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Instrument Systems Compasses (11.5.1.3) Learning Objectives 11.5.1.3.1 Describe the purpose and operation of direct-reading and remote-reading compasses (Level 2). 11.5.1.3.2 Describe the effects aircraft magnetism can have on compasses (Level 2). 11.5.1.3.3 Describe compass compen...
Instrument Systems Compasses (11.5.1.3) Learning Objectives 11.5.1.3.1 Describe the purpose and operation of direct-reading and remote-reading compasses (Level 2). 11.5.1.3.2 Describe the effects aircraft magnetism can have on compasses (Level 2). 11.5.1.3.3 Describe compass compensation and adjustment (Level 2). 11.5.1.3.4 Describe precautions involved with compass systems and components (Level 2). 11.5.1.3.5 Describe basic magnetic principles (S). 11.5.1.3.6 Describe basic compass terminology (S). 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 117 of 356 CASA Part Part 66 - Training Materials Only Magnetic Principles Introduction to Magnetic Principles The direct-reading compass was the rst of many airborne ight and navigational instruments to be tted to aircraft. The prime function of the compass was (and still is) to display the direction in which the aircraft is heading in respect to the Earth’s magnetic ux. Most light aircraft use the magnetic compass as the primary heading reference. In aircraft that employ a remote indicating compass system and radio navigational aids, it plays the role of a standby compass, used as a backup heading reference if the primary navigation systems fail. Terrestrial Magnetism The surface of the Earth is surrounded by a weak magnetic eld which terminates in two internal magnetic poles situated near the north and south geographic poles. The total magnetic force is resolved into a horizontal component and a vertical component. Only the horizontal component is useful in determining magnetic heading. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 118 of 356 CASA Part Part 66 - Training Materials Only The Earth’s Magnetic Field and the Magnetic Poles The Earth is a great sphere spinning in space, but it is also a huge permanent magnet with a magnetic north and a magnetic south pole. A freely suspended permanent magnet on the surface of the Earth will align itself with the lines of ux linking the two magnetic poles, and it will maintain this alignment anywhere on the surface of the Earth. The magnetic pole in the northern hemisphere is a magnetic south pole, and vice versa. This explains how the north pole of a suspended bar magnet (or a compass needle magnet) points its north pole towards north. The geographic and the magnetic poles are not located together. The Earth’s magnetic south pole is located at about 74°N 101°W. That location is 2000 km from the geographic North Pole. Another complicating factor is that over a period of years, the magnetic poles and the lines of ux that link them will shift. This slowly changing physical property of the Earth is not a major problem, but it is signicant enough that aeronautical charts must be periodically updated. © Aviation Australia Magnetic poles 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 119 of 356 CASA Part Part 66 - Training Materials Only Magnetic Variation or Declination The grid of an aeronautical navigation chart is aligned to the geographic north pole, i.e., towards True North (TN). A magnetic compass aligns to the Earth's magnetic ux, which may be oriented either side of true north. This is termed Magnetic North (MN). The difference between true north and magnetic north at a geographic location is termed magnetic variation, variation or declination. Variation is measured in degrees of error east or west of true north. Variation changes with geographic location, but it is not affected by aircraft heading. An installed compass and a compass that has been removed from the aircraft are both equally affected by variation. The magnetic variation map below shows lines of equal variation, called isogonic lines (or isogonals). Anywhere along an isogonic line, there is a constant angle between the magnetic and geographic north poles. Magnetic variation map of Australia and New Zealand region Brisbane is situated about halfway between a 10°E and a 12°E isogonal. This enables us to determine that the Brisbane area has a magnetic variation of approximately 11° east. Aeronautical charts also show isogonic lines. The pilot uses this information to calculate the magnetic heading that must be own to achieve any required true heading. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 120 of 356 CASA Part Part 66 - Training Materials Only East and West variation There is no practical way to compensate (adjust) a direct reading standby compass for variation because variation changes as the aircraft travels. Summary of Magnetic Variation Variation is the difference between true north and magnetic north at a specic geographical location (aircraft position). Variation does not change as aircraft heading changes. A compass cannot be compensated for variation. The pilot must take variation into account and correct for it when navigating. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 121 of 356 CASA Part Part 66 - Training Materials Only Magnetic Inclination or Dip In a magnet, the lines of force emerge vertically at the north magnetic pole and descend vertically into the south magnetic pole. The Earth's lines of force behave in a similar manner; the lines emerge at the South Pole and return at the North Pole. Since these lines of force form great arcs around the surface of the Earth, they will only be horizontal to the surface at the place known as the magnetic equator. The angle the lines of force make with the Earth’s surface at any given place is called magnetic inclination or magnetic dip. The angle varies from zero degrees at the magnetic equator to 90 degrees at the magnetic poles. © Aviation Australia Magnetic inclination 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 122 of 356 CASA Part Part 66 - Training Materials Only Direct-Reading Compass Introduction to Direct-Reading Compass A direct-reading compass is a device which incorporates both the magnetic sensing element and display element in one component. The primary function of a direct-reading magnetic compass is to show the direction in which an aircraft is heading with respect to the Earth's magnetic meridian. On the modern jet, they serve as the standby compass, whilst on the light aircraft, they act as the primary heading reference. There are two common types of direct reading compasses in regular use: Vertical panel mounted compass of American design Panel mounted type Suspended type of British design. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 123 of 356 CASA Part Part 66 - Training Materials Only Suspended Compass Type Both compasses have similar construction details but differ in size and shape. Direct-Reading Compass Construction Direct-reading compass The compass consists of a non-magnetic metal or plastic case which houses the magnet system. This is a lightweight alloy or plastic azimuth card or dial that is mounted upon the oat assembly and an annular cobalt steel magnet. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 124 of 356 CASA Part Part 66 - Training Materials Only Magnet Systems Magnet systems consist solely of a single annular cobalt-steel magnet, to which is attached a compass card. The suspension consists of an iridium-tipped pivot secured to the centre of the magnet system and resting in a sapphire cup supported in a holder or stem. The use of iridium and sapphire in combination provides hard-wearing properties and reduces pivot friction to a minimum. The compass card is graduated in 5- or 10-degree increments identied every 30 degrees and having the four main cardinal points identied by the letters N, S, E and W for north, south, east and west. When read, the compass card is referenced against a lubber line xed to the interior of the bowl and lying on or parallel to the longitudinal axis when the compass is installed in an aircraft. Magnet system assembly To be able to determine magnetic heading, a compass must align itself with the horizontal component of the lines of force. To maximise this requirement, the compass magnet system is mounted pendulously, with the magnet below the point of suspension to counter the effect of dip. Gravity acting on the magnet assembly pulls it into a smaller angle. This method reduces the apparent dip angle in aircraft compasses to approximately 3 degrees between the latitudes 60° north and south. Above these latitudes, the dip angle (the vertical component of the lines of force) makes a magnetic compass unreliable. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 125 of 356 CASA Part Part 66 - Training Materials Only © Aviation Australia Magnetic inclination It is also normal practice for the compass magnet system to be counterbalanced for the region in which the compass is operated. A direct-reading compass counterbalanced for use in the higher latitudes of the northern hemisphere would be virtually unreadable in Southern Australian states. This is because the counterbalance weight would in fact be amplifying the dip angle. Always check that a new compass from overseas has been calibrated for operation in southern latitudes before use on Australian aircraft. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 126 of 356 CASA Part Part 66 - Training Materials Only © Aviation Australia Pendulous card assembly Compass Fluid The primary reason for lling compasses with a liquid is to make them aperiodic. The magnet system of any compass must be designed so as to be as dead beat or aperiodic in its operation as possible, which means that it must follow the magnetic meridians in the minimum of time without oscillating. This is achieved by using damping devices such as laments or wires (often referred to as the spider) and liquid. Movement of the magnet system and its spider does not cause swirling in the liquid as a whole, but rather, it sets up small eddies which are quickly dissipated. The other reasons are that the liquid will steady the magnet system and give it buoyancy, thus reducing the weight on the pivot and lowering the effects of friction and wear. The liquids are normally mineral or alcohol; however, some special silicone liquids are now being used. Desirable properties of a liquid include having a low freezing point, having a low viscosity, having no corrosive properties and remaining free from discolouration. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 127 of 356 CASA Part Part 66 - Training Materials Only Liquid Volume Compensation Compass liquids are subject to expansion and contraction with changes in temperature, and the resulting changes in their volume need to be accommodated. Every liquid-lled compass contains a exible element such as a bellows or a corrugated diaphragm which forms the rear part of the bowl. When the compass is subject to low temperatures, the volume of the liquid decreases and the exible element is able to expand to compensate. Likewise, an increase of temperature causes the exible element to compress to compensate for the increased liquid volume. External leaks will result from liquid expansion, and low temperatures contract the liquid, causing air bubbles to appear. In both cases, the compass becomes unserviceable. Compass Illumination In order to be useable for night-ying or ying in low light situations, the direct-reading compass will be illuminated by a small bulb. Because the passage of an electric current causes a magnetic eld which will affect the accuracy of the compass, very low power lighting is used. Power to supply this is usually fed through either screened cable or a twisted pair. Both types of cable minimise the magnetic eld produced by current ow in the wire. Deviation Deviation is an error in the indication of an installed magnetic compass caused by local magnetic elds in the aircraft. A compass that has been removed from the aircraft is not affected by deviation. Deviation error changes as aircraft heading changes. Deviation error is measured during a compass calibration. If the measured deviation is out-of-limits, it can be minimised by adjusting the compass. The adjustment phase of the calibration procedure is called compass compensation (compass swinging). The process involves aligning the aircraft on a series of known headings and adjusting the compensating magnets to obtain the minimum possible residual deviation error. The recorded deviation for each Magnetic Heading (MH) is used to calculate the corresponding Compass Heading (CH). This information is compiled into a compass correction card that is mounted close to the compass. During ight, the pilot can correct for deviation errors by using the compass correction card (compass calibration card). Compass correction card 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 128 of 356 CASA Part Part 66 - Training Materials Only Direct Reading Compass Compensation Any components tted to an aircraft that are magnetisable, i.e., contain iron, have the potential to distort the Earth's magnetic eld. This distortion, named deviation, is sensed by the compass and erroneous readings will occur. Every compass is tted with a compensator which is calibrated during a compass swinging process to minimise the effects of the local magnetic disturbances. The compensator on direct-reading compasses is a mechanical device. It consists of two pairs of magnets, with each pair being tted in bevel gears made of a non-magnetic material. The gears are mounted one above the other so that in the neutral condition, one pair of magnets lies longitudinally and the other pair lies laterally. Compensator construction Production of magnetic elds required for correction is obtained by rotating a small bevel pinion which meshes with the bevel gears, causing them to rotate in opposite directions. The magnets are thus made to open up in the manner of a pair of scissors, producing a magnetic eld between the poles, which is in a direction dependent on the direction of rotation of the pinion. When the compass is adjusted during a compass compensation (compass swing), the laterally orientated magnets are adjusted to compensate for an error that affects the compass on north/ south headings and is known as coefcient C. The longitudinally orientated magnets are adjusted for an error that affects the compass on east/west headings and is known as coefcient B. The adjusting pinions are accessible on the front face of the compass and identied appropriately. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 129 of 356 CASA Part Part 66 - Training Materials Only Direct-reading compass B/C compensators Direct-reading compass B/C compensators 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 130 of 356 CASA Part Part 66 - Training Materials Only In-Flight Errors Earlier in this section, it was stated that a compass card is made pendulous to minimise the effects of magnetic dip. Pendulous card This allowed the compass to operate at the higher latitudes without an excessive tilt angle which compromises accuracy. Unfortunately, any manoeuvre of the aircraft during ight introduces a component of aircraft acceleration, causing the pendulous card to be displaced from the vertical. The magnet system is now inuenced by the vertical component of the Earth's magnetic eld and attempts to rotate to align with it, even though the aircraft’s heading might not have changed. The effect of acceleration on the pendulous card can be classied into two distinct errors: Turning error Acceleration error. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 131 of 356 CASA Part Part 66 - Training Materials Only Turning Error When an aircraft is in a coordinated turn, the forces acting on the centre of gravity will cause the pendulous card to remain vertical to the plane of the aircraft. As the aircraft turns, the card pivot point is carried along with the aircraft, but the cards centre of gravity is caused to swing outward and rotate due to the force of centrifugal acceleration produced by the turn. Whenever the magnet system is tilted, as in any turn, the card’s magnet system is free to rotate under the inuence of the vertical component of the Earth's magnetic eld. The extent and direction of turning error is dependent on the aircraft heading, the hemisphere in which the aircraft is ying, the magnetic angle of dip (vertical component) and the angle of tilt of the magnetic card. Turning errors are greatest when an aircraft is turning from either a northerly or southerly heading. The error generated in the southern hemisphere is a mirror image to that produced in the northern hemisphere. Turning error 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 132 of 356 CASA Part Part 66 - Training Materials Only Acceleration Error Any speed change of the aircraft will be directly matched by the card suspension point, but the cards inertia, due to being pendulous, will lag behind the change in acceleration. The card will tilt in response to these forces and the card’s magnet assembly is free to rotate and align with the vertical component of the Earth's magnetic eld. The extent and direction of acceleration error is dependent on the aircraft heading, the hemisphere in which the aircraft is ying, the magnetic angle of dip (vertical component) and the angle of tilt of the magnetic card. Acceleration error is greatest when an aircraft is accelerating in either a westerly or easterly heading. The error generated in the southern hemisphere is a mirror image to that produced in the northern hemisphere. If an aircraft in the southern hemisphere on an easterly heading increases its speed, the acceleration force displaces the magnet system and allows it to rotate in a anticlockwise direction, indicating an apparent turn towards the south. When the aircraft decelerates, the reverse action takes place and the effect is for the magnet system to rotate in a clockwise direction, giving an apparent turn to the north. Acceleration error 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 133 of 356 CASA Part Part 66 - Training Materials Only Remote-Reading Compass Introduction to Remote-Reading Compass An instrument panel usually has a concentration of magnetic elds around it due to the location of electrical wiring looms in the vicinity of the cockpit. So the instrument panel is not the ideal place to install a direct-reading compass. To eliminate the problem of the interfering magnetic elds, it is much better to mount the magnetic compass sensing element as far away as possible from the inuence of soft and hard iron magnetism and electrical interference. A pure remote-indicating compass system consists solely of a magnetic eld detector and a heading indicator. This simple system suffers many inherent problems. As electronics and gyro systems have developed signicantly, it is now standard for remote-indicating compass systems to incorporate a directional gyro. The gyro’s inherent stability, especially during turns and acceleration changes, coupled with a magnetic eld detector or ux valve, provides respectively short- and long-term reliability of the display of the aircraft’s magnetic heading. Slaved remote compass 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 134 of 356 CASA Part Part 66 - Training Materials Only Flux Valve The ux valve, of similar design in all remote indicating compasses, detects the Earth's magnetic eld as an electromagnetically induced voltage. The sense element is pendulous and suspended from a central point of the case by a universal joint known as a Hooke’s joint. This allows the element to tilt to about 25°. There is no freedom in azimuth, so turning and acceleration errors are non-existent. As with a direct-reading compass, navigating in the vicinity of the poles remains difcult. The whole sensing element is enclosed in a sealed bowl and immersed in a damping oil to minimise pendulous jarring caused by rapid attitude changes. Flux valve assembly The ux valve sense element consists of a central point or hub, to which is attached three spokes set 120° apart. Each spoke has a top and bottom leg, insulated from each other. Each spoke has a collector horn which acts as one of three individual ux collectors. The spokes and horns are manufactured from laminated Permalloy. The characteristic property of this material is that it is easily magnetised, but it loses all its magnetism once the external force is removed. So it is a soft iron magnet. A coil, wound around the hub of the wheel, is the primary winding, which is energised with an AC voltage. Each spoke has a secondary winding, centre tapped together and providing a three wire output. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 135 of 356 CASA Part Part 66 - Training Materials Only Flux valve construction When the ux valve is not subjected to any external magnetic eld, the magnetic eld in each spoke, induced by the primary winding, induces an equal voltage into each of the secondary windings. With the ux valve exposed to the Earth's magnetic eld, the collector horns help concentrate the magnetic eld, which takes the path of least resistance through the legs. The direction and intensity through each leg depends on the angular relationship of the ux detector to the Earth's magnetic eld. With the primary winding energised, the voltage induced into each of the secondary coils is modied by the inuence of the Earth's magnetic eld. This change in voltage output from the secondary coils is very small, but has the effect of producing an output indicative of the aircraft’s heading. This output is based on the same principle as an AC synchro output and, once amplied, can be distributed to all aircraft systems dependent upon magnetic heading information. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 136 of 356 CASA Part Part 66 - Training Materials Only Flux valve operation 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 137 of 356 CASA Part Part 66 - Training Materials Only Flux Valve Location Flux valves should be positioned well away from any magnetic inuences. This is usually in the wingtips or at the top of the tailn. Aircraft designers will select the most appropriate place for ux valve positioning. The ux valve location, mounting screws and panel screws in the vicinity of the ux valve must have non-magnetic properties to minimise the effects of local magnetism. Some aircraft have two ux valves tted to provide for redundancy, with each one supplying its own remote compass system. One system provides compass information to the pilot’s displays and the second system for the co-pilot’s displays. Flux valve location Slaved Gyro Most remote compass system commonly use an electrically driven directional gyro, often remotely mounted, to provide the short term stability of magnetic heading presentation. Remote compass system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 138 of 356 CASA Part Part 66 - Training Materials Only The gyro erection or levelling system maintains the inner gimbal perpendicular to the outer gimbal of the directional gyro. The erection system uses a position switch, or gravity sensing mercury switch, mounted on the inner gimbal and an erection torque motor The gyro slaving system detects any mismatch between the ux valve and the gyro heading output caused by real and apparent gyro drift. The error signal, after amplication, is applied to an inner gimbal torque motor to precess the outer gimbal in azimuth, at a rate between 1 and 2 degrees per minute. When the remote compass system is initially energised, the slaving system automatically enters a fast-align mode to slave the gyro to align with the magnetic heading. Once the error signal is within dened limits, the slaving system reverts to normal operation, and the compass system is said to be synchronised. When synchronised, the outer gimbal azimuth position is relayed by a synchro to the heading indicator and can be monitored by the pilot. This process of slaving the gyro to the magnetic heading is the origin of the name slaved gyro. Remote directional gyro 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 139 of 356 CASA Part Part 66 - Training Materials Only Indicating Elements Magnetic heading information is presented in a vertical card format on a variety of instruments depending on the system. The compass card in various systems is driven by a servomechanism which rotates in response to heading changes. The aircraft’s magnetic heading is read off under the lubber line placed at the top of the dial. The compass system’s synchronisation with the Earth's magnetic eld is monitored by a dot/cross ag mounted in the bezel. When the pointer is between the dot/cross, the compass system is synchronised. A knob on the bezel, when pushed and turned, allows rapid manual synchronisation of the card heading to the ux valve heading. Compass indicating elements The indicators shown above (from the left) are Horizontal Indicator (HI) and Course Deviation Indicator (CDI). 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 140 of 356 CASA Part Part 66 - Training Materials Only Compass Controller On more complex systems, a separate compass control panel may be incorporated. This has the advantage of de-cluttering the instrument bezel/display. Selections available are same as those previously covered. Synchronising indicator Mode selector – DG, slaved or synchronised (used in conjunction with a synchronisation adjustment knob) When DG is selected on the compass panel, the following happens: DG is free of MAG inputs. The directional gyro may be slewed to any position at two different speeds. In this case, the SYNC operator does not work because there are not magnetic indicators to synchronise. When Slaved (SLA) is re-selected, the DG will align to magnetic north as received from the MAG Flux at a rapid rate. Warning light – when not synchronised Course/heading set knob – controls heading bug on indicator Latitude setting knob as described in overcoming problems associated with navigation in polar regions. Compass control panel 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 141 of 356 CASA Part Part 66 - Training Materials Only Aircraft Magnetism and Its Effect on Compasses Introduction to Aircraft Magnetism and Its Effect on Compasses All aircraft are themselves in possession of magnetism in varying amounts. Such magnetism is of course a potential source of error (deviation) in the indications of compasses installed in any type of aircraft and is unavoidable. The two types of aircraft magnetism can be divided in the same way that magnetic materials are classied according to their ability to be magnetised, namely, hard iron and soft iron. Hard Iron Hard iron magnetism can be described as the resident permanent magnetic elds present within the aircraft. Iron and steel parts of the aircraft structure become magnetised due to the Earth's magnetic eld building itself into the ferrous parts during construction or when the aircraft is left on one heading for lengthy periods. The strength of these components will not vary with heading or change of latitude but may vary with time due to a weakening of the magnetism in the aircraft. The effect of hard iron or permanent magnetism deviation is a single-cycle error; all readings are distorted in the direction of the magnet, crossing the axis of “no error” at the two points (one complete cycle) where the magnetic vector is aligned with the Earth's magnetic eld. Hard iron errors may be created by magnets in speakers, generators or DC motors, electrical current in a wire or even the alignment of the magnetism in the fuselage of an aircraft sitting or heading in the same direction for a long period of time. Hard iron errors 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 142 of 356 CASA Part Part 66 - Training Materials Only Soft Iron Metals which are easy to magnetise (silicon iron for example) and generally lose their magnetic state once the magnetising force is removed are classied as soft. Soft iron magnetism is of a temporary nature and is caused by metallic parts of the aircraft which are magnetically soft becoming magnetised due to induction by the Earth's magnetic eld. The effect of this type of magnetism is dependent on aircraft heading and attitude in its geographical position. An aircraft is effectively a cross of wings and the fuselage. So when the aircraft is heading north or south, the Earth's magnetic eld runs directly down through the fuselage and doesn’t induce any errors because it is aligned with the Earth's magnetic eld. The same occurs on east and west headings, but in this case, the Earth's magnetic eld runs directly down through the wings, again not inducing any error in the compass reading. The greatest soft iron magnetism errors are induced on NE, SE, SW and NW headings when the Earth's magnetic eld is distorted with lines of ux bending to run through the fuselage and wings, inducing errors into the compass. This error is represented by a two-cycle error. Soft iron 2 cycle error Deviation Coefcients Before steps can be taken to minimise the deviations caused by hard and soft iron components of aircraft magnetism, their values on each heading must be obtained and quantitatively analysed into coefcients of deviation. In total, there are ve coefcients, designated A, B, C, D and E. Practical compass compensation deals only with coefcients A, B and C. Coefcient A This refers to installation error and is corrected by rotating the compass or ux valve until the compass reads the corrected heading. These components have slotted mounting holes to allow movement in azimuth. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 143 of 356 CASA Part Part 66 - Training Materials Only Coefcient B Coefcient B refers to deviation produced by soft and hard iron magnetism creating an imaginary magnet lying along the fore and aft axis of the aircraft fuselage. Because the Earth's lines of ux travel north-south, when the aircraft is heading north or south, the fuselage magnetism will not modify the magnetic meridian; it will only strengthen or weaken it because the Earth's and aircraft’s magnetic ux are in alignment. So the magnetism in the fuselage, which is termed coefcient B error, affects compass accuracy predominantly on east-west headings and has no effect on north-south headings. One pair of compensator magnets are located in the lateral axis across the aircraft and, when adjusted, will correct for coefcient B. Coefcient C Coefcient C refers to deviation produced by an imaginary magnet lying along the aircraft’s lateral axis or wingspan. When ying east or west, lateral magnetism will not modify the magnetic meridian. It will only strengthen or weaken it because the Earth's and aircraft’s magnetic ux are in alignment. When ying on the north or south headings, the Earth's eld is distorted through the aircraft by some value. So the magnetism in the wingspan, which is termed coefcient C error, affects compass accuracy predominantly on north-south headings and has no effect on east-west headings. One pair of compensator magnets are located in the longitudinal axis and, when adjusted, will correct for coefcient C. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 144 of 356 CASA Part Part 66 - Training Materials Only Compass Calibration Proforma Use this proforma, as it simplies compass compensation process. Compass calibration proforma Now that the compensation process is complete, the compass must be calibrated, that is, all residual deviations must be identied and recorded on a compass calibration card. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 145 of 356 CASA Part Part 66 - Training Materials Only Check calibration 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 146 of 356 CASA Part Part 66 - Training Materials Only Compass Compensation and Adjustment Introduction to Compass Compensation and Adjustment This procedure is performed when the following occurs: The compass system has reason to be suspected of inaccuracies At specied time intervals in the maintenance manual or by regulation After an airframe or engine component change that may affect the compass readings After a major check or repair procedure has been performed on the aircraft After a compass has been removed and replaced After a lightning strike After a major modication to an aircraft system. Methods of Compass Swinging There are three possible methods of carrying out a compass swing, using as a reference: The comparison method An electronic compass calibrator A datum or landing compass. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 147 of 356 CASA Part Part 66 - Training Materials Only Remote Compass Compensation Even though the ux valve is remotely mounted, it is still inuenced by the effects of magnetic materials in the aircraft. These inuences are detected during a compass swing and corrected in the same manner as for direct reading compasses. However, most ux valve compensators consist of an electronic circuit instead of positioning magnets. Two variable potentiometers are electrically connected to the secondary output windings of the ux detector. The potentiometers correspond to the co-efcient ‘B’ (deviations on east/west headings) and ‘C’ (deviations on north/south headings) magnets of the mechanical compensator. When the potentiometers are rotated with respect to calibrated dials, they inject very small DC currents into the ux detector coils. The polarity and magnitude of the elds produced by the currents are sufcient to oppose those causing the deviations. Electronic compensator Comparison Method Instead of using an outside reference, the comparison method uses the aircraft’s higher level system to swing a lower level compass. For example, an inertial navigation system can be used to swing a remote or direct reading compass. Likewise, a remote compass can be used to swing a standby compass. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 148 of 356 CASA Part Part 66 - Training Materials Only Electronic Compass Calibrator Instead of physically repositioning the aircraft to each heading, the electronic calibrator alters the alignment of a magnetic eld around a ux valve so it can be checked on all headings. Complex aircraft operated by large organisations may have access to an electronic compass calibrator system. Datum Compass The datum compass or landing compass is either tripod mounted or hand-held and used at a certied compass swinging site. The aircraft is positioned at the centre of the site and aligned to each heading sequentially. At each heading, the datum compass operator takes a reading by sighting an external reference point on the aircraft to obtain a magnetic heading unaffected by local deviation. The datum compass reading is compared with the aircraft compass, and the aircraft’s magnetic heading and any deviation are recorded. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 149 of 356 CASA Part Part 66 - Training Materials Only Compass Systems and Components Precautions Introduction to Compass Systems and Components Precautions Use only non-magnetic tools when performing any maintenance on or around direct-reading compasses or ux valves. Use non-ferrous tools to adjust a compass; otherwise, the compass will veer signicantly, and you will be unable to make the compass read what you have calculated it should read. Never expose direct-reading compasses or ux valves to magnetic elds. Only non-ferrous panels and screws may be tted in vicinity of direct-reading compasses and ux valves. To reduce magnetic interference from compass light electrical leads, ensure leads are either screened cable or twisted pair to cancel magnetic effects. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 150 of 356 CASA Part Part 66 - Training Materials Only Compass Swing Precautions An aircraft compass needs to be swung in an area which is free from all unusual magnetic inuences other than those derived from the aircraft. The area should not be near any hangars or other buildings, electronic transmitting devices, underground or above-ground power cabling, piping or plumbing or any other metal objects including concrete reinforcing, which will cause distortion of the Earth's magnetic eld. The area needs to be large enough to allow the aircraft to be manoeuvred around and have the datum compass approximately 50 metres away. The aircraft should be prepared in the normal in-ight condition; that is, all equipment in the correct stowage position. Never leave any ferrous material in the vicinity of a direct reading compass, or a ux valve, during a calibration or compensation, as this will corrupt the accuracy of the compass. For example, when you remove a spanner at completion of the calibration, the compass calibration will no longer be accurate. This is because the system is only compensated while the spanner remains where it sat during the process. Some aircraft require a nal check to be carried out with the engine running. If this is the case, an authorised engine runner should be available. Use only non-magnetic tools for adjusting the correctors. Usually there will be a surveyed compass swing site established at major airelds and compass swings should only be performed at this site. Compass swing site 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 151 of 356 CASA Part Part 66 - Training Materials Only Compass Check Swing Procedure Sometimes a check is required rather than a full swing. Measurements are made at the cardinal points (north, south, east and west) then compared with previous compass swing records to determine serviceability. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 152 of 356 CASA Part Part 66 - Training Materials Only Compass Terminology Terminology Calibration is the measurement of the deviation of a compass installed in an aircraft, any necessary compensation of this deviation and the recording of the residual deviation. Compensation is the adjustment of a compass to minimise deviation errors caused by the magnetic properties of an aircraft. It is also referred to as Compass Swinging. Compass Correction Card is a card mounted near a compass in full sight of the pilot. It indicates the difference between the compass reading and the actual magnetic heading. It is also referred to as a Compass Calibration Card. Deviation is an error in the indication of a magnetic compass caused by local magnetic elds in the aircraft. To correct for deviation, the pilot must algebraically add the angle to a compass reading to obtain the aircraft magnetic heading. Direct Reading Compass is a compass which has the magnetic sensing element and heading indication located in the one instrument. Remote Reading Compass (non-stabilised) is a remote indicating compass without gyroscopic means of stabilisation or smoothing. It is referred to in this text as a pure remote indicating compass system. Remote Reading Compass (stabilised) is a compass system which has the magnetic sensing element located remotely from the indicator(s) together with gyroscopic means to stabilise or smooth the heading indications. Residual Deviation is the deviation remaining after compensation. Standby Compass is a direct-reading compass which is not used as the primary heading reference. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 153 of 356 CASA Part Part 66 - Training Materials Only Instrument Systems Other (11.5.1.4) Learning Objectives 11.5.1.4.1 Describe the term angle of attack (S). 11.5.1.4.2 Describe the term stall (S). 11.5.1.4.3 Describe the purpose and operation of angle of attack indication systems and components (Level 2). 11.5.1.4.4 Describe the purpose and operation of stall warning systems and components (Level 2). 11.5.1.5 Describe the features and benets of an aircraft glass cockpit for commercial and general aviation applications (Level 2). 11.5.1.6 Describe the purpose and operation of other aircraft indicating systems and components (Level 2). 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 154 of 356 CASA Part Part 66 - Training Materials Only Angle of Attack Indication Introduction to Angle of Attack Indication The stall angle is the angle of attack at which the airow over the wing is no longer even but instead starts to break away and create a turbulent ow, destroying the lift. For most wing congurations, there is a set angle at which this occurs. Effect of excessive angle of attack on an aerofoil To sense these angles, a system of angle of attack sensors has been incorporated into aircraft design. Angle of Attack The Angle of Attack (AoA) is the angle between the chord line of the aerofoil and the direction of the relative wind. It is important in the production of lift. Lift acts perpendicular to the relative wind regardless of the angle of attack. Measuring angle of attack During ight, the angle of attack is changed any time the control column is moved forward or aft, and the coefcient of lift is changed at the same time. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 155 of 356 CASA Part Part 66 - Training Materials Only As AoA increases, lift increases. This continues to a point where lift peaks (CL Max). In the example below, this point of maximum lift is at about 17° (most aerofoils are between 12° and 18°). If the maximum lift angle is exceeded, lift decreases rapidly and the wing stalls. For a given aircraft, a stall always occurs at the same angle of attack regardless of airspeed, ight attitude or weight. This is the stalling or critical angle of attack. It is important to remember that an aircraft can stall at any airspeed, in any ight attitude or at any weight. Lift versus angle of attack 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 156 of 356 CASA Part Part 66 - Training Materials Only Stall Any aircraft wing will produce lift when air passes over it. The value of the lift is directly related to: Density of the air Size and shape of the aerofoil Angle of attack. Speed and loading affect the aircraft stall, in that at lower speed, a higher AOA must be maintained to provide sufcient lift to keep the aircraft at a constant altitude. If the aircraft is heavy, a higher AOA or higher airspeed (to produce more lift) is required and must be maintained. So more weight or slower airspeeds mean an aircraft is at a higher AOA and therefore closer to the critical attack angle, causing a stall. It is the AOA which causes the stall, not the weight or airspeed. When the AOA reaches the point of stalling, the air is separating from the leading edge, and the wing is producing no lift. This angle is termed the stalling or critical angle of attack. Note: The wing begins to stall as the separation moves forward (nearing critical AOA), and the wing is only fully stalled at the critical AOA. Stall progression as angle of attack increases To recover from a stall, smooth airow must be restored. The only way to do this is to decrease the angle of attack to a point below the stalling or critical angle of attack. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 157 of 356 CASA Part Part 66 - Training Materials Only Angle of Attack Indication An aircraft will, in its own characteristic manner, provide warning of a stalled condition by buffeting, gentle or severe pitch down attitude change and/or wing drop. Although recoverable, in a situation such as an approach, when an aircraft is running out of airspace beneath itself, these inherent warnings could come too late. It is therefore necessary to provide a means whereby the angle of attack or Alpha angle can be sensed directly and at some value just below that at which a stalled condition can occur. One of the methods utilised by pilots to monitor their aircrafts’ ying characteristics is to provide them with an indication of AOA. This is achieved by measuring the angle between the aircraft’s chord line and the surrounding airow. The AOA detecting element must therefore be able to detect and measure the angle at which the aircraft is cutting through the air. Angle of attack indicators There is no standard requirement for angle of attack indicators to be installed in aircraft. When selected for installation, they must only be used as a supplement to an appropriate type of aural stall warning, with or without a stick-shaker system, and a stall avoidance stick-pusher system. The pilot can move a bug around the indicator bezel to set up the optimum attitude or pitch angle. The attitude angles change for climb, cruise, descent and landing. In aircraft having electronic ight instrument display systems, the data can be programmed into computers and displayed against a vertical scale. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 158 of 356 CASA Part Part 66 - Training Materials Only Angle of Attack Sensors The sensors are located on the side of the fuselage, normally below the cockpit oor level, and mounted from the inside. To compensate for asymmetric airows during turns or large crosswinds, two sensors are installed on either side of the fuselage. The sensor output, produced by a potentiometer or transducer, is used to drive a pointer in the AOA indicator and is often shared with the stall warning, autopilot and windshear systems. The most common angle of attack sensor or probe is the air pressure chamber type. Other sensor types are the vane or air ow type. The Probe Sensor The AOA probe type sensor is installed so that it senses the airow relative to the fuselage datum line. When equal airow pressure is passing into the two equal-sized slots in the leading edge of the probe, the air pressure on either side of the vane is equal, and the vane takes up a null position. When the aircraft takes up another attitude, the airow in the two slots becomes unequal, and the vane will be pushed from its null position to one side by the unbalanced air pressure. Attached at the end of the vane pivot shaft is a potentiometer, and any rotation of the shaft will produce a changed electrical output. The potentiometer output is fed to the cockpit indicator, and the pointer will take up a corresponding attitude position. Probe-type sensor 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 159 of 356 CASA Part Part 66 - Training Materials Only The Vane Type Sensor The vane type consists of a precision counterbalanced aerodynamic vane which positions the rotor of a synchro or the wiper of a potentiometer. The vane is protected against ice formation by an internal heater element. The complete unit is accurately aligned by means of index pins at the side of the front fuselage section of an aircraft. The airow passes over a wedge-shaped vane attached to a pivot arm. When the airow is equal on either side of the vane, the vane will be stationary. As the aircraft takes up another attitude, the airow changes, causing the vane to rotate on its pivot, moving the position transmitter attached to its shaft. A course scale may be found on the exterior housing near the vane arm pivot point. Vane type sensor Angle of Attack Sensor Precautions AoA sensors incorporate a heater element to prevent inight freezing. In some cases, the heater element is switched to half power when the AoA system senses the wheels are on the ground. This prevents overheating and the burning out of the element. Inight excess heat is removed by the airow. Aviation Australia Caution Care must be taken when handling the probe-type sensor to avoid damage to the calibrated slots. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 160 of 356 CASA Part Part 66 - Training Materials Only When moving the vane-type sensor, care must be taken to avoid damage to the ne movement of the transmitter shaft. AoA probes must be aligned and located correctly when being installed. Ensure datum points, lines and direction arrows are observed. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 161 of 356 CASA Part Part 66 - Training Materials Only Stall Warning Systems Stagnation Point The stagnation point is where the air separates to go above or below the wing’s leading edge. As AoA increases, the stagnation point gradually moves down leading edge of wing. Stagnation point A vibrating reed-type device is triggered by low pressure at stagnation point. The vane type is triggered by upward moving air when the stagnation point moves below the vane. Stall warning systems are required in all aircraft. They are designed to provide a clear warning when the angle of attack sensor detects a critical angle of attack. Low-speed general aviation aircraft are tted with one of the following: Vibrating reed Vane-operated switch. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 162 of 356 CASA Part Part 66 - Training Materials Only Vibrating Reed Operation A reed, like that found in a saxophone, produces a sound when the air is sucked through it. The reed is connected by tubing to a small hole in the leading edge of the wing near the stagnation point. With an increase in angle of attack, the low-pressure region over the wing moves into the area where the reed inlet is located, causing it to produce a sound to warn the pilot of an impending stall. The vibrating reed system requires no outside power source. Vibrating reed stall warning 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 163 of 356 CASA Part Part 66 - Training Materials Only Vane-Operated Switch The vane-operated switch consists of a hinged vane mounted in the leading edge of the wing so that the vane protrudes into the airstream, sensing the angle at which the air ows over the wing. When ying with the AOA well below the critical angle, the airow over the vane is downward and holds the internal electrical switch in the vane open. At higher angles of attack, the stagnation point moves downward until the airow over the vane is upward. This point would be just below the critical AOA, thereby warning the pilot of the impending stall condition before the wing actually stalls. The vane closes a switch to operate a stall warning device such as a: Horn Light Stick shaker. Vane-operated stall warning sensor 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 164 of 356 CASA Part Part 66 - Training Materials Only Vane-operated stall warning sensor 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 165 of 356 CASA Part Part 66 - Training Materials Only
