected for instrument or system errors. temperature to the CAS. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 17  BASIC AIRCRAFT INSTRUMENTS Some aircraft are equipped with true ASIs that have a temperature-compensated aneroid bellows inside the instrument case. This bellows modifies the movement of the rocking shaft inside the instrument case so the pointer shows the actual TAS. The TAS indicator provides both true and IAS. These instruments have the conventional airspeed mechanism, with an added subdial visible through cutouts in the regular dial. A knob on the instrument allows the pilot to rotate the subdial and align an indication of the Figure 3-11. Mechanism of an outside air temperature with the pressure Airspeed Indicator. altitude being flown. This alignment causes the instrument pointer to indicate the TAS on the subdial. [Figure 3-12] MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 18  BASIC AIRCRAFT INSTRUMENTS Therefore, in this case, airspeed is not entirely adequate to warn the pilot of the impending problems. Mach number is more useful. Mach number is the ratio of the TAS of the aircraft to the speed of sound in the same atmospheric conditions. An aircraft flying at the speed of sound is flying at Mach 1.0. Some older mechanical Machmeters not driven from an air data computer use an altitude aneroid inside the instrument that converts pitot-static pressure into Mach number. These systems assume that the temperature at any altitude is standard; Figure 3-12. A true airspeed indicator allows the pilot to correct IAS for nonstandard therefore, the indicated Mach number is temperature and pressure. inaccurate whenever the temperature deviates from standard.These systems are called indicated Machmeters. Modern electronic Machmeters use Mach Number information from an air data computer system to As an aircraft approaches the speed of sound, correct for temperature errors. These systems the air flowing over certain areas of its surface display true Mach number. speeds up until it reaches the speed of sound, and shock waves form. The IAS at which these conditions occur changes with temperature. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 19  BASIC AIRCRAFT INSTRUMENTS The speed of sound varies with the air temperature. If the aircraft were flying at Mach.83 at 10,000 feet where the air is much warmer, its airspeed would be 530 knots. Maximum Allowable Airspeed Some aircraft that fly at high subsonic speeds are equipped with maximum allowable ASIs like the one in Figure 3-14. This instrument looks much like a standard air-speed indicator, calibrated in knots, but has an additional Figure 3-13. A Machmeter shows the ratio of pointer colored red, checkered, or striped. The the speed of sound to the TAS the aircraft is flying. maximum airspeed pointer is actuated by an aneroid, or altimeter mechanism, that moves it Most high-speed aircraft are limited to a to a lower value as air density decreases. By maximum Mach number at which they can fly. keeping the airspeed pointer at a lower value This is shown on a Machmeter as a decimal than the maximum pointer, the pilot avoids the fraction. [Figure 3-13] For example, if the onset of transonic shock waves. Machmeter indicates.83 and the aircraft is flying at 30,000 feet where the speed of sound under standard conditions is 589.5 knots, the airspeed is 489.3 knots. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 20  BASIC AIRCRAFT INSTRUMENTS Magnetism The Earth is a huge magnet, spinning in space, surrounded by a magnetic field made up of invisible lines of flux. These lines leave the surface at the magnetic north pole Lines of magnetic flux have two important characteristics: any magnet that is free to rotate will align with them, and an electrical current is induced into any conductor that cuts across them. Most direction indicators installed in aircraft make use of one of these two Figure 3-14. A maximum allowable airspeed characteristics. indicator has a movable pointer that indicates the never-exceed speed, which changes with altitude to avoid the onset of transonic shock waves. Airspeed Color Codes The dial of an ASI is color coded to alert the pilot, at a glance, of the significance of the speed at which the aircraft is flying. These colors and their associated airspeeds are shown in Figure 3-15. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 21  BASIC AIRCRAFT INSTRUMENTS The Basic Aviation The card is marked with letters representing the Magnetic Compass cardinal directions, north, east, south, and west, and a number for each 30° between these One of the oldest and simplest instruments for letters.The final “0” is omitted from these indicating direction is the magnetic compass. It is directions; for example, 3 = 30°, 6 = 60°, and 33 also one of the basic instruments required by 14 = 330°. There are long and short graduation CFR part 91 for both VFR and IFR flight. marks between the letters and numbers, with each long mark representing 10° and each short mark representing 5°. Magnetic Compass Overview A magnet is a piece of material, usually a metal containing iron, which attracts and holds lines of magnetic flux. Regardless of size, every magnet has two poles: a north pole and a south pole. When one magnet is placed in the field of another, the unlike poles attract each other and like poles repel. An aircraft magnetic compass, such as the one in Figure 3-16, has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid similar to kerosene. A graduated scale, called a card, is wrapped around the float Figure 3-16. A Magnetic Compass. The vertical line and viewed through a glass window with a is called the lubber line. lubber line across it. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 22  BASIC AIRCRAFT INSTRUMENTS Magnetic Compass Note that in Figure 3-16, the pilot sees the Construction compass card from its backside. When the pilot is The float and card assembly has a hardened steel flying north as the compass shows, east is to the pivot in its center that rides inside a special, pilot’s right, but on the card “33”, which represents spring-loaded, hard-glass jewel cup. The 330° (west of north), is to the right of north. The buoyancy of the float takes most of the weight off reason for this apparent backward graduation is the pivot, and the fluid damps the oscillation of that the card remains stationary, and the compass the float and card. This jewel-and-pivot type housing and the pilot turn around it, always mounting allows the float freedom to rotate and viewing the card from its backside. tilt up to approximately 18° angle of bank. At A compensator assembly mounted on the top or steeper bank angles, the compass indications are bottom of the compass allows an aviation erratic and unpredictable. maintenance technician (AMT) to create a The compass housing is entirely full of compass magnetic field inside the compass housing that fluid. To prevent damage or leakage when the cancels the influence of local outside magnetic fluid expands and contracts with temperature fields. This is done to correct for deviation error. changes, the rear of the compass case is sealed The compensator assembly has two shafts whose with a flexible diaphragm, or with a metal ends have screwdriver slots accessible from the bellows in some compasses. front of the compass. Each shaft rotates one or two small compensating magnets. The end of one shaft Magnetic Compass Theory of Operations is marked E-W, and its magnets affect the compass The magnets align with the Earth’s magnetic when the aircraft is pointed east or west. The other field and the pilot reads the direction on the shaft is marked N-S and its magnets affect the scale opposite the lubber line. compass when the aircraft is pointed north or south. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 23  BASIC AIRCRAFT INSTRUMENTS Magnetic Compass Induced Figure 3-17 shows the isogonic lines that identify the Errors number of degrees of variation in their area. The The magnetic compass is the simplest line that passes near Chicago is called the agonic instrument in the panel, but it is subject to a line. Anywhere along this line the two poles are number of errors that must be considered. aligned, and there is no variation. East of this line, the magnetic pole is to the west of the geographic Variation pole and a correction must be applied to a compass The Earth rotates about its geographic axis; indication to get a true direction. maps and charts are drawn using meridians of longitude that pass through the geographic poles. Directions measured from the geographic poles are called true directions. The north magnetic pole to which the magnetic compass points is not collocated with the geographic north pole, but is some 1,300 miles away; directions measured from the magnetic poles are called magnetic directions. In aerial navigation, the difference between true and magnetic directions is called variation. This same angular difference in surveying and land navigation is called declination. Figure 3-17. Isogonic lines are lines of equal variation. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 24  BASIC AIRCRAFT INSTRUMENTS Flying in the Washington, D.C. area, for Variation error cannot be reduced or changed, example, the variation is 10° west. If the pilot but deviation error can be minimized when a wants to fly a true course of south (180°), the pilot or AMT performs the maintenance task variation must be added to this resulting in a known as “swinging the compass.” magnetic course to fly of 190°. Flying in the Los Most airports have a compass rose, which is a Angeles, CA area, the variation is 14° east. To fly series of lines marked out on a taxiway or ramp a true course of 180° there, the pilot would at some location where there is no magnetic have to subtract the variation and fly a interference. Lines, oriented to magnetic north, magnetic course of 166°. The variation error are painted every 30°, as shown in Figure 3-18. does not change with the heading of the aircraft; it is the same anywhere along the isogonic line. Deviation The magnets in a compass align with any magnetic field. Local magnetic fields in an aircraft caused by electrical current flowing in the structure, in nearby wiring or any magnetized part of the structure, conflict with the Earth’s magnetic field and cause a compass error called deviation. Deviation, unlike variation, is different on each Figure 3-18. Utilization of a Compass heading, but it is not affected by the Rose Aids Compensation for Deviation Errors. geographic location. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 25  BASIC AIRCRAFT INSTRUMENTS The pilot or AMT aligns the aircraft on each The corrections for variation and deviation must magnetic heading and adjusts the be applied in the correct sequence and is shown compensating magnets to minimize the below starting from the true course desired. difference between the compass indication and Step 1: Determine the Magnetic Course the actual magnetic heading of the aircraft. Any True Course (180°) ± Variation (+10°) = Magnetic error that cannot be Course (190°) removed is recorded on a compass correction The Magnetic Course (190°) is steered if there is card, like the one in Figure 3-19, and placed in a no deviation error to be applied. The compass cardholder near the compass. If the pilot wants card must now be considered for the compass to fly a magnetic heading of 120° and the course of 190°. aircraft is operating with the radios on, the pilot Step 2: Determine the Compass Course Magnetic should fly a compass heading of 123°. Course (190°, from step 1) ± Deviation (-2°, from correction card) = Compass Course (188°) NOTE: Intermediate magnetic courses between those listed on the compass card need to be interpreted. Therefore, to steer a true course of 180°, the pilot would follow a compass course of 188°. To find the true course that is being flown Figure 3-19. A compass correction when the compass course is known: Compass card shows the deviation Course ± Deviation = Magnetic Course ± Variation correction for any heading. = True Course MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 26  BASIC AIRCRAFT INSTRUMENTS Dip Errors The lines of magnetic flux are considered to The vertical component of the Earth’s magnetic leave the Earth at the magnetic north pole and field pulls the north-seeking end of the magnet enter at the magnetic South Pole. At both to the right, and the float rotates, causing the locations the lines are perpendicular to the card to rotate toward west, the direction Earth’s surface. At the magnetic equator, which opposite the direction the turn is being made. is halfway between the poles, the lines are [Figure If the 3-20] turn is made from north to west, the parallel with the surface. The magnets in a aircraft banks to the left and the compass card compass align with this field, and near the poles tilts down on the left side. The magnetic field they dip, or tilt, the float and card. The float is pulls on the end of the magnet that causes the balanced with a small dip-compensating card to rotate toward east. This indication is weight, so it stays relatively level when again opposite to the direction the turn is being operating in the middle latitudes of the made. The rule for this error is: when starting a northern hemisphere. This dip along with this turn from a northerly heading, the compass weight causes two very noticeable errors: indication lags behind the turn. northerly turning error and acceleration error. The pull of the vertical component of the Earth’s magnetic field causes northerly turning error, which is apparent on a heading of north or south. When an aircraft flying on a heading of north makes a turn toward east, the aircraft banks to the right, and the compass card tilts to the right. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 27  BASIC AIRCRAFT INSTRUMENTS Figure 3-20. Northerly Turning Error. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 28  BASIC AIRCRAFT INSTRUMENTS Figure 3-21. The Effects of Acceleration Error. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 29  BASIC AIRCRAFT INSTRUMENTS When an aircraft is flying on a heading of south As soon as the speed of the aircraft stabilizes, the and begins a turn toward east, the Earth’s card swings back to its east indication. If, while magnetic field pulls on the end of the magnet flying on this easterly heading, the aircraft that rotates the card toward east, the same decelerates, the inertia causes the weight to direction the turn is being made. If the turn is move ahead and the card rotates toward south made from south toward west, the magnetic until the speed again stabilizes. pull starts the card rotating toward west— When flying on a heading of west, the same again, in the same direction the turn is being things happen. Inertia from acceleration causes made. The rule for this error is: When starting a the weight to lag, and the card rotates toward turn from a southerly heading, the compass north. When the aircraft decelerates on a heading indication leads the turn. of west, inertia causes the weight to move ahead In acceleration error, the dip-correction weight and the card rotates toward south. causes the end of the float and card marked N (the south-seeking end) to be heavier than the Oscillation Error opposite end. When the aircraft is flying at a Oscillation is a combination of all of the other constant speed on a heading of east or west, errors, and it results in the compass card swinging the float and card is level. The effects of back and forth around the heading being flown. magnetic dip and the weight are approximately When setting the gyroscopic heading indicator to equal. If the aircraft accelerates on a heading of agree with the magnetic compass, use the east [Figure 3-21], the inertia of the weight average indication between the swings. holds its end of the float back and the card rotates toward north MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 30  BASIC AIRCRAFT INSTRUMENTS The Vertical Card Magnetic Compass The Flux Gate Compass System The floating magnet type of compass not only As mentioned earlier, the lines of flux in the has all the errors just described, but also lends Earth’s magnetic field have two basic itself to confused reading. It is easy to begin a characteristics: a magnet aligns with these lines, turn in the wrong direction because its card and an electrical current is induced, or generated, appears backward. East is on what the pilot in any wire crossed by them. would expect to be the west side. The vertical card magnetic compass eliminates some of the errors and confusion. The dial of this compass is graduated with letters representing the cardinal directions, numbers every 30°, and marks every 5°. The dial is rotated by a set of gears from the shaft-mounted magnet, and the nose of the symbolic airplane on the instrument glass represents the lubber line for reading the heading of the aircraft from the dial. Eddy currents induced into an aluminum-damping cup damp oscillation of the magnet. [Figure 3- 22] Figure 3-22. Vertical Card Magnetic Compass. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 31  BASIC AIRCRAFT INSTRUMENTS Gyroscopic Systems Flight without reference to a visible horizon can Power Sources be safely accomplished by the use of gyroscopic Aircraft and instrument manufacturers have instrument systems and the two characteristics designed redundancy in the flight instruments of gyroscopes, which are rigidity and so that any single failure will not deprive the precession. These systems include attitude, pilot of the ability to safely conclude the flight. heading, and rate instruments, along with their Gyroscopic instruments are crucial for power sources. These instruments include a instrument flight; therefore, they are powered gyroscope (or gyro) that is a small wheel with by separate electrical or pneumatic sources. its weight concentrated around its periphery. When this wheel is spun at high speed, it Pneumatic Systems becomes rigid and resists tilting Pneumatic gyros are driven by a jet of air or turning in any direction other than around its impinging on buckets cut into the periphery of spin axis. the wheel. On many aircraft this stream of air is obtained by evacuating the instrument case Attitude and heading instruments operate on with a vacuum source and allowing filtered air the principle of rigidity. For these instruments, to flow into the case through a nozzle to spin the gyro remains rigid in its case and the the wheel. aircraft rotates about it. Rate indicators, such as turn indicators and turn coordinators, operate on the principle of precession. In this case, the gyro processes (or rolls over) proportionate to the rate the aircraft rotates about one or more of its axes. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 32  BASIC AIRCRAFT INSTRUMENTS Venturi Tube Systems Aircraft that do not have a pneumatic pump to evacuate the instrument case can use venturi tubes mounted on the outside of the aircraft, similar to the system shown in Figure 3-27. Air flowing through the venturi tube speeds up in the narrowest part and, according to Bernoulli’s principle, the pressure drops. This location is connected to the instrument case by a piece of tubing. The two attitude instruments operate on approximately 4" Hg of suction; the turn- and-slip indicator needs only 2" Hg, so a pressure-reducing needle valve is used to decrease the suction. Air flows into the instruments through filters built into the instrument cases. In this system, ice can clog Figure 3-27. A venturi tube system that provides necessary the venturi tube and stop the instruments vacuum to operate key instruments. when they are most needed. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 33  BASIC AIRCRAFT INSTRUMENTS Vacuum Pump Systems Wet-Type Vacuum Pump Dry Air Vacuum Pump Steel-vane air pumps have been used for many As flight altitudes increase, the air is less dense years to evacuate the instrument cases. The and more air must be forced through the vanes in these pumps are lubricated by a small instruments. Air pumps that do not mix oil with amount of engine oil metered into the pump the discharge air are used in high flying aircraft. and discharged with the air. In some aircraft the Steel vanes sliding in a steel housing need to be discharge air is used to inflate rubber deicer lubricated, but vanes made of a special boots on the wing and empennage leading formulation of carbon sliding inside carbon edges. To keep the oil from deteriorating the housing provide their own lubrication in a rubber boots, it must be removed with an oil microscopic amount as they wear. separator like the one in Figure 3-28. The vacuum pump moves a greater volume of air than is needed to supply the instruments with the suction needed, so a suction-relief valve is installed in the inlet side of the pump. This spring-loaded valve draws in just enough air to maintain the required low pressure inside the instruments, as is shown on the suction gauge in the instrument panel. Filtered air enters the instrument cases from a central air filter. As long as aircraft fly at relatively low altitudes, enough air is drawn into the instrument cases to spin the gyros at a sufficiently high speed. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 34  BASIC AIRCRAFT INSTRUMENTS Pressure Indicating Systems Electrical Systems Figure 3-29 is a diagram of the instrument Many general aviation aircraft that use pneumatic system of a twin-engine general pneumatic attitude indicators use electric rate aviation airplane. Two dry air pumps are used indicators and/or the reverse. Some with filters in their inlet to filter out any instruments identify their power source on contaminants that could damage the fragile their dial, but it is extremely important that carbon vanes in the pump. The discharge air pilots consult the POH/AFM to determine the from the pump flows through a regulator, power source of all instruments to know what where excess air is bled off to maintain the action to take in the event of an instrument pressure in the system at the desired level. The failure. Direct current (D.C.) electrical regulated air then flows through inline filters to instruments are available in 14- or 28-volt remove any contamination that could have models, depending upon the electrical system been picked up from the pump, and from there in the aircraft. A.C. is used to operate some into a manifold check valve. If either engine attitude gyros and autopilots. Aircraft with only should become inoperative or either pump D.C. electrical systems can use A.C. instruments should fail, the check valve isolates the via installation of a solid-state D.C. to A.C. inoperative system and the instruments are inverter, which changes 14 or 28 volts D.C. into driven by air from the operating system. After three-phase 115-volt, 400-Hz A.C. the air passes through the instruments and drives the gyros, it is exhausted from the case. The gyro pressure gauge measures the pressure drop across the instruments. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 35  BASIC AIRCRAFT INSTRUMENTS Figure 3-29. Twin-Engine Instrument Pressure System Using a Carbon- Vane Dry-Type Air Pump. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 36  BASIC AIRCRAFT INSTRUMENTS Gyroscopic Instruments Attitude Indicators The first attitude instrument (AI) was originally A bank index at the top of the instrument referred to as an artificial horizon, later as a shows the angle of bank marked on the banking gyro horizon; now it is more properly called an scale with lines that represent 10°, 20°, 30°, attitude indicator. Its operating mechanism is a 45°, and 60°. [Figure 3-30] small brass wheel with a vertical spin axis, spun at a high speed by either a stream of air impinging on buckets cut into its periphery, or by an electric motor. The gyro is mounted in a double gimbal, which allows the aircraft to pitch and roll about the gyro as it remains fixed in space. A horizon disk is attached to the gimbals so it remains in the same plane as the gyro, and the aircraft pitches and rolls about it. On early instruments, this was just a bar that represented the horizon, but now it is a disc with a line representing the horizon and both pitch marks and bank-angle lines. The top half of the instrument dial and horizon disc is blue, Figure 3-30. The dial of this attitude indicator has representing the sky; and the bottom half is reference lines to show pitch and roll. brown, representing the ground. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 37  BASIC AIRCRAFT INSTRUMENTS Heading Indicators A magnetic compass is a dependable A knob on the front of the instrument, below instrument used as a backup instrument. the dial, can be pushed in to engage the Although very reliable, it has so many inherent gimbals. This locks the gimbals allowing the errors that it has been supplemented with pilot to rotate the gyro and card until the gyroscopic heading indicators. The gyro in a number opposite the lubber line agrees with heading indicator is mounted in a double the magnetic compass. When the knob is pulled gimbal, as in an attitude indicator, but its spin out, the gyro remains rigid and the aircraft is axis is horizontal permitting sensing of rotation free to turn around the card. about the vertical axis of the aircraft. Gyro heading indicators, with the exception of slaved Heading indicators like the one in Figure 3-31 gyro indicators, are not north seeking, therefore work on the same principle as the older they must be manually set to the appropriate horizontal card indicators, except that the gyro heading by referring to a magnetic compass. drives a vertical dial that looks much like the Rigidity causes them to maintain this heading dial of a vertical card magnetic compass. The indication, without the oscillation and other heading of the aircraft is shown against the errors inherent in a magnetic compass. nose of the symbolic aircraft on the instrument Older directional gyros use a drum-like card glass, which serves as the lubber line. A knob in marked in the same way as the magnetic the front of the instrument may be pushed in compass card. The gyro and the card remain and turned to rotate the gyro and dial. The rigid inside the case with the pilot viewing the knob is spring loaded so it disengages from the card from the back. This creates the possibility gimbals as soon as it is released. This the pilot might start a turn in the wrong instrument should be checked about every 15 direction similar to using a magnetic minutes to see if it agrees with the magnetic compass. compass. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 38  BASIC AIRCRAFT INSTRUMENTS Turn-and-Slip Indicator The first gyroscopic aircraft instrument was the turn indicator in the needle and ball, or turn- and-bank indicator, which has more recently been called a turn-and-slip indicator. [Figure 3- 33] Figure 3-31. The heading indicator is not north seeking, but must be set periodically (about every 15 minutes) to agree with the magnetic compass. Turn Indicators Attitude and heading indicators function on the principle of rigidity, but rate instruments such as the turn-and slip indicator operate on Figure 3-32. Precession causes a force applied to a precession. Precession is the characteristic of a spinning wheel to be felt 90° from the point of gyroscope that causes an applied force to application in the direction of rotation. produce a movement, not at the point of application, but at a point 90° from the point of application in the direction of rotation. [Figure 3-32] MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 39  BASIC AIRCRAFT INSTRUMENTS The inclinometer in the instrument is a black glass ball sealed inside a curved glass tube that is partially filled with a liquid for damping. This ball measures the relative strength of the force of gravity and the force of inertia caused by a turn. When the aircraft is flying straight-and- level, there is no inertia acting on the ball, and it remains in the center of the tube between two wires. In a turn made with a bank angle that is too steep, the force of gravity is greater than the inertia and the ball rolls down to the inside of the turn. If the turn is made with too shallow a bank angle, the inertia is greater than gravity and the ball rolls upward to the outside Figure 3-33. Turn-and-Slip Indicator. of the turn. The inclinometer does not indicate the amount of bank, nor does it indicate slip; it only indicates the relationship between the angle of bank and the rate of yaw. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 40  BASIC AIRCRAFT INSTRUMENTS The turn indicator is a small gyro spun either by air or by an electric motor. The gyro is mounted in a single gimbal with its spin axis parallel to the lateral axis of the aircraft and the axis of the gimbal parallel with the longitudinal axis. [Figure 3-34] Figure 3-34. The rate gyro in both turn-and-slip indicator and turn coordinator. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 41  BASIC AIRCRAFT INSTRUMENTS Turn Coordinator The major limitation of the older turn-and-slip indicator is that it senses rotation only about the vertical axis of the aircraft. It tells nothing of the rotation around the longitudinal axis, which in normal flight occurs before the aircraft begins to turn. A turn coordinator operates on precession, the same as the turn indicator, but its gimbals frame is angled upward about 30° from the longitudinal axis of the aircraft. [Figure 3-34] This allows it to sense both roll and yaw. Therefore during a turn, the indicator first Figure 3-35. A turn coordinator senses rotation about shows the rate of banking and once stabilized, both roll and yaw axes. the turn rate. Some turn coordinator gyros are The inclinometer, similar to the one in a turn- dual powered and can be driven by either air or and-slip indicator, is called a coordination ball, electricity. Rather than using a needle as an which shows the relationship between the bank indicator, the gimbal moves a dial that is the angle and the rate of yaw. The turn is rear view of a symbolic aircraft. The bezel of the coordinated when the ball is in the center, instrument is marked to show wings-level flight between the marks. The aircraft is skidding and bank angles for a standard rate turn. when the ball rolls toward the outside of the [Figure 3-35] turn and is slipping when it moves toward the inside of the turn. A turn coordinator does not sense pitch. This is indicated on some instruments by placing the words “NO PITCH INFORMATION” on the dial. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 42  BASIC AIRCRAFT INSTRUMENTS Flight Support Systems Attitude and Heading Reference System (AHRS) As aircraft displays have transitioned to new The ADC outputs this information in a digital technology, the sensors that feed them have format that can be used by a variety of aircraft also undergone significant change. Traditional systems including an EFIS. Modern ADCs are gyroscopic flight instruments have been small solid-state units. Increasingly, aircraft replaced by Attitude and Heading Reference systems such as autopilots, pressurization, and Systems (AHRS) improving reliability and FMS utilize ADC information for normal thereby reducing cost and maintenance. operations. NOTE: In most modern general aviation systems, both the AHRS and ADC are The function of an AHRS is the same as integrated within the electronic displays gyroscopic systems; that is, to determine which themselves thereby reducing the number of way is level and which way is north. By knowing units, reducing weight, and providing the initial heading the AHRS can determine simplification for installation resulting in both the attitude and magnetic heading of the reduced costs. aircraft. Air Data Computer (ADC) An Air Data Computer (ADC) [Figure 3-37] is an aircraft computer that receives and processes pitot pressure, static pressure, and temperature to calculate very precise altitude, IAS, TAS, and air temperature. Figure 3-37. Air Data Computer (Collins). MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 43  BASIC AIRCRAFT INSTRUMENTS Analog Pictorial Displays Horizontal Situation Indicator (HSI) The HSI is a direction indicator that uses the output from a flux valve to drive the dial, which acts as the compass card. This instrument, shown in Figure 3-38, combines the magnetic compass with navigation signals and a glide slope. This gives the pilot an indication of the location of the aircraft with relationship to the chosen course Figure 3-38. Horizontal Situation Indicator (HSI). Figure 3-36. The Kearfott Attitude Heading Reference System (AHRS) on the left incorporates a Monolithic Ring Laser Gyro (MRLG) (center), which is housed in an Inertial Sensor Assembly (ISA) on the right. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 44  BASIC AIRCRAFT INSTRUMENTS Attitude Direction Indicator (ADI) Advances in attitude instrumentation combine For the purpose of the practical test standards, the gyro horizon with other instruments such as any flight instrument display that utilizes LCD or the HSI, thereby reducing the number of picture tube like displays is referred to as separate instruments to which the pilot must “electronic flight instrument display” and/or a devote attention. The attitude direction glass flight deck. In general aviation there is indicator (ADI) is an example of such typically a primary flight display (PFD) and a technological advancement. multi-function display (MFD). Although both displays are in many cases identical, the PFD A flight director incorporates the ADI within its provides the pilot instrumentation necessary system, which is further explained below (Flight for flight to include altitude, airspeed, vertical Director System). However, an ADI need not velocity, attitude, heading and trim and trend have command cues; however, it is normally information. equipped with this feature. Glass flight decks (a term coined to describe electronic flight instrument systems) are Electronic Flight Instrument Systems becoming more widespread as cost falls and Modern technology has introduced into dependability continually increases. These aviation a new method of displaying flight systems provide many advantages such as being instruments, such as electronic flight lighter, more reliable, no moving parts to wear instrument systems, integrated flight deck out, consuming less power, and replacing displays, and others. numerous mechanical indicators with a single glass display. Because the versatility offered by glass displays is much greater than that offered by analog displays, the use of such systems will only increase with time until analog systems are eclipsed. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 45  BASIC AIRCRAFT INSTRUMENTS Primary Flight Display (PFD) Synthetic Vision PFDs provide increased situational awareness Synthetic vision provides a realistic depiction of to the pilot by replacing the traditional six the aircraft in relation to terrain and flight path. instruments used for instrument flight with an Systems such as those produced by Chelton easy-to-scan display that provides the horizon, Flight Systems, Universal Flight Systems, and airspeed, altitude, vertical speed, trend, trim, others provide for depictions of terrain and rate of turn among other key relevant course. Figure 3-46 is an example of the indications. Examples of PFDs are illustrated in Chelton Flight System providing both 3- Figure 3-45. dimensional situational awareness and a synthetic highway in the sky, representing the desired flight path. Synthetic vision is used as a PFD, but provides guidance in a more normal, outside reference format. Figure 3-45. Two Primary Flight Displays (Avidyne on the Left and Garmin on the Right). MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 46  BASIC AIRCRAFT INSTRUMENTS Multi-Function Display (MFD) In addition to a PFD directly in front of the pilot, The key to ADS-B is GPS, which provides three- an MFD that provides the display of information dimensional position of the aircraft. As an in addition to primary flight information is used simplified example, consider air-traffic radar. within the flight deck. [Figure 3-47] Information The radar measures the range and bearing of an such as a moving map, approach charts, Terrain aircraft. The bearing is measured by the Awareness Warning System, and weather position of the rotating radar antenna when it depiction can all be illustrated on the MFD. For receives a reply to its interrogation from the additional redundancy both the PFD and MFD aircraft, and the range by the time it takes for can display all critical information that the the radar to receive the reply. other normally presents thereby providing redundancy (using a reversionary mode) not normally found in general aviation flight decks. Advanced Technology Systems Automatic Dependent Surveillance—Broadcast (ADS-B) Although standards for Automatic Dependent Surveillance (Broadcast) (ADS-B) are still under continuing development, the concept is simple: aircraft broadcast a message on a regular basis, which includes their position (such as latitude, longitude and altitude), velocity, and possibly other information. Other aircraft or systems can Figure 3-47. Example of a Multi-Function Display (MFD). receive this information for use in a wide variety of applications. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 47  BASIC AIRCRAFT INSTRUMENTS An ADS-B based system, on the other hand, would listen for position reports broadcast by the aircraft. [Figure 3-48] These position reports are based on satellite navigation systems. These transmissions include the transmitting aircraft’s position, which the receiving aircraft processes into usable pilot information. The accuracy of the system is now determined by the accuracy of the navigation system, not measurement errors. Furthermore the accuracy is unaffected by the range to the aircraft as in the case of radar. With radar, detecting aircraft speed changes require tracking the data and changes can only be detected over a period of several position updates. With ADS-B, speed changes are broadcast almost instantaneously and received by properly equipped aircraft. Figure 3-48. Aircraft equipped with Automatic Dependent Surveillance—Broadcast (ADS-B) continuously broadcast their identification, altitude, direction, and vertical trend. The transmitted signal carries significant information for other aircraft and ground stations alike. Other ADS- equipped aircraft receive this information and process it in a variety of ways. It is possible that in a saturated environment (assuming all aircraft are ADS equipped), the systems can project tracks for their respective aircraft and retransmit to other aircraft their projected tracks, thereby enhancing collision avoidance. At one time, there was an Automatic Dependent Surveillance—Addressed (ADS-A) and that is explained in the Pilot’s Handbook of Aeronautical Knowledge. MAINTENANCE TRAINING PROGRAM FOR TRAINING PURPOSES ONLY 48