Aircraft Instruments Types and Cockpit Layout 2

Transcription

1 Aircraft Instruments Types and Cockpit Layout Introduction An aircraft is a very complex machine, which has to be monitored and controlled, either manually by the pilot(s) or by the dedicated flight control computers. In the latter case, however, pilot(s) will have the final authority to fly the aircraft. The modern aircraft has a large number of transducers which convert physical parameters such as airspeed, altitude, attitude, temperatures, engine para meters, etc., and present them to the pilot in the most convenient way for him to see, comprehend and act to fly the aircraft in a safe and purposeful manner. In the early days, a scarf around the pilot s neck provided vital data on the attitude (pitch, roll and yaw) of the aircraft, angle of attack, side slip, etc. Arrangement of instruments in the cockpit is such that pilot very naturally sees effortlessly most vital and flight-critical data, namely, airspeed, altitude, attitude and vertical speed. Such data are presented to the pilot just below the wind shield in the main instrument panel which is discussed in more detail later in this chapter. It is also very important, that he comprehends the data he sees with minimum mental effort. Therefore, it is necessary to arrange the instruments, taking into account the importance of the data as well as their location, and the format of the displays. This becomes the more important in military fighter aircraft capable of flying at low altitudes and high speeds. This deals with broad overview of aircraft instruments their type and location and cockpit layouts of modern aircraft.

2 AIRCRAFT INSTRUMENTS TYPES AND COCKPIT LAYOUT 11 ASI Air Speed GH Gyro Horizon VSI Vertical Speed ALTI DI TBI Altitude Direction Turn and Bank Fig. 2.2 Basic six grouping of aircraft instruments in earlier aircraft now outdated. panel meaning pilots do not rely on outside cues which could be misleading; instead, they are trained to rely always on aircraft instruments, keeping blind eye to the view outside. In basic-six instruments, Gyro Horizon, which shows the aircraft s attitude (displaying pitch, roll and yaw motions of aircraft) occupies the central top position. By far this is the most important aircraft instrument which is relied upon by the pilots. The attitude of the aircraft, in turn depends on the air speed and aircraft s vertical speed, hence they are positioned on the two sides of the Gyro Horizon as shown in Figure 2.2. Another important instrument is the Direction (DI) which gives to the pilot, in which direction, the aircraft is heading. Directional changes are achieved by rolling (banking) and yawing (turning) the aircraft. Hence very naturally, a Turn and Bank is positioned at the right side of DI. To the left of DI is situated the Altitude (ALTI) giving information of flight level, usually in flight level units of 100 feet; for example a flight level of 300 represents an altitude of 30,000 feet. Another grouping of instruments is known as basic T, which is of more recent origin than the basic six, and is shown in Figure 2.3. This is the present standard even in modern aircraft of recent origin. In larger civil transport aircraft both pilot and co-pilot have independently, such identical display elements. After reviewing modern instruments, requirements of newer generation of aircraft, and pilot feedbacks, designers arrived at this basic T grouping of most important flight instruments required for safely flying the aircraft, without much effort and eye scan by the pilots. Most important indicators are: Air Speed (ASI), Attitude Director (ADI) and Altitude ALTI, which from the horizontal bar of T. Horizontal Situation (HSI) is at the centre and makes up the vertical bar of T, as shown in Figure 2.3. HSI gives directional information to the pilots. On the right side of HSI is located a Vertical Speed (VSI), which gives rate of climb or descent of the aircraft not really so flight critical. A Radio

3 12 AIRCRAFT INSTRUMENTATION AND SYSTEMS ASI Air Speed ADI Attitude Direction ALTI Altitude RMI HSI VSI Radio Magnetic Horizontal Situation Vertical Speed Fig. 2.3 Basic T arrangement of aircraft instruments. Magnetic (RMI) is located on the left side of HSI, and gives: (i) magnetic heading derived from flux gates which detect the direction of the aircraft with reference to earth s magnetic field and (ii) bearings to two radio stations, located on ground, hence the joint name Radio and Magnetic or Radio Magnetic. In some designs, RMI position is filled up by a TBI the Turn and Bank or by another instrument of recent origin called as Turn coordinator (TC). In aircraft manufactured after 1980, more sophisticated, all Electronic Flight Instruments Systems (EFIS) replace individual ADI and HSI. Present day aircraft (2009) use just one AMLCD colour monitor for each of the pilot and co-pilot, located directly in front of them. A third shared colour monitor displays all Engine s and Crew Alert System (EICAS). Such monitors replace a large number of cluster of instruments, which makes pilot invest considerable effort and eye scan movements to see, understand, analyse and take consequential steps for safe and desired flight of the aircraft. All the computer-generated dial instruments follow the basic T configuration. The on-board computers automatically decide and select as to which instruments need to be presented to the pilot on a need-to-know basis, depending on the phase of the flight. There are various well-identified phases of flight such as ground taxiing from departure point, take off, climb, cruise, descent and ground taxiing to arrival terminal. Pilot(s) should at all times be able to easily read and interpret the data presented to him by aircraft instruments in order to either maintain the aircraft in a steady and stable condition of flight or changing conditions of flight through manoeuvres. Aircraft instruments and display systems play an extremely important role in assisting the pilot to fly the aircraft safely and in a desired attitude. We now consider the aircraft display systems. Figure 2.4 shows the classification of displays along with some examples. The qualitative and quantitative displays are further described.

4 AIRCRAFT INSTRUMENTS TYPES AND COCKPIT LAYOUT 13 Classification of Aircraft Instruments QUALITATIVE DISPLAYS QUANTITATIVE DISPLAYS > Represents data as a symbol/moving bar/ command bar/horizon bar, etc. > Examples: Electronic Attitude Director (EADI) as shown below: > Represents quantitatively in numbers For example, dial reading on ALTI will give aircraft s altitude in feet > Some examples are ALT DME Circular scales Linear scales F o LOC RA ASI EGT of 4 Engines > Other qualitative displays include: 1) Flight Director (FD) 2) Attitude Director (ADI) 3) Horizontal Situation (HSI) 4) Electronic Horizontal Situation (EHSI) 5) Head Up Display (HUD) 6) Flight control surface positions as shown below FQI VSI RPM of 4 Engines LEFT RIGHT UP UP DN DN ALTI ELEVATOR L SPOILER R RUDDER UPPER LOWER Colour codes in circular displays Red Max limit Yellow Cautionary Green Normal Red arc Prohibited range Fig. 2.4 Classification of aircraft instruments.

5 14 AIRCRAFT INSTRUMENTATION AND SYSTEMS 2.3 Aircraft Display Types Aircraft displays form an important link between the pilots and the aircraft (man machine loop). Some of the more important requirements of the display system are: 1. They must be easy to interpret. 2. The display should be unambiguous. 3. They must follow natural sense of pilots. 4. Reliability should be very high. 5. Pilot effort should be minimum to read and absorb data content. 6. Accuracy of indication should be high. 7. Adequate sensitivity is required to sense small deviations. 8. Repeatability should be high to reduce repeated calibration efforts. There are broadly two types of display: 1. Quantitative display, and 2. Qualitative display Quantitative Displays In this type of displays, the data is displayed quantitatively as numbers either using a pointer-scale instrument or using an alphanumeric LED/LCD type numeric displays. Examples of quantitative displays are: air speed indicator, altitude indicator, vertical speed indicator, etc. All of them provide numeric display of concerned parameters. Some examples of quantitative displays are shown in Figure 2.5. The circular scales are good for ASI, ALTI, engine oil pressure and temperature, etc. Sometimes clustered straight scale as in Figure 2.5(b) is ideal, for example, to indicate exhaust gas temperatures (EGT) of a 4-engined large transport aircraft. A quick glance shows how the temperatures vary in comparison with each other and to know if any particular engine is malfunctioning and its EGT is wildly straying off the normal value. In circular scales the range can be extended by having a dynamic counter as in Figure 2.5(d) for an altimeter. One full rotation of pointer advances the counter main scale by one. For example, the reading shown is 34,400 feet above sea level, and after the main pointer increases to full scale of 10, the main counter reaches 35,000 feet. In addition, there is a static counter at the bottom of the circular scale. The static counter is used to adjust the atmospheric barometer pressure to the appropriate ambient pressure value by using the BARO knob located at the bottom left of the instrument. More on altimeter will be covered in subsequent chapters. Such BARO corrections are required frequently as will be explained later. The high range can also be accomplished by having another smaller circular scale and pointer as shown in Figure 2.6. The full range, here is split into two concentric scales; the inner scale is an extension of outer scale. A common aircraft instrument using this type of instrument is the engine speed indicator the large outer scale has a multiplication factor of 100 rpm, while the

6 16 AIRCRAFT INSTRUMENTATION AND SYSTEMS RPM Inner high range scale Fig. 2.6 Concentric scale, in RPMI. inner scale has a 1000 rpm. These instruments are however replaced by modern digital, unambiguous indicators, where RPM value is digitally displayed with no room for confusion. Linear and Non-Linear Scales Some physical parameters are non-linear and therefore direct linkage results in a non-linear scale. For example, air speed measures a differential pressure q = ( p t p s ), where p t is the total pilot pressure and p s is the static pressure and q = ½ ρv 2, where ρ is atmospheric density and v is air speed which is being measured. The capsule responds to the pressure only. Figure 2.7 shows capsule deflection vs. air speed, which of course follows square law. Capsule deflection is proportional to the pressure and pressure in turn depends on square of air speed. Observe that for same speed change Δv, deflection δ 2 is much larger, at high end of scale. If the capsule deflection is directly magnified and linked to pointer, the display will be non-linear as indicated in Figure 2.7(b). It will be crowded at the low end of scale and gets expanded at higher speeds. Sometimes this non-linear displays are useful because the readings are expanded Air speed knots* *knot is a measure of δ 1 speed (1 knot = 1.18 mph = 1.82 kmph) Δv δ 2 δ, capsule deflection knots (a) Air speed vs q (b) Direct magnification (c) Non-linear magnification or capsule deflection (non-linear markings) to compensate the square law dependence of pressure (linear markings) Fig. 2.7 Non-linear scale displays. 200 knots

7 AIRCRAFT INSTRUMENTS TYPES AND COCKPIT LAYOUT 17 in the range of interest (i.e. between knots) to the pilot. However, the non-linearity can be compensated by using non-linear magnification to compensate for parameter non-linearity. If this is done, the display will be linear as shown in Figure 2.7(c). Sometimes logarithmic scale is preferred to open up at low end and to get crowded at near full scale, as in vertical speed indicator. (More information will be presented in later chapters.) A typical VSI instrument is shown in Figure 2.8. Scale is opened up at low end, which is often of importance. Such representation provides improved readability near level flight, i.e. near zero rate of climb. Note: In an aircraft the units are funny and use of knots for speed, feet for altitude and feet per minute for rate of climb, because the pilots are used to them. Provides better readability, near level flight UP 2 2 RATE OF CLIMB 3 4 FEET PER MINUTE DN 3 4 Mounting holes Fig. 2.8 Vertical speed indicator logarithmic scale Display Colour and Markings Pilots should be able to easily interpret, comprehend and be alerted about certain parameters exceeding maximum limits. In order to achieve this, there are coloured arcs, radial lines and sectors, in order to highlight the limits of operation. A pilot can also set bugs which may be manually moved around to fix desired limits of performance, he has chosen by experience or aircraft manufacturer. The markings use the following standard colour conventions: Markings Red markings on scale Yellow arc Green arc Red arc Purpose Maximum and minimum limits Take-off/precautionary sectors Safe and normal operational zone Prohibited zone

8 AIRCRAFT INSTRUMENTS TYPES AND COCKPIT LAYOUT 19 be evident in subsequent chapters. Both qualitative and quantitative indications are made available on a single multi-coloured electronic display. Most aircraft are equipped with a standard set of instruments, which informs the pilot about the attitude (pitch, roll and yaw) air speed and altitude, of the aircraft. Many aircraft will have these basic flight instruments: 1. Altimeter: giving the aircraft s height above some reference level by measuring the local air pressure. The altimeter has a provision to adjust to local barometric pressure which must be correctly set to obtain accurate altitude data, (BARO adjustment). This adjustment is mandatory after a certain prescribed altitude by all aircraft so that they are all properly separated vertically during cruise. 2. Attitude : shows the pitch, and roll and angles relative to the horizon. This is also known as the artificial horizon. By reading this instrument, pilot will be able to know whether the wings are level and if the aircraft nose is pointing above or below the horizon. This is a primary instrument for instrument flight and is very useful in poor visibility conditions. In modern aircraft EADI replaces this altitude indicator. 3. Air Speed (ASI): This instrument displays the aircraft s speed in knots: (1 knot = 1.18 mph = 1.85 kmph) relative to the surrounding air. The indicated speed should be corrected for air density (which varies with altitude, temperature, and humidity) in order to get the True Air Speed TAS, and further corrected for wind conditions to obtain the ground speed. 4. Magnetic Compass: The magnetic compass is used to indicate the aircraft s heading relative to the earth s magnetic north, to know which direction the aircraft is flying with respect to the magnetic north. While the compass shows reliable readings in steady and level flight, it gives faulty indications when turning, climbing, descending or accelerating. This faulty indication can be compensated by using the gyro- stabilised heading indicator. For navigational purposes, it is necessary to correct the magnetic direction to obtain direction with respect to true geographic north (which points to the earth s axis of rotation). Note that magnetic north is wandering and slightly to the left of geographic North. Magnetic compass acts as a standby unit when other direction indicators malfunction. 5. Heading : (also known as Directional Gyro-DG). It is based on the gyro stability and precession, and is therefore subject to drift errors, which must be periodically corrected by calibrating it with respect to the magnetic compass. In modern aircraft, the DG is replaced by a Horizontal Situation (HSI), which provides the same heading information, but also helps in navigation. 6. Turn and Bank (TBI): The TBI is a gyroscopic instrument, displaying the direction and the rate of turn, (in degrees per minute). Internally mounted inclinometer shows the turn quality, i.e. whether turn is properly coordinated (i.e. no slip out or skid in) as opposed to an uncoordinated turn. This instrument has become a turn coordinator in newer aircraft, which are manufactured after Turn Coordinator: typically displays the rate and direction of roll while the aircraft is rolling; displays rate and direction of turn, while the aircraft is not rolling. Internally housed inclinometer also displays the turn quality. The turn coordinator has replaced the good old TBI in modern aircraft, and shows the rate of turn, but it does not indicate pitch information.

9 20 AIRCRAFT INSTRUMENTATION AND SYSTEMS 8. Vertical Speed (VSI): Displays the rate of climb or descent, usually in feet per minute. Vertical speed is indicated by sensing the changing air pressure during ascent or descent. 2.4 Instrument Grouping Basic T Grouping As previously mentioned modern aircraft have four of the flight instruments arranged as a T called the basic T as shown in Figure Instruments are located as described below. The ADI is in the top centre, ASI is to the left, ALTI is at the right, HSI is situated right below ADI in the bottom row. The turn coordinator or RMI is positioned to the left of HSI. VSI is located to Magnetic compass (below wind shield) AIR SPEED INDICATOR (ASI) PFD ATTITUDE DIRECTOR INDICATOR (ADI) ALTIMETER (ALTI) T grouping of 4 primary flight instruments TURN COORDINATOR OR RADIO MAGNETIC INDICATOR (ND) HORIZONTAL SITUATION INDICATOR (HSI) VERTICAL SPEED INDICATOR (VSI) Navigation display (ND) Fig Basic T configuration of Main Instrument Panel (MIP). the right of HSI. The magnetic compass will be located above the instrument panel, often on the windscreen centre post. In some cases, the position taken by the Turn Coordinator is replaced by the Radio Magnetic RMI which incorporates magnetic compass plus the bearing of the aircraft with respect to ground stations of radio navigation systems such as: VOR (Very high frequency Omni Range) and Automatic Direction Finder (ADF). In the newer aircraft with glass cockpit instruments also, the basic T grouping is generally followed. Brief details of glass cockpit is given below.

10 22 AIRCRAFT INSTRUMENTATION AND SYSTEMS Computer keybord Wind shield EADI EHSI Magnetic compass Overhead panel Wind shield Fuel indicators Engine & Crew Alter System (ELCAS) Computer monitor display MFD MFD Rudder pedal Side stick Foot rest Pilot Seat Co-pilot Seat Engine thrust lever Flight Management System (FMS) Fig All-glass cockpit of a modern aircraft.

11 AIRCRAFT INSTRUMENTS TYPES AND COCKPIT LAYOUT 23 Early glass cockpits, as in Macdonald Douglas MD80/90, Boeing s 737, 757 and 767, Airbus s A-300 and A-310, used EFIS for displaying attitude and navigational parameters only, and they continued to use mechanical gauges for air speed, altitude and vertical speed. Modern glass cockpits have now replaced totally all the mechanical gauges and warning systems, which were present in previous generation aircraft. Modern glass cockpit aircraft are: Boeing 737NG, , , 777 and 787; Airbus A-320, A-330 and the very new largest transport aircraft A-380. Future cockpit displays constitute a true departure from all of the above displays. They can customise the cockpits to various end users, to a greater extent than previous generations, for example they deploy a track ball, thumb pad or joystick as a pilot input device in a computer-like environment. Greater situational awareness, and man machine interface are now emphasised. Figure 2.13 shows the all- glass cockpit of a modern aircraft. Note that 5 in-line display units are used to present flight data to both pilot (left seat) and co-pilot. Also included in line is a Fuel Quantity (FQI) and control surface position indicators. The flight data includes basic T grouping, generally. However, one of the displays shows EADI data and another LRU (Line Replaceable Unit) presents EHSI data. The central display shows EICAS data, viewable by both pilots. Sometimes, instead of using two separate displays for EADI and EHSI, a single LRU is used to display both EADI and EHSI data. It is noteworthy that all display units are similar and can be plugged in any of the 5 positions. This greatly simplifies maintenance and reduces spare part counts. We now proceed to discuss the broad functionalities of some newer aircraft instruments Attitude Director (ADI) ADI is a Primary Flight Display (PFD), displaying all information critical to flight. Earlier, the Flight Director (FD) or Artificial Horizon (AH) or Gyro Horizon was used in an aircraft to indicate to the pilot about the orientation of the aircraft relative to earth. It indicates pitch (nose up/ down) and roll (wings not level) of the aircraft and constitutes the most important instrument for flight in the so called IMC* (Instrument Meteorological Conditions). Attitude indicators also have significant role in VMC,* Visual Meteorological Conditions. Figure 2.14 shows a typical attitude indicator (gyro horizon). The pilot actions will be made in the natural sense so that there is no confusion or ambiguity to manoeuvre the aircraft. For example, if the pilot desires a level flight he rotates the control wheel clockwise as shown in Figure 2.14(b) to make the aircraft fly straight and level (Figure 2.14(c)) Electronic Attitude Director An improvement was made in EADI (Electronic Attitude Director ), which in addition to displaying the attitude, issues commands using movable command bars. This is shown in the * IMC and VMC are types of air navigation, that enables pilots to land an aircraft. IMC conditions refer to landing the aircraft under poor visibility. VMC refers to visual conditions permitting a visual landing, i.e. the pilots can visually see the runway.

12 AIRCRAFT INSTRUMENTS TYPES AND COCKPIT LAYOUT 25 Stabilised bank pointer Horizon bar Roll scale Pitch scale Command bars Fixed aircraft symbol (a) A/C flying straight and level (b) A/C flying nose up (c) Nose up command. A/C must fly with nose up to satisfy the command (d) Fly left command. Fly the a/c to left to satisfy command bar (e) Fly right command. Fly the aircraft to right to satisfy command bar (f) Commands have been satisfied Fig Different command bar settings to be obeyed. Note the following: 1. ADI is basically similar to Attitude Director (or gyro horizon). 2. Horizon bar is driven by servo motors driven by remotely located gyroscopes. The sense of movement is natural to the pilot. 3. Aircraft symbol and the bank pointer are fixed to the aircraft. (See Figure 2.16(a)) 4. Both horizon bar and roll pointer are gyro stabilised and provide a spatial reference to the changing aircraft attitude. 5. Command bar displays are preset by pilot in the Mode Select Panel and enables him to fly the aircraft in a predetermined fashion, by matching the aircraft symbol coincide with command bars. These bars are particularly useful during take-off and landing phases of flight. They ensure flight safety. 6. The scale at left of EADI of Figure 2.15 refers to glide slope of ILS (Instrument Landing System) and indicates to the pilot whether he is flying above or below glide slope beam, which directs him to fly right into the runway during landing. Similarly, the scale at the bottom relates to localiser beam, for lateral alignment of the aircraft, to fly the aircraft aligned to the centre of the runway. ILS ensures flight safety under poor visibility conditions Horizontal Situation (HSI) The HSI is primarily a Navigation Display (ND), and it is an aircraft instrument located in Main Instrument Panel (MIP), just below the ADI (Attitude Director ). (See Figure 2.11),

13 S 26 AIRCRAFT INSTRUMENTATION AND SYSTEMS replacing the conventional DG (directional gyro). In the electronic flight instrumentation system, it is known as EHSI (Electronic Horizontal Situation ). HSI provides the plan view (map view) of aircraft motion which is important to navigate to the desired airport. Thus, HSI provides a basic horizontal view of the aircraft s navigation around the earth. It provides an excellent picture for precise navigation. EHSI displays: (i) magnetic heading, (ii) bearing and distance to navigation aid (VOR or ADF) and (iii) CDI (course deviation indicator). An HSI is a combination of two familiar cockpit instruments: the directional gyro (DG) with a heading memory bug and (ii) a VOR (VHF Omni directional Receiver) / ILS (Instrument Landing System) indicator. EHSI reduces pilot s effort otherwise the pilot has to eye scan many instruments individually. Figure 2.17 shows the EHSI, which incorporates the following: Distance TO/FROM arrow Glide slope pointer and scale Compass card Heading select knob 378 MILES HDG N W COURSE E COURSE Lubber line (reference line) Selected course counter Heading select bug Course deviation indicator (CDI) and scale (small circles) Aircraft symbol Lateral deviation bar and scale Reciprocal course Course select knob Fig Electronic horizontal situation indicator. 1. Glide slope needle for guiding the aircraft into the airport runway, in a vertical direction. 2. Aircraft Symbol. 3. Course Deviation (CDI) is shown relative to airplane symbol, in azimuth (plan) direction. 4. Interlinked to autopilot to hold altitude and while approaching follow glide slope all the way to decision height (DH) and beyond. 5. Course deviation indicator (CDI) and scale. 6. Compass card operated by DG (directional gyro) and a lubber reference line. 7. Heading select memory bug to aid the pilot to navigate to destination. Combining the DG and NAV indicator into a single instrument, reduces the pilot workload by providing the following vital information: heading which direction the aircraft is going, course reference, which direction the aircraft has to go, course deviation, glide slope information; while landing.

14 28 AIRCRAFT INSTRUMENTATION AND SYSTEMS resolved. Modern aircraft use Twisted Active Matrix Liquid Crystal Displays (TAMLCD), which replaces earlier CRT displays Multi Function Display (MFD) MFD is primarily a Navigation Display (ND) unit. It combines, however, weather data superimposed on the map Engine Indications and Crew Alert Systems (EICAS) EICAS shows information regarding the aircraft s systems such as fuel, electrical, engines, etc. EICAS also alerts pilots of unusual or hazardous situations like low engine lubricating oil pressure, engine overheat, autopilot malfunction, loss of emergency/utility power, etc Mode Control Panels Pilots can select display range and mode as well he can enter data, using the control panel. For example, command bars mentioned earlier are set using knobs in Mode Control Panels Display Data Processors The visual display of an EFIS is made possible through the SGU (Symbol Generator Unit), which gets data from the pilot, sensors and format selected. SGU is also called display processor, or display electronics unit. Advantages of EFIS are: 1. The same display can be made to function as PFD, ND, offering versatility. 2. Software upgradable to latest versions without changing hardware. 3. Should one of the display fail, the other can take over and thus act as a redundant system. 4. Any LRU can be plugged in any one of the five slots (see Figures 2.13a and b). EFIS has become a standard equipment for all modern aircraft like aircraft from Boeing (B-767, B-777 and future 787) and Airbus (A-320, A-330, and the more recent A-380). With the advent of low-cost computers, liquid crystal colour displays and inexpensive NAVsensor s Fibre optic gyros, GPS and AHRS; EFIS can be adapted to even low-cost general aviation aircraft, which was unthinkable a few years ago. Figure 2.18 is the cockpit of A-330, a large civil transport aircraft and Figure 2.19 is the cockpit of the A-330, which is the largest commercial transport plane today. Once again it should be reiterated, all of the above information in chapter 2 is to provide a broad overview and requires considerable knowledge to fully understand. Revisit this chapter after covering the full textbook.