Angle of Attack PDF

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...

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 benets 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 airow over the wing is no longer even but instead starts to break away and create a turbulent ow, destroying the lift. For most wing congurations, 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 coefcient 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 sufcient 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 airow 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 airow. 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 airows 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 airow relative to the fuselage datum line. When equal airow 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 airow 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 airow passes over a wedge-shaped vane attached to a pivot arm. When the airow is equal on either side of the vane, the vane will be stationary. As the aircraft takes up another attitude, the airow 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 inight 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. Inight excess heat is removed by the airow. 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 airow 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 airow 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  Stick Shaker A stick shaker system is designed to provide the pilot with a feeling of buffeting of the separating air, indicating to the pilot that immediate action is required. A stick shaker is not normally tted to light general aviation aircraft. Stick shaking is accomplished by a motor which is secured to the control column and drives a weighted ring that is deliberately unbalanced to set up vibrations of the column. This form of warning is simply more demanding than a light or audible warning. The output of a vane-type stall warning detector energises the motor while sounding a warning horn and illuminating a light, warning the pilot that a stall is about to occur. Stall warning stick shaker 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 166 of 356 CASA Part Part 66 - Training Materials Only  Stall Warning Computer Commercial aircraft stall warning systems are designed to integrate more parameters, such as actual angle of attack, ap conguration, engine thrust settings and airspeed to provide a more comprehensive calculation of a stall warning trigger point. The type of AOA sensor normally used for these systems is of the same physical design as the vane- type sensor described above. However, the unit uses a more reliable synchro output. In most cases, two AOA detecting systems are installed and located on each side of the front fuselage section. Likewise, there are two computers, each driving a stick shaker on each pilot’s control column and providing a signal to an aural warning unit. The stall warning computer receives angle of attack information from the angle of attack sensor, temperature and airspeed information from the air data computer and ap and slat position information from the respective position sensors. Processing circuits determine the trip point at which the stall warning stick shaker and horn will activate. The system is energised whenever the aircraft is airborne and is deactivated on the ground by a squat switch or Weight-On-Wheels (WOW) switch. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 167 of 356 CASA Part Part 66 - Training Materials Only  Stall Warning Computer Operation When the aircraft is on the ground and electrical power is on, the contacts of the landing gear micro switches complete a DC circuit to a sensing relay K shown below. The AC ‘bias off’ voltage from the relay ensures any signal output from the amplier is attenuated to prevent the operation of the stick-shaker motor. The vane heater element circuit is also isolated from its AC supply by the opening of the second set of contacts of K1. The sensor synchro is supplied directly from the AC power source. At take-off, the air ground switch operates to de-energise relay K, and the system becomes fully activated. The only signal now supplied to the amplier and demodulator is the signal from the angle of attack sensor, modied by the ap position transmitter. If the aircraft’s attitude should approach that of a stalled condition, the demodulator produces a voltage which triggers the switch SS1 to connect a 28-V DC supply direct to the stick-shaker motor, which then starts vibrating the control column. Stall warning computer schematic 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 168 of 356 CASA Part Part 66 - Training Materials Only  Stall Avoidance Systems In some types of aircraft, particularly those with rear-mounted engines and a ‘T’-tail conguration, it is possible for what is termed a ‘deep’ or ‘super’ stall situation to develop. When such aircraft rst get into a stalled condition, the air owing from the wings is of a turbulent nature, and if the angle is such that the engines are subjected to this airow, loss of power will occur because of surging and possible ame-out. If the stall develops still further, the horizontal stabiliser will also be subjected to the turbulent airow with a resultant loss of pitch control. The aircraft then sinks rapidly in the deep stalled attitude, from which recovery is difcult, if not impossible. This was a lesson that was learned with tragic results during the ight testing of two of the earliest T- tail types of commercial aircraft, namely, the BAC 1-11 and HS Trident. To prevent the development of a deep stall situation, systems are installed which, in addition to stick- shaking, use the sensor signals and actuators which cause a forward push on the control columns, resulting in downward deection of the elevators. The way this is accomplished varies: A linear actuator is mechanically connected to the articial feel and centring unit of the elevator control system. Fly-by-wire aircraft have a signal transmitted to the elevator control channel of the ight control computer. Stall avoidance system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 169 of 356 CASA Part Part 66 - Training Materials Only  Temperature-Measuring Systems Introduction to Temperature-Measuring Systems Temperature is one of the basic parameters used to establish data vital to the control and performance monitoring of aircraft systems and engines. Temperature-Indicating Systems How high a temperature measurement needs to be governs the choice of substance property. On an aircraft, the range of temperature measurements can vary from an outside ambient air temperature of minus 60 °C to an engine exhaust temperature as high as 1000 °C. The basis of most forms of temperature measurement is the variation of a property of a substance with temperature. The substance property used provides a convenient means to classify an instrument measuring temperature. Expansion type (liquid or solid) Vapour-pressure type Electrical type (resistance or thermo-electric) Radiation type. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 170 of 356 CASA Part Part 66 - Training Materials Only  Expansion Type (Liquid or Solid) Most substances expand as their temperature rises. As a result, a measure of temperature is obtainable by taking equal amounts of expansion to indicate equal increments of temperature. Expansion type 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 171 of 356 CASA Part Part 66 - Training Materials Only  Bimetallic Temperature Indicator The sensor is made up of two metals, each having different temperature coefcients of linear expansion, welded together to form a spiral. The metals most used are brass and invar, which has a very low expansion coefcient. This type of indicator is normally found on aircraft that y at or below 150 knots. Above that speed, their location out in the slipstream interrupts the airow and causes unnecessary drag. One end of the spiral mounts to the end of the vented outer case, which protrudes into the airow, while the other end of the spiral attaches to a pointer, which is free to move over a dial. The dial mounts into the bezel assembly, which also supports the vented outer case and provides a mounting boss for the instrument. As the temperature varies, the spiral winds or unwinds, causing the pointer to move over the scale indicating the temperature. Helix-type bimetallic temperature indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 172 of 356 CASA Part Part 66 - Training Materials Only  Vapour-Pressure Type Many liquids, when subjected to a temperature rise, experience such motion of their molecules that there is a change of state from liquid to vapour. Temperature can be measured by taking equal increments of the pressure of the vapour to represent equal increments of temperature. The system consists of a bourdon tube indicator, a suitable length of capillary tubing and a bulb shaped sensor. The sensor contains a quantity of a volatile liquid, usually methyl chloride, and the capillary tube extends into the uid. The capillary tube and bourdon tube has all the air evacuated and lled with the uid before the system is sealed. Vapour-pressure type The capillary tube is usually made of annealed copper, which has lightly wound copper wire as a protective cover. Care is required to prevent tight bends, as the tube may crack causing a leak or kink and close off the tube, rendering the unit inoperable. The operation of the system is such that when the sensor is heated, the liquid vaporises, and the increase in pressure bears down on the uid, forcing it into the capillary tube and extending the bourdon tube. The lever attached to the end of the bourdon tube operates the sector gear to rotate the pinion, which moves the pointer over a scale marked in units of temperature. A reduction in temperature cools the vapour, which condenses, reducing the pressure in the bulb allowing the bourdon tube to contract. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 173 of 356 CASA Part Part 66 - Training Materials Only  Vapour-pressure temperature indicator 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 174 of 356 CASA Part Part 66 - Training Materials Only  Electrical-Type Temperature-Measuring Devices Resistance Temperature System A variable resistance temperature system consists of a sensor unit, generally referred to as a bulb, consisting of a coil of either nickel or platinum wire and a moving-coil-type indicator. The changing resistance in an electrical circuit modies the current driving the indicator. Resistance Temperature Bulb Conductive materials when subjected to changing temperatures also change their resistivity. Generally, these materials are categorised as either PTC (Positive Temperature Coefcient) or NTC (Negative Temperature Coefcient). PTC describes materials which increase their resistance with an increase in temperature. NTC encompasses materials which decrease their resistance with an increase in temperature. The two metals most commonly used in aircraft resistance thermometry are nickel and platinum, both manufactured to a high degree of purity and reproducibility of resistance characteristics. Platinum is a precious metal with a very stable and near linear resistance versus temperature function. The resistance value of both nickel and platinum increase with an increase in temperature and are PTC. Effect of temperature on resistance The bulb has the resistance element wound on an insulated former which connects to a two-pin plug via contact strips. The resistance element is at the bottom end of its former so that it is closest to sensed liquid or gas, thus minimising errors due to radiation and conduction losses. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 175 of 356 CASA Part Part 66 - Training Materials Only  A calibrating or balancing coil (resistor) achieves a standard constant temperature/resistance characteristic for each bulb and compensates for any slight change in the physical characteristics of the sensor material. The casing, which serves to protect and seal the coil assembly from the outside environment, is a stainless-steel tube closed at one end and secured to a union nut at the other. The union nut secures the complete unit to the mounting point. Resistance temperature bulb 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 176 of 356 CASA Part Part 66 - Training Materials Only  Wheatstone Bridge The most common method of measuring resistance is by means of the well-known Wheatstone bridge network. The circuit is made up of four resistances arms: R1 , R2 , R3 and Rx. A moving-coil or moving-spot galvanometer is connected across points B and D, and a source of low voltage is connected across points A and C. Current ows in the directions indicated by the arrows, dividing at point A and owing through R3 and Rx at strengths which we may designate respectively as 1 and 2. At point C, the currents reunite and ow back to the voltage source. Wheatstone bridge When the bulb is subjected to temperature variations, the resistance will vary, thus varying the current ow through the meter movement which is calibrated to indicate temperature proportional to the resistance of the temperature bulb. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 177 of 356 CASA Part Part 66 - Training Materials Only  Indicator There are different types of moving coil indicators. The commonly adopted conguration is the ratiometer. The system requires DC power supplied from a relevant busbar or by rectication of a single-phase AC supply. The theoretical circuit of the ratiometer system shows that the moving coil assembly consists of two coils mounted on a common former and connected in a manner so that the torques produced by the coils are in opposition. Ratiometer construction The two coils of insulated wire are wound on a common former attached to a spindle and pointer, supported in bearings and free to rotate within a non-uniform air gap containing a magnetic eld, produced by a permanent magnet and concentrated by an iron core. Ratiometer circuit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 178 of 356 CASA Part Part 66 - Training Materials Only  When the temperature of the sensor Rx increases, its resistance will increase, causing a decrease in the current owing in winding B and a corresponding decrease in the force created by it. With the current ratio altered, the constant torque of winding A will rotate the measuring element so that both windings rotate round the air gap. As a result, winding B advances further into the stronger part of the eld, and winding “A” moves toward the weaker part. When the sensor temperature stabilises at its new value, the torques produced by both windings will once again balance at a new current ratio, and the angular deection of the measuring element will be proportional to the temperature change. Thermocouple Systems If the ends of two dissimilar metals of similar length have their ends joined together to form a continuous circuit and one joint is heated, the circuit generates an Electromotive Force (EMF). Thermocouple circuit The magnitude of the EMF and the resulting current produced depends upon the combination of the metals, the difference in temperature between the hot and cold junctions of the metals and the resistance of the circuit. The indicator is an ammeter or current meter calibrated in temperature units. High temperature indication For their operation, thermocouple systems depend on electrical energy produced by the direct conversion of heat energy at the source of measurement. Thus, unlike variable resistance systems, they are independent of any external electrical supply. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 179 of 356 CASA Part Part 66 - Training Materials Only  The use of a thermocouple system is appropriate for very high temperature measurement and control applications, for example, engine turbine temperature and cylinder head temperature. As temperature is a critical variable of engine operation, the system can provide information for electronic fuel control units and or the crew can monitor the temperature of the engine at various critical locations to determine overall engine operating conditions. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 180 of 356 CASA Part Part 66 - Training Materials Only  Thermocouple Materials and Combinations There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance and application compatibility. Type J, K and T are “base metal” thermocouples, the most common types of thermocouples. K Type (Chromel-Alumel) Type K is the most common type of thermocouple. It is inexpensive, accurate, reliable and has a wide temperature range. Temperature Range -270 to 1260 °C (-454 to 2300 °F) Commonly used in Turbine Inlet Temperature (TIT) and Exhaust Gas Temperature (EGT) indicating systems. J Type (Iron-Constantan) Type J is very common. It has a smaller temperature range and a shorter lifespan at higher temperatures than type K. It is equivalent to type K in terms of expense and reliability. Temperature Range -210 to 760 °C (-346 to 1400 °F) Commonly used in EGT indicating systems in piston engines. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 181 of 356 CASA Part Part 66 - Training Materials Only  T Type (Copper-Constantan) Type T is a very stable thermocouple and is often used in extremely low temperature applications such as cryogenics or ultra-low freezers. Temperature Range -270 to 3700 °C (-454 to 7000 °F) Commonly used in Cylinder Head Temperature (CHT) indicating systems in piston engines. Colour standards for thermocouples 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 182 of 356 CASA Part Part 66 - Training Materials Only  Specialised Resistance Temperature Systems Air Temperature Measurement Static Air Temperature Measurement One very important parameter is the accurate provision of static air temperature for the calculation of functions like true airspeed, Mach number and engine power settings. Outside Air Temperature Measurement For low-speed aircraft, a conventional platinum resistance wire bulb or bimetallic type sensor protruding into the airstream is adequate for providing Outside Air Temperature (OAT) information. Generally, the OAT of the boundary layer air of an aircraft ying at speeds below 0.2 Mach number is very close to Static Air Temperature (SAT). On high-speed aircraft, the temperature reading becomes corrupt because of adiabatic compression and friction heat, and it becomes necessary to make provisions for this effect. OAT indicator Ram Air Temperature Measurement At higher Mach numbers, the boundary layer, slowed down or stopped relative to the aircraft, experiences adiabatic compression that raises the air temperature to a value appreciably higher than SAT. Additionally, friction of the high-speed airow across the sensor also raises the air temperature. The term describing the temperature increase is ram rise, and the name for the temperature indicated under such conditions is Ram Air Temperature (RAT), in other words, RAT equals SAT plus ram rise. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 183 of 356 CASA Part Part 66 - Training Materials Only  The proportion of ram rise is dependent on the design and location of the sensor, and the term recovery factor refers to its ability to sense or recover the ram rise. For example, a sensor that has a recovery factor of 0.80 will measure SAT plus 80% of the ram rise. One method used is a ush-mounted bulb. Flush-mounted bulb The recovery factor is determined during testing and forms the basis of tables or graphs in the operations or ight manual for an aircraft type. The observed RAT indication, correlated with the current Mach number, plotted on the tables or graphs allows SAT to be determined. In the case of air data computers, there is an automatic application of a correction signal. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 184 of 356 CASA Part Part 66 - Training Materials Only  OAT, SAT and RAT indicators 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 185 of 356 CASA Part Part 66 - Training Materials Only  Total Air Temperature Measurement Currently, for aircraft operating at high Mach numbers, it is customary to sense and measure the maximum temperature rise that is possible. This is referred to as Total Air Temperature (TAT) and is obtained when the air is brought to rest (or nearly so) without addition or removal of heat. A probe supplies information to an indicator on the ight deck instrument panel or air data computer. The probe is in the form of a small strut to support an air intake outside of any boundary layer that may exist. It is made of nickel-plated beryllium copper that gives good thermal conductivity and strength and is secured to the aircraft skin at a pre-determined location in the fuselage nose section. In ight, the air pressure within the probe is higher than that outside, and the ow of air is in the manner indicated. Inside the air intake, the airow turns through ninety degrees for the separation of water particles from the air before passing around the sensing element. A pure platinum wire resistance element, hermetically sealed within two concentric platinum tubes, uses the inner tube as the element former to ensure a close match of thermal expansion and minimisation of thermal strain. Some probes have an auxiliary element with one element used for the indicator and the other for air data computations. An axial wire wound heating element is mounted integral with the probe to prevent ice formation. TAT probe 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 186 of 356 CASA Part Part 66 - Training Materials Only  Total Air Temperature Probe Handling TAT probes are sensitive items that require care when working around them. Observe the following precautions: Do not use probes as hand holds. Do not hang equipment on probes. Fit covers for personnel safety (protrusion hazard) and to stop insect contamination. Temperature probes can be heated; do not touch them unless necessary and always test for presence of heat with the back of the hand. Radiation Type The radiation emitted by any body at any wavelength is a function of the temperature of the body. This property is termed its emissivity. If, therefore, a body of known emissivity has its radiation measured, the temperature of the body can be determined. The name for this measuring technique is radiation pyrometry. Radiation pyrometer type 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 187 of 356 CASA Part Part 66 - Training Materials Only  As used in turbine engines 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 188 of 356 CASA Part Part 66 - Training Materials Only  Cabin Pressurisation Introduction to Cabin Pressurisation Gases of the atmosphere have weight just like that of solid matter. The downward force exerted by of a column of air stretching from the surface into space is called atmospheric pressure. The pressure of this column of air at sea level is approximately 14.7 psi. This atmospheric pressure decreases proportionally as altitude increases. When atmospheric pressure falls, the partial pressure of oxygen also decreases, resulting in a lack of breathable oxygen, known as hypoxia. People breathe easier at low altitudes, but ight is more efcient at high altitudes. A means of preventing hypoxia must be provided. This can be achieved by pressurising the cabin, increasing air pressure and therefore increasing the available oxygen to passengers. In pressurised aircraft it is important that the pilot and crew have an accurate indication that the cabin altitude corresponding to the maximum differential pressure conditions is being maintained. To meet this requirement, simple altimeters, calibrated to the same pressure/altitude law as normal altimeters, are provided on the main instrument panel or on the pressurisation system control panel, with their measuring elements responding directly to the prevailing cabin air pressure. Cabin Altimeters Cabin altitude is generally maintained below 10 000 ft (no oxygen necessary for passengers) although aircraft y around above 30 000 ft, but all aircraft have a maximum differential pressure measured with reference to the pressurised cabin and outside atmospheric pressure. This differential pressure must never be exceeded, or the aircraft can pop like an over inated balloon. Cabin pressure altimeter 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 189 of 356 CASA Part Part 66 - Training Materials Only  Altitude Alerting Systems In certain aircraft systems, the control and operating conditions are related to one specic altitude; for example, in a cabin-pressurisation system, the necessity arises for an indication of a possible increase of cabin altitude above the desired level while the aircraft is at its normal operating altitude. To cater for the appropriate requirements therefore, it is usual to employ altitude switching units capable of transmitting altitude signals to a separate alerting unit. Altitude switching units normally consist of an aneroid-capsule measuring element like that used in altimeters, but in lieu of a pointer actuating mechanism, the capsule is so designed that at a preset altitude, its expansion actuates an electrical contact assembly to complete a circuit to a warning light or aural warning device. The capsule may be housed in a separate unit, or contacts can be incorporated in the indicator. Contacts within an indicator introduce a greater load on the capsule sensing element, and this would be the major limiting factor as to whether the instrument is designed to house the contacts or whether a separate sensing and warning device is utilised. Cabin altitude warning system 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 190 of 356 CASA Part Part 66 - Training Materials Only  Accelerometer An aircraft structure is designed and built to withstand a certain load, and an accelerometer on the instrument panel gives the pilot an indication of the load imposed on the airframe in terms of load factors. Non-aerobatic aircraft use an accelerometer as a means of recording the loads placed on an airframe during its life. The unit is remotely mounted near the aircraft’s normal centre of gravity, and the information is logged automatically. G Force indicator Accelerometers are calibrated in g units, and when at rest, the instrument should read 1 g positive. They measure an aircraft's acceleration in the pitch axis (g-force). Most often they are used to determine how tightly the aircraft is turning or in achieving a 0-g dive for maximum aircraft acceleration. The number of g indicates the apparent gravitational force being applied to the aircraft and pilot in the pitch axis. One g is the normal force of gravity and is what is experienced in normal straight and level ight. So in a 5-g turn, the plane and pilot experience a force ve times the force of gravity. Positive g indicates a force toward the bottom of the aircraft; negative g indicates a force towards the top of the aircraft. 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 191 of 356 CASA Part Part 66 - Training Materials Only  “G” meter 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 192 of 356 CASA Part Part 66 - Training Materials Only  Aircraft Clock An aircraft clock performs the same function as any other clock but must be very accurate, display hours, minutes and seconds and will normally have internal lighting incorporated. Aircraft clocks may be digital or analogue and powered mechanically or off the battery bus. Aircraft clocks have a stopwatch function incorporated. Aircraft certied for Instrument Flight Rules ight must have a serviceable clock installed as they are critical for timing navigation functions. They are also used for timing events such as auxiliary power unit start cycle and engine cool down times. Aircraft clocks (A320 and B737) 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 193 of 356 CASA Part Part 66 - Training Materials Only  Glass Cockpit Introduction to Glass Cockpit A glass cockpit is an aircraft cockpit that features electronic (digital) instrument displays, typically large LCD screens, rather than the traditional style of analogue dials and gauges. While a traditional cockpit relies on numerous mechanical gauges to display information, a glass cockpit uses several displays driven by ight management systems that can be adjusted to display ight information as needed. This simplies aircraft operation and navigation and allows pilots to focus only on the most pertinent information. As aircraft displays have modernised, the sensors that feed them have modernised as well. Traditional gyroscopic ight instruments have been replaced by electronic Attitude and Heading Reference Systems (AHRS) and Air Data Computers (ADCs), improving reliability and reducing cost and maintenance. In Commercial Aviation The improved concepts enable aircraft makers to customise cockpits to a greater degree than previously. All the manufacturers involved have chosen to do so in one way or another, such as using a trackball, thumb pad or joystick as a pilot-input device in a computer-style environment. Many of the modications offered by the aircraft manufacturers improve situational awareness and customise the human-machine interface to increase safety. Modern glass cockpits might include Synthetic Vision (SVS) or Enhanced Vision systems (EVS). Synthetic vision systems display a realistic 3D depiction of the outside world (like a ight simulator) based on a database of terrain and geophysical features in conjunction with the attitude and position information gathered from the aircraft navigational systems. Enhanced vision systems add real-time information from external sensors, such as an infrared camera. Airbus A380 glass cockpit 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 194 of 356 CASA Part Part 66 - Training Materials Only  General Aviation Many modern general aviation aircraft are available with glass cockpits. Systems such as the Garmin G1000 are now available on many new GA aircraft, including the classic Cessna 172. Many small aircraft can also be modied post-production to replace analogue instruments. Garmin G1000 displays in a Cessna 182 2023-01-18 B1-11f Turbine Aeroplane Aerodynamics, Structures and Systems Page 195 of 356 CASA Part Part 66 - Training Materials Only