Ó Aircraft Instruments— Types and Cockpit Layout 2 2.1 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 fl ight control computers. In the latter case, however, pilot(s) will have the fi nal authority to fl y the aircraft. The modern aircraft has a large number of trans- ducers 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, compre- hend and act to fl y 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 fl ight-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 fi ghter aircraft capable of fl ying at low altitudes and high speeds. This deals with broad overview of aircraft instruments—their type and location and cockpit lay- outs of modern aircraft. AIRCRAFT INSTRUMENTS—TYPES AND COCKPIT LAYOUT 11 ASI GH VSI Vertical Air Speed Gyro Horizon Speed Indicator Indicator ALTI DI TBI Turn Altitude Direction and Bank Indicator Indicator Indicator Fig. 2.2 Basic six grouping of aircraft instruments in earlier aircraft–now outdated.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 Indicator (DI) which gives to the pilot, in which direction, the aircraft is heading. Directional changes are achieved by rolling (banking) and yaw- ing (turning) the aircraft. Hence very naturally, a Turn and Bank Indicator is positioned at the right side of DI. To the left of DI is situated the Altitude Indicator (ALTI) giving information of fl ight level, usually in fl ight level units of 100 feet; for example a fl ight 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 fl ight instruments required for safely fl ying the aircraft, without much effort and eye scan by the pilots. Most important indicators are: Air Speed Indicator (ASI), Attitude Director Indicator (ADI) and Alti- tude Indicator—ALTI, which from the horizontal bar of T. Horizontal Situation Indicator (HSI) is at the centre and makes up the vertical bar of T, as shown in Figure 2.3. HSI gives direc- tional information to the pilots. On the right side of HSI is located a Vertical Speed Indicator (VSI), which gives rate of climb or descent of the aircraft—not really so fl ight critical. A Radio 12 AIRCRAFT INSTRUMENTATION AND SYSTEMS ASI ADI ALTI Attitude Air Speed Altitude Direction Indicator Indicator Indicator RMI HSI VSI Radio Horizontal Vertical Magnetic Situation Speed Indicator Indicator Indicator Fig. 2.3 Basic T arrangement of aircraft instruments. Magnetic Indicator (RMI) is located on the left side of HSI, and gives: (i) magnetic heading derived from fl ux gates which detect the direction of the aircraft with reference to earth’s mag- netic fi eld and (ii) bearings to two radio stations, located on ground, hence the joint name Radio and Magnetic Indicator—or Radio Magnetic Indicator. In some designs, RMI position is fi lled up by a TBI—the Turn and Bank Indicator 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 Indicators 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 fl ight of the aircraft. All the computer-generated dial instruments follow the “basic T” confi guration. 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 fl ight. There are various well-identifi ed phases of fl ight 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 fl ight or changing conditions of fl ight through manoeuvres. Aircraft instruments and display systems play an extremely important role in assisting the pilot to fl y the aircraft safely and in a desired attitude. We now consider the aircraft display systems. Figure 2.4 shows the classifi cation of displays along with some examples. The qualitative and quantitative displays are further described. Classification of Aircraft Instruments QUALITATIVE DISPLAYS QUANTITATIVE DISPLAYS > Represents data as a symbol/moving bar/ > Represents quantitatively in numbers command bar/horizon bar, etc. For example, dial reading on ALTI > Examples: Electronic Attitude Director will give aircraft’s altitude in feet Indicator (EADI) as shown below: > Some examples are LAYOUT COCKPIT AND INSTRUMENTS—TYPES AIRCRAFT ALT DME Circular scales Linear scales F ASI EGT of o 4 Engines LOC RA > Other qualitative displays include: FQI 1) Flight Director (FD) RPM of 2) Attitude Director Indicator (ADI) 4 Engines 3) Horizontal Situation Indicator (HSI) 4) Electronic Horizontal Situation Indicator (EHSI) 5) Head Up Display (HUD) VSI 6) Flight control surface positions as shown below LEFT RIGHT UP UP ALTI DN DN ELEVATOR Colour codes in circular displays Red – Max limit LRSPOILER Yellow – Cautionary UPPER Green – Normal LOWER Red arc – Prohibited range 13 RUDDER Fig. 2.4 Classifi cation of aircraft instruments. 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. 2.3.1 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 pro- vide 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 tempera- tures (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.