Aeronautical engineering - Strumentazione aeronautica

Completed notes of the course - English version

Complete course

Aeronautical Instrumentation 2026 Complete course notes made by Eliott Daniel based on Professor Alberto Luigi Michele Rolando's Instrumentation class. Although I have carefully reviewed the material and successfully used it to pass the exam, some errors may still be present. Feel free to make modifications or improvements by copying the document into Google Docs: https://docs.google.com/document/d/1iNjpfvtOKUSL6Kuv1-gtVxgJHpBkbKJQHBLDa5g-9Zw/edit?usp=sharing Aeronautical Instrumentation — Lecture 1.......................................................................... 6 Introduction to the Course .................................................................................................. 6 1. Course Overview and Motivation ................................................................................... 6 2. Course Topics ................................................................................................................. 8 3. Course Activities ............................................................................................................. 9 4. Exam Structure ............................................................................................................... 9 5. Fundamental Concepts: The Mission and the Role of Instrumentation ........................ 10 Aeronautical Instrumentation — Lecture 2........................................................................ 14 History of Cockpit Instrumentation & Avionics Architecture ............................................. 14 1. Uses of On-Board Data Beyond the Cockpit ................................................................ 14 2. Historical Evolution of Cockpit Instrumentation ............................................................ 15 3. Reliability, Redundancy, and Failure Management ...................................................... 19 4. Human Factors in Instrumentation Design ................................................................... 22 5. Avionics Architecture .................................................................................................... 22 Aeronautical Instrumentation — Lecture 3........................................................................ 25 The Standard Atmosphere, Altimetry, and Air Data Systems ........................................... 25 1. The International Standard Atmosphere (ISA) ............................................................. 25 2. Altimetry: Reference Pressures and Flight Levels ....................................................... 29 3. Air Data: Definition and Importance ............................................................................. 31 4. Air Data Probes ............................................................................................................ 33 5. Pressure Capsule Instruments (Mechanical Altimeter / Airspeed Indicator) ................ 39 6. Envelope Protection and AoA: The AF447 and Fly-By-Wire Context .......................... 40 7. Environmental Qualification of Probes and Avionics .................................................... 40 Aeronautical Instrumentation — Lecture 4........................................................................ 41 Barometric Altimeter, Temperature Measurement, Variometer, and Airspeed Definitions 41 1. The Barometric Altimeter .............................................................................................. 41 2. Temperature Measurement .......................................................................................... 45 3. The Variometer (Vertical Speed Indicator — VSI) ........................................................ 46 Page 1 4. Anemometry: Airspeed Measurement .......................................................................... 49 Aeronautical Instrumentation — Lecture 5........................................................................ 53 Airspeed Corrections, ASI Coloured Arcs, Machmeter, Air Data Computer, and Head-Up Display .............................................................................................................................. 53 1. The IAS → CAS → EAS → TAS Correction Chain (Detailed) ...................................... 53 2. The Airspeed Indicator (ASI) — Coloured Arc System ................................................ 56 3. The Machmeter ............................................................................................................ 59 4. The Air Data Computer (ADC) ..................................................................................... 62 5. The Head-Up Display (HUD) ........................................................................................ 65 6. Environmental Qualification Standards ........................................................................ 68 Aeronautical Instrumentation — Lecture 6........................................................................ 69 Air Data Computer Architecture, Electromagnetic Compatibility, Navigation Fundamentals, and the Magnetic Compass ..................................................................... 69 1. Air Data Computer (ADC) — Internal Architecture ....................................................... 69 2. Electromagnetic Compatibility (EMC) ........................................................................... 73 3. Navigation Fundamentals ............................................................................................. 75 4. The Magnetic Compass ............................................................................................... 79 Aeronautical Instrumentation — Lecture 7........................................................................ 83 Compass Errors, Gyroscope Fundamentals, and Apparent Precession .......................... 83 1. Magnetic Compass — Errors and Compensation (Continued) .................................... 83 2. Magnetic Dip and Acceleration/Turn Errors ................................................................. 87 3. The Transition to Gyroscopic Heading References ...................................................... 89 4. Gyroscope Fundamentals ............................................................................................ 89 5. Apparent Precession of Gyroscopes ............................................................................ 92 6. The Rate Gyroscope .................................................................................................... 94 Aeronautical Instrumentation — Lecture 8........................................................................ 96 Directional Gyro, Artificial Horizon, Slaved Compass, and Turn Instruments .................. 96 1. Directional Gyro (DG) — Heading Indicator ................................................................. 96 2. The Artificial Horizon (Attitude Indicator) ...................................................................... 99 3. The Slaved Compass ................................................................................................. 103 4. Turn Indicator and Turn Coordinator .......................................................................... 107 Aeronautical Instrumentation — Lecture 9...................................................................... 111 Inertial Navigation: Gyro-Stabilised Platforms, Strapdown Systems, and Alignment ..... 111 1. The Inertial Navigator — Concept and Purpose ......................................................... 111 2. The Gyro-Stabilised (Gimballed) Inertial Platform ...................................................... 112 3. Strapdown Inertial Platform ........................................................................................ 115 4. Platform Alignment ..................................................................................................... 117 5. Inertial Navigation Errors ............................................................................................ 119 6. GPS–INS Complementarity and Sensor Fusion ........................................................ 120 7. Attitude and Heading Reference System (AHRS) ...................................................... 120 8. INS Modes of Operation and System Architecture ..................................................... 122 Aeronautical Instrumentation — Lecture 10 .................................................................... 123 Radio Navigation Fundamentals, ADF/NDB, and Introduction to VOR .......................... 123 1. Electromagnetic Waves — Relevant Properties for Avionics ..................................... 123 Page 2 2. Radio Navigation — Principles and Geometry ........................................................... 128 3. Automatic Direction Finder (ADF) and Non-Directional Beacon (NDB) ..................... 131 4. VOR — VHF Omnidirectional Range: Introduction .................................................... 136 Aeronautical Instrumentation — Lecture 11 .................................................................... 138 VOR Operating Principles, TO/FROM Logic, DME, Area Navigation, and the HSI ....... 138 1. VOR — Signal Generation and Radial Encoding ....................................................... 138 2. VOR Cockpit Indication — CDI, OBS, and TO/FROM Logic ...................................... 140 3. VOR Accuracy and Limitations ................................................................................... 143 4. VOR Integration with the RMI ..................................................................................... 143 5. DME — Distance Measuring Equipment .................................................................... 144 6. Position Fix Using VOR and DME .............................................................................. 147 7. Area Navigation (RNAV) — Moving Beyond Ground Station Waypoints ................... 148 8. Horizontal Situation Indicator (HSI) ............................................................................ 149 Aeronautical Instrumentation — Lecture 12 .................................................................... 152 ILS, MLS, and Introduction to GNSS/GPS ..................................................................... 152 1. Regulatory and Operational Context: MEL and Stakeholders .................................... 152 2. ILS — Instrument Landing System ............................................................................. 153 3. MLS — Microwave Landing System .......................................................................... 160 4. GPS-Based Landing Systems (GLS/GBAS) — Introduction ...................................... 161 5. GNSS — Global Navigation Satellite Systems ........................................................... 161 Aeronautical Instrumentation — Lecture 13 .................................................................... 163 GPS Architecture, Signal Structure, Navigation Message, and Time Reference ........... 163 1. GPS System Architecture: Three Segments .............................................................. 163 2. GPS Measurement Principle: Pseudoranges and Multilateration .............................. 166 3. GPS Time Reference ................................................................................................. 169 4. Navigation Message ................................................................................................... 170 5. GPS Signal Structure ................................................................................................. 172 6. Time to First Fix (TTFF) .............................................................................................. 175 7. Cryptography and Signal Security (Contextual Note) ................................................. 176 Aeronautical Instrumentation — Lecture 14 .................................................................... 177 GPS Observables, Error Sources, DOP, and GNSS Integrity ........................................ 177 1. GPS Observables ....................................................................................................... 177 2. GNSS Performance Characteristics: The Four Pillars ............................................... 179 3. GPS Error Budget ...................................................................................................... 180 4. Dilution of Precision (DOP) ........................................................................................ 185 5. Introduction to GNSS Integrity: The Challenge .......................................................... 188 6. The GNSS Constellations: Galileo and BeiDou ......................................................... 188 Aeronautical Instrumentation — Lecture 15 .................................................................... 190 RAIM, Differential GPS, SBAS, GBAS, and Primary Surveillance Radar ...................... 190 1. RAIM — Receiver Autonomous Integrity Monitoring .................................................. 190 2. Differential GPS (DGPS) ............................................................................................ 191 3. SBAS — Satellite-Based Augmentation System ........................................................ 193 4. GBAS — Ground-Based Augmentation System ........................................................ 197 5. GNSS Navigation Performance by Flight Phase ........................................................ 199 Page 3 6. Primary Surveillance Radar (PSR) ............................................................................. 199 Aeronautical Instrumentation — Lecture 16 .................................................................... 206 Secondary Surveillance Radar, Mode S Transponder, and Radio Altimeter .................. 206 1. Secondary Surveillance Radar (SSR) and Transponder ............................................ 206 2. Radio Altimeter (RA) ................................................................................................... 211 3. Integration in the Modern Cockpit .............................................................................. 215 Aeronautical Instrumentation — Lecture 17 .................................................................... 217 Airborne Radar Systems: Military Fire-Control Radars and the Meteorological Radar ..217 1. Airborne Radar on Civil vs. Military Aircraft ................................................................ 217 2. Challenges of Airborne Target Acquisition: Ground Clutter ........................................ 217 3. Modern Fire-Control Radar Antennas: Beamforming ................................................. 218 4. The Airborne Weather Radar ..................................................................................... 220 5. The Weather Radar Equation ..................................................................................... 221 6. The dBZ Scale ............................................................................................................ 223 7. Antenna, Installation, and Controls ............................................................................ 225 Aeronautical Instrumentation — Lecture 18 .................................................................... 227 Wind Shear Detection, Controlled Flight Into Terrain, and Traffic Collision Avoidance ... 227 1. Wind Shear and the Microburst .................................................................................. 227 2. Wind Shear Detection Systems .................................................................................. 230 3. Controlled Flight Into Terrain (CFIT) ........................................................................... 231 4. Ground Proximity Warning System (GPWS) .............................................................. 232 5. Enhanced GPWS / Terrain Awareness and Warning System (EGPWS / TAWS) ...... 233 6. Traffic Collision Avoidance System (TCAS) ............................................................... 236 Aeronautical Instrumentation — Lecture 19 .................................................................... 242 TCAS II — Detailed Operation: Surveillance, Tau, Sensitivity Levels, Resolution Advisory Logic, and Avionics Certification .................................................................................... 242 1. Cooperative Architecture and the Non-Transponder Gap .......................................... 242 2. TCAS Generations and Alert Types ........................................................................... 243 3. Coordinated RAs and the Überlingen Rule ................................................................ 243 4. The Closest Point of Approach and Tau ..................................................................... 244 5. The Slow-Overtake Problem and the Distance Modifier ............................................ 244 6. Sensitivity Levels ........................................................................................................ 246 7. The Protected Volume ................................................................................................ 247 8. Control Panel Modes .................................................................................................. 248 9. Display and Pilot Interface .......................................................................................... 248 10. Vocal Announcements .............................................................................................. 249 11. Continuous Monitoring and RA Revision .................................................................. 249 12. Pilot Reaction and Design Margins .......................................................................... 251 13. Avionics Certification: TSO / ETSO .......................................................................... 251 Aeronautical Instrumentation — Lecture 20 .................................................................... 252 ADS-B, UAT, Data-Link Services, and Introduction to Air Traffic Management .............. 252 1. The Surveillance Gap and the ADS-B Concept ......................................................... 252 2. Expanding the Acronym ............................................................................................. 253 Page 4 3. Accuracy Advantage Over Primary Radar ................................................................. 253 4. Signal Structure: Squitters and the Extended Squitter ............................................... 254 5. ADS-B Message Types (Extended Squitter Payload) ................................................ 254 6. Cybersecurity: The Open-Standard Problem ............................................................. 255 7. ADS-B IN and ADS-B OUT ........................................................................................ 255 8. UAT — Universal Access Transceiver ........................................................................ 256 9. ADS-B Re-Broadcast (ADS-R) and System Architecture ........................................... 257 10. Multilateration ........................................................................................................... 258 11. Air Traffic Management — Overview ........................................................................ 258 12. Airspace Classification ............................................................................................. 259 13. Airspace Structure in Italy ........................................................................................ 260 14. Control Tower and Remote Tower ............................................................................ 261 15. Controller–Pilot Data Link Communication (CPDLC) ............................................... 261 16. Future Air Navigation System — Free Flight ............................................................ 262 Aeronautical Instrumentation — Lecture 21 .................................................................... 262 ACARS, CPDLC, Flight Director, and Autopilot .............................................................. 262 1. ACARS — Aircraft Communication Addressing and Reporting System .................... 263 2. CPDLC — Controller–Pilot Data Link Communication ............................................... 266 3. The Flight Director ...................................................................................................... 268 4. The Autopilot .............................................................................................................. 270 Aeronautical Instrumentation — Lecture 22 .................................................................... 271 Flight Management System, Area Navigation (RNAV), and Performance-Based Navigation (PBN) ............................................................................................................ 271 1. The Flight Management System — Role and Motivation ........................................... 271 2. FMS Architecture and Interfaces ................................................................................ 272 3. FMS Databases .......................................................................................................... 274 4. FMS Mission Management Functions ........................................................................ 276 5. Area Navigation (RNAV) ............................................................................................. 277 6. Waypoints — Definition and Naming .......................................................................... 278 7. Performance-Based Navigation (PBN) ....................................................................... 279 Aeronautical Instrumentation — Lecture 23 .................................................................... 283 Avionics Data Buses: Theory, ARINC 429, and MIL-STD-1553B .................................. 283 1. Why Data Buses? The Federated Avionics Architecture ........................................... 283 2. Data Bus Classification .............................................................................................. 284 3. Multiplexing and Channel Allocation .......................................................................... 284 4. Network Access Techniques ...................................................................................... 285 5. Error Protection Techniques ....................................................................................... 286 6. ARINC 429 — Mark 33 Digital Information Transfer System ..................................... 288 7. MIL-STD-1553B ......................................................................................................... 290 8. Comparison: ARINC 429 vs. MIL-STD-1553B ........................................................... 292 9. Legacy and Persistence ............................................................................................. 293 Aeronautical Instrumentation — Lecture 24 .................................................................... 294 AFDX (ARINC 664 Part 7) and Health & Usage Monitoring Systems (HUMS) .............. 294 1. AFDX — Avionics Full-Duplex Switched Ethernet ..................................................... 294 Page 5 2. HUMS — Health and Usage Monitoring System ........................................................ 297 Aeronautical Instrumentation — Lecture 1 Introduction to the Course 1. Course Overview and Motivation 1.1 The Cockpit as the Most Evolved Part of the Aircraft Page 6 Consider two photographs side by side: the rollout of the Boeing 747-100 (the first of its kind) and the Airbus A380 in flight. After forty years of development, the external shape and even the engines have changed relatively little. The 747 was one of the first aircraft equipped with a turbofan engine; the A380 has a much higher bypass ratio — but the form factor is broadly similar. The same cannot be said for what is inside the cockpit. In the instructor's view, the cockpit has undergone more disruptive generational leaps than aerodynamics or propulsion. From panels crowded with electromechanical gauges — as found on the original 747-100 — aviation has moved to the glass cockpit : fully integrated digital displays that consolidate and clarify information, enabling a fundamental reorganisation of flight operations and crew roles. A noteworthy example of recent propulsion progress is the CFM LEAP engine , used on the Airbus A320neo family. Beyond its higher bypass ratio, it incorporates a gearbox (reduction gear) that allows the fan to rotate at a lower speed than the core shaft, which are normally rigidly coupled. This decoupling enables a more aerodynamically efficient fan and contributes to a 15–20% reduction in fuel consumption compared to the previous generation. 1.2 The Microprocessor as the Enabling Technology The glass cockpit and virtually all modern avionics are made possible by the microprocessor , invented in 1970 by Federico Faggin , an Italian physicist working at Intel Page 7 in California. After eight to nine months of development, Intel released the result as the Intel 4004 — the first commercially available microprocessor in history. This invention transformed electronics in aviation, consumer electronics, and every other field of modern life. A philosophical note: the progress of science and technology is cumulative and shared. Had Faggin not made this invention when he did, someone else would have done so shortly after. Individual breakthroughs emerge from a common substrate of accumulated knowledge. 1.3 Scope and Philosophy of the Course This is an 8-credit course. The objective is to survey, within those limits, the instrumentation typically installed on a commercial aircraft — covering: ● The physical principles underlying each instrument ● The architecture and interconnection of instrument systems ● The historical evolution that explains why current instruments look and behave as they do (design choices inherited from earlier technological constraints) ● The impact on flight operations : how instrumentation shapes the pilot's workload, the organisation of the airline, and the broader air transport system The reference vehicle throughout the course is a commercial jet transport (passenger or cargo), which represents the largest and most technically sophisticated segment of civil aviation. Unmanned Aerial Vehicles (UAVs/UAS) will also receive some attention, as they are a rapidly growing sector. 2. Course Topics The course covers the following subjects (not exhaustively listed here): 1. Air data — measurement of speed, altitude, and atmospheric parameters 2. Magnetic compass — principles and limitations 3. Attitude and heading — note that in aeronautics, attitude refers to two angles (pitch and roll), while heading is treated separately; this differs from the convention in classical mechanics 4. Autonomous navigation — inertial navigation systems 5. Assisted navigation — GNSS, radio navigation aids 6. Air Traffic Control (ATC) — operational context and communication requirements 7. Safety and risk-reduction systems — equipment designed to minimise mission risk or maximise effectiveness A recurring theme is that instruments which could be designed differently from scratch today are not redesigned, because pilots are trained to read them in their established form , and radical changes to the human-machine interface carry their own risks. Page 8 3. Course Activities ● Lectures : the primary delivery method ● Guided exercises : worked through together in class ● Flight simulator session : the department has a procedural simulator (not full-motion, but with representative cockpit controls). The simulator does not replicate an exact aircraft type, but experience shows it is highly effective — understanding an artificial horizon or a compass conceptually is very different from observing them during a simulated flight. Due to the large number of enrolled students, the class will be split into two groups. ● Industry seminars : guest speakers from the aeronautical industry will complement classroom material ● Company visits : planned, subject to availability 4. Exam Structure 4.1 Written Test (Online Quiz) ● Delivered via the Politecnico di Milano Moodle platform ● Students must bring their own laptop and have working access to the Polimi network (eduroam or Polimi protected WiFi) ● Questions are drawn randomly from a bank of approximately 790 questions, so each student receives a different quiz ● Questions are weighted by difficulty on a scale of 1 to 3 points ● For single-answer questions: full marks for a correct answer, zero otherwise ● For multiple-answer questions: each correct answer contributes a positive fractional score; each incorrect answer contributes a negative fractional score — but the total score for a question is floored at zero (no negative total per question) ● A speed bonus of +1 point is awarded to students who finish significantly early and achieve ≥ 90% correct answers ● The result is expressed as a percentage and is visible immediately upon submission ● Access thresholds for the oral exam : ○ ≥ 65%: admitted to the oral ○ 60–64.9%: case-by-case evaluation ○ < 60%: must retake the quiz at the next session 4.2 Oral Exam ● Conducted on the same day as the written test (typically a Saturday) ● Language: English or Italian, student's choice ● Order of examination follows the order of registration to the exam session (exchanges between students are permitted by mutual agreement) ● Duration: variable, depending on total enrolments ● Weight in final grade : written test 65%, oral 35% (plus possible speed bonus) Page 9 Oral exam evaluation criteria (in order of emphasis): 1. Answering the question asked — do not deflect to a related general topic 2. Demonstrated understanding — not only recitation of slides 3. Correct technical vocabulary — use the precise engineering terms 4. Clarity of exposition 4.3 Progress Assessment Tests (PATs) — "Fastlane" An optional fast-track pathway. Four intermediate tests are administered during the course, each covering roughly one quarter of the material. Students who pass all four with a sufficient average may register the final grade directly, without sitting the standard exam. Borderline averages (e.g., 18–19 or 29–30) require a brief supplementary oral. ● PATs are not a guaranteed right; they are an additional opportunity ● Each PAT must be passed individually; a failed PAT does not eliminate eligibility if the overall average remains sufficient, but attendance at all sessions is strongly encouraged for practice ● PATs must be taken in person — remote participation is not permitted ● Approximately half of students who attempt all four PATs have historically passed via this route 5. Fundamental Concepts: The Mission and the Role of Instrumentation 5.1 The Flight Mission Every flight constitutes a mission : a planned operation in four dimensions (three spatial dimensions plus time). The time dimension is critical for air traffic management — planning which sectors will be occupied at which times allows ATC to allocate controllers, subdivide Page 10 sectors, and regulate traffic flow. The phases that occupy only a small fraction of total flight time account for a disproportionately large share of accidents. This has direct implications for instrumentation development: effort should be directed toward the phases with the highest accident rates . 5.2 Flight Safety Trends Aviation accident rates (accidents per million departures) have fallen dramatically over the past decades, even as total traffic has grown continuously. Key data point: around 1960–1962, there was a sudden and sharp reduction in accident rates. This was due to a major technological innovation: jet aircraft powered by reliable turbofan/turbojet engines. The absolute number of accidents per year may not decrease as fast as the rate, because the total number of departures is growing rapidly — particularly driven by Asia (China, India), where a large fraction of the population has not yet entered the air travel market. Reference document: Boeing Statistical Summary of Commercial Jet Airplane Accidents (publicly available). Contains accident rate data broken down by flight phase, region, and cause category. Page 11 5.3 The Pilot's Three Tasks: Aviate, Navigate, Communicate A widely used framework for pilot tasks: 1. Aviate — control the aircraft: maintain attitude, heading, airspeed, and configuration compatible with the current flight phase. This is the innermost, fastest control loop. 2. Navigate — determine current position, compare it to the planned position, and apply corrections to close the gap. This is an outer loop, operating at a lower frequency. 3. Communicate — maintain continuous interaction with ATC. In the current organisation of airspace, separation between aircraft (preventing collisions) is delegated to an external entity (ATC), not managed autonomously on board. This is a historical architectural choice; if the system were being designed today, autonomous on-board separation — inspired, for example, by studies of how storks avoid collisions without coordination — might be considered. There is also an informal fourth task: communicating with passengers , which pilots report finding generally unrewarding. 5.4 Situation Awareness Definition (Endsley, 1988): Situation awareness is the perception of elements in the environment within a volume of time and space, the comprehension of their meaning, and the projection of their status in the near future. The three levels are: 1. Perception — sensing the current state of the environment 2. Comprehension — understanding what those states mean for the mission 3. Projection — anticipating the near-future state Instrumentation directly impacts situation awareness. The goal of good instrument design is to minimize the cognitive resources required for human-machine interaction, so that the pilot can dedicate maximum cognitive capacity to building and maintaining situation awareness. A key case study illustrating the consequences of lost situation awareness: Air France Flight 447 (June 2009). The aircraft descended into the Atlantic Ocean at approximately 12,000 ft/min, with a pitch attitude of ~40°, from cruise altitude — while the crew apparently did not correctly perceive their actual flight state (aerodynamic stall), due to initial corrupted airspeed indication (frozen pitot tubes). All 228 people on board were killed. Page 12 The TCAS (Traffic Collision Avoidance System) is an example of a system explicitly designed to support the projection component of situation awareness: it alerts the crew to conflicting traffic and, when a collision risk is detected, issues a Resolution Advisory (RA) specifying the corrective manoeuvre. 5.5 Instrumentation Design Principles Instrumentation must be: ● Clear — unambiguous, easy to read at a glance ● Unequivocal — no possible misinterpretation ● Simple to use — minimising cognitive load Early electromechanical cockpits failed on all three counts. The complexity was such that a third crew member — the flight engineer — was required to manage aircraft systems. The glass cockpit addressed many of these issues. Modern displays often include a de-clutter function: a button that, when pressed, removes all non-essential information from the display, leaving only the minimum representation needed for the current flight phase. 5.6 The Basic Instrumentation Paradigm To conduct a flight safely, the pilot — or the autopilot — requires reliable data. These data must be acquired through appropriate sensors, processed, and then presented in a usable form. In this context, instrumentation is defined as: Every device or system that provides the controller of the air vehicle — whether human or automatic — with parameters that may affect the success of the mission. Over the past decades, the amount of data available on board aircraft has increased dramatically, mainly due to the development of more powerful acquisition and processing systems. As a result, modern instruments are capable of performing increasingly complex tasks, enabling a high level of automation in mission management and shifting the pilot’s role toward supervision. The information generated by on-board systems is intended for two main recipients: ● the crew ● automatic systems Consequently, this information must be properly processed, formatted, and distributed according to the specific requirements of each recipient. Page 13 The trend is toward increasing automation: autopilots and flight management systems are increasingly the primary consumers of sensor data, with human pilots acting in a supervisory role. End of Lecture 1 notes. Aeronautical Instrumentation — Lecture 2 History of Cockpit Instrumentation & Avionics Architecture 1. Uses of On-Board Data Beyond the Cockpit Sensor data collected in flight is not consumed solely by pilots and autopilots. Over the last few decades, a major secondary use has emerged: maintenance and prognostics . 1.1 Flight Data Recorder and Flight Data Monitoring (FDM) The Flight Data Recorder (FDR) — colloquially the "black box" (actually orange) — is mandated by ICAO on all commercial aircraft. It records a minimum set of parameters at specified frequencies. Airlines quickly realised that systematically downloading FDR data allowed them to analyse exceedances (moments when the aircraft operated outside the envelope defined by the operator) and to perform prognostic analysis — predicting the future health of the aircraft or diagnosing past anomalies. However, the FDR was designed for accident investigation, not for routine data access; it required physical connector attachment for each download. This led to the development of parallel on-board recording systems. The FDR has been retained for its regulatory purpose, while a separate layer of systems is used for operational monitoring. This practice is now formalised as Flight Data Monitoring (FDM) , universally adopted by commercial operators. 1.2 Data Volumes: The Scale of the Problem The volume of data generated per flight hour has grown dramatically: Page 14 Aircraft Data per flight hour Boeing 737-800 ~50 MB Boeing 737 MAX ~600 MB Boeing 787 Dreamliner ~1 GB The volume is now so large that data analysis has become a bottleneck. Techniques borrowed from other industries — broadly grouped under the term data mining — are applied to extract operationally relevant signals from this mass of data. 1.3 Quick Access Recorders (QAR) To make routine data access practical without disturbing the FDR: ● Quick Access Recorders (QAR) : a cartridge-based system with a slot on the cockpit pedestal (the horizontal console between the two pilot seats), allowing a technician to swap media rapidly ● Wireless QAR systems : activate automatically when the aircraft is on the ground (to avoid radio interference in flight) and transmit data via TCP/IP to the operator's operations centre — even when the aircraft is away from its home base 1.4 Applications of FDM Data ● Safety analysis : identifying patterns of procedural error (e.g., a particular approach procedure consistently flown outside parameters) — data is typically anonymised to avoid punitive use ● Engine health monitoring : an illustrative example is a cargo operator (MD-11 fleet, 3 aircraft) that used FDM data to feed a GE-provided engine condition monitoring programme. Parameters such as vibration (accelerometer data) and coast-down time (the time for the engine to spool down from idle to a stop across high-pressure and low-pressure spools) were used to predict impending failures. In one case, the programme identified that an engine was approaching failure, allowing a planned replacement — avoiding an AOG (Aircraft on Ground) event in a remote location, which at the time could cost on the order of €500,000 for a shop visit. 2. Historical Evolution of Cockpit Instrumentation Page 15 2.1 The Era of the Senses In early aviation, pilots relied exclusively on their own senses to manage flight: ● Vision — the dominant sense (the visual cortex occupies a disproportionately large fraction of the brain relative to other sensory areas) ● Auditory — engine sound, airframe noise ● Tactile — stick/rudder feedback ● Olfactory — detecting unusual smells ● Vestibular (balance) — the sense of acceleration and orientation This last point was captured by the famous remark attributed to Niki Lauda: "a car is driven with the ass" — meaning that felt accelerations are a primary information channel for the skilled driver (or pilot). As aircraft performance grew, however, the human sensory system became insufficient for safe flight management. 2.2 The First Instruments The first instrument introduced in aviation was the tachometer (rev counter) for the engine. Airspeed was initially estimated by the pilot feeling the ram pressure of the wind. Gradually, instruments were added one by one until cockpits became densely packed. Page 16 2.3 The Electromechanical Era and its Limitations In the fully developed electromechanical cockpit (typified by aircraft such as the Concorde ), there was a one-to-one (bi-univocal) relationship between each sensor and each indicator: ● Each instrument on the panel was physically connected to exactly one sensor ● Each instrument could only display one parameter (at most two or three) ● If the instrument failed, the associated information was completely lost ● If the Attitude Director Indicator (ADI) failed, the pilot was entirely deprived of attitude information The density of information became so large that pilots had to perform a continuous periodic scan of all instruments — most of which, most of the time, showed nominal values. This was cognitively exhausting. When system complexity grew to the point that managing aircraft systems became a full-time task, a third crew member — the Flight Engineer — was introduced to manage aircraft systems while the two pilots focused on aviate and navigate. An intermediate solution was the integrated indicator : electromechanical instruments (using moving needles, flags, and magnets) that combined several parameters into one display — the Attitude Director Indicator (ADI) and Horizontal Situation Indicator (HSI) are early examples. However, the fundamental one-to-one sensor-to-display constraint remained. Page 17 2.4 The Glass Cockpit Revolution The introduction of digital computers on board — enabled by the microprocessor (Faggin, 1970) — broke the one-to-one sensor-to-indicator constraint. In a glass cockpit: The representation of a quantity is decoupled from its source , with a computer acting as an intermediary layer. This decoupling has profound consequences: Advantages: 1. Display redundancy : any information can be routed to any screen. If the primary display fails, the same data can be presented on another screen 2. Information fusion : since all data are available digitally in one place, multiple parameters can be combined into a single aggregate representation — giving the pilot all necessary information in one glance, adapted to the current flight phase 3. Decluttering : the displayed information can be filtered to show only what is relevant at any given moment. For systems pages (electrical, hydraulic, pneumatic), a summary page shows "all normal" when everything is fine; detailed sub-pages are available on demand Page 18 4. Electronic checklists and procedures : following a failure, the appropriate Emergency/Abnormal checklist can be displayed directly on screen — what was previously found only in the paper Aircraft Flight Manual (AFM) can now appear automatically 5. Elimination of the flight engineer : the reduced cognitive load of managing systems allowed the crew to be reduced to two pilots Disadvantages / Risks: 1. Single point of failure — mitigated by redundancy : if the central computer fails, all derived displays fail simultaneously. The solution is to install multiple computers (typically three: one serving the captain's side, one the first officer's side, and one switchable between them) 2. Information not designed in = information inaccessible : if a parameter is not explicitly routed to a display page, it cannot be seen even if the underlying sensor is working correctly. The Air France 447 accident is the paradigmatic example: three Angle of Attack (AoA) sensors were functioning, AoA data reached the computers — but no display of AoA had been designed for the pilot-facing screens. The crew had no way to observe their actual AoA 3. Common-mode failure (see below) 2.5 The Backup / "Get-Home" Display Regulatory authorities require a minimum backup instrument — sometimes called the get-home display — that is fully independent of the main avionics suite. It must have: ● Its own sensors ● Its own power supply ● Its own computer It provides at minimum: attitude , altitude , airspeed , and heading . Even these backup instruments are now implemented electronically (glass). The practical utility is debated — if all electrical power is lost on a modern jetliner, the situation goes well beyond what a backup display can address. 3. Reliability, Redundancy, and Failure Management 3.1 Regulatory Framework: Failure Severity Classification The certification standards CS-25 (large aircraft) and CS-23 (smaller aircraft) establish a five-level classification of failure consequences, each associated with a maximum allowable probability of occurrence per flight hour: Page 19 Severity level Description Max probability (per flight hour) Catastrophic Loss of the aircraft; multiple fatalities < 10 ⁻⁹ Hazardous Large reduction in safety margins; crew impairment < 10 ⁻⁷ Major Significant reduction in capability or crew workload increase < 10 ⁻⁵ Minor Slight reduction in safety margins < 10 ⁻³ No safety effect e.g., in-flight entertainment failure no target The most demanding target — 10 ⁻⁹ per flight hour for catastrophic failures — cannot be met by any single electronic device with current technology. This forces the use of redundancy . 3.2 Redundancy Redundancy is the only practical technique to achieve 10 ⁻⁹-class reliability with real-world electronic components. How it works : multiple independent instances of a system perform the same function. A majority voting scheme is used to determine which output is accepted (e.g., in a triple-redundant system: 2-out-of-3 vote). Costs of redundancy: ● Additional weight (the redundant units serve no operational purpose in normal flight) ● Additional complexity, wiring, and connectors ● Increased average failure rate of the system as a whole: a system with N redundant units has N times as many components that can fail, each triggering a degraded-mode situation even if total loss requires multiple failures This last point explains the industry trend toward twin-engine aircraft (A350, B777): with four engines, the statistical probability of at least one engine failure per flight is higher, even if the probability of losing all four is far lower. Page 20 3.3 Common-Mode Failures: The Limit of Redundancy Redundancy does not protect against common-mode failures — failures that affect all redundant instances simultaneously and identically. Two classes: Hardware common-mode : fire or physical damage that destroys all redundant units at once. Mitigated by physical separation of redundant units (different bays, different sides of the aircraft). Software/design common-mode : a systematic error embedded in the design affects all instances equally. A redundant majority vote cannot detect this because all units produce the same incorrect output. Illustrative example — Ariane 6 (first launch failure): The attitude control computer of the Ariane 6 launch vehicle was running software ported from Ariane 5, which had accumulated 140 successful launches. Unit testing of the software passed. However, Ariane 6 was physically larger than Ariane 5, and the vibration amplitudes measured by on-board accelerometers during launch were an order of magnitude higher than anything in the Ariane 5 flight database. The Ariane 5-derived software experienced a numerical overflow when processing these vibration values, causing an unhandled exception that crashed the primary attitude control computer. The hot-standby redundant computer — running the same software on the same hardware — experienced the identical failure . The launcher lost attitude control and had to be destroyed. Lesson learned : during the unit testing phase, software must be fed with data representative of the actual operational environment in which it will run — not the environment for which it was originally designed. The test environment must encompass the full operational envelope of the new system. Mitigation — Dissimilar Redundancy: To protect against common-mode software/hardware errors, some critical systems use dissimilar redundancy : ● Different processor hardware ● Different software written in different languages by different development teams Example : the NH90 helicopter (NHIndustries consortium, first fly-by-wire helicopter in its class) used two dissimilar flight control computers. The fly-by-wire architecture replaced the mechanical/hydraulic linkage between pilot controls and the swashplate (the mechanism that tilts to modulate the blade pitch collectively and cyclically) with potentiometers detecting control positions, electrical motors on the swashplate, and flight control computers. Drawback : dissimilar redundancy nearly doubles development and certification cost, and requires maintaining two parallel hardware and software supply chains. For commercial aircraft, this is economically unviable. Most commercial aircraft use fully identical redundant computers (e.g., three identical computers on Airbus/Boeing/Embraer glass cockpit aircraft). Dissimilar redundancy is used only where cost is not the overriding constraint. Page 21 4. Human Factors in Instrumentation Design 4.1 Definition and Scope Human factors (also called ergonomics or HF/E) is the multidisciplinary field studying the interaction between human beings and the systems they use. It draws on psychology, physiology, and cognitive science alongside engineering. Per the FAA definition, human factors principles apply to any tool or system: "...tools as simple as a pen or writing instrument to systems as complex as a space shuttle." In the context of avionics, human factors governs the design of displays, controls, alerts, and procedures — with the aim of minimising workload and maximising situation awareness under all conditions, including degraded and emergency situations. A canonical example of human factors applied to organisations and procedures (not just equipment): the Linate airport disaster (8 October 2001) , in which a SAS MD-87 and a Cessna Citation collided on the ground in dense fog. Analysis showed that the conditions (ATC procedures, ground markings, crew briefings) were organised in such a way that an error was not merely possible but structurally probable — anyone in the position of those who erred would likely have erred in the same way. This illustrates that human factors extends to organisational design. 4.2 Workload Quantification and Certification Regulatory authorities require manufacturers to quantify and demonstrate acceptable pilot workload , particularly in failure scenarios. Certification standards require that the aircraft remain manageable not just after a first failure, but after a second, independent failure occurring on top of the first. The workload after this double failure must still be within the capacity of a two-person crew. This requirement drove the original dual-pilot certification of commercial transport aircraft. Example — A successful mid-twin helicopter (over 1,000 delivered). During its certification programme, the FAA accepted a glass cockpit configuration with three displays. When EASA sought type validation, they required a fourth display , citing human factors arguments about display loss scenarios. This added significant cost to an already mature design, illustrating how human factors considerations — even partially subjective ones — carry regulatory weight. 5. Avionics Architecture 5.1 Definition of Avionics Page 22 Avionics = aviation + electronics . The term refers to all electronic systems installed on an aircraft. A modern commercial jetliner's avionics system is functionally analogous to a large distributed computing system. 5.2 The Federated Architecture and ARINC Standards The dominant architecture for commercial aircraft avionics is the federated architecture , in which each function is implemented by a dedicated, self-contained hardware unit. The architecture was standardised by ARINC (Aeronautical Radio, Incorporated ), founded by the airline association (IATA) to define interoperability standards for avionics equipment. ARINC standards allow operators to mix equipment from different suppliers, avoiding vendor lock-in. Key elements of the federated architecture: 5.2.1 Line Replaceable Unit (LRU) Definition : An LRU is an avionics unit designed to be removed and replaced at the flight line — at the aircraft parking stand — without requiring the aircraft to enter a maintenance hangar. The name breaks down as: ● Line = flight line (the apron/parking area) ● Replaceable = removable and swappable ● Unit = a self-contained functional module Operational rationale : aircraft generate revenue only when flying. The unit must be replaceable during a standard turnaround — in 15–20 minutes in the case of aggressive low-cost operators. A technician arrives, removes the failed unit, installs the spare, runs the built-in test, and clears the aircraft for departure. The failed unit is then sent to a repair shop. Physical location : LRUs are installed in the avionics bay , located below the cockpit floor. Access is via a hatch. Units are racked in standardised frames. Page 23 5.2.2 Built-In Test (BIT) Each LRU contains Built-In Test (BIT) functionality — self-diagnostic circuitry. Three types: BIT type When executed Depth Power-on BIT At aircraft power-up, aircraft on ground Deepest — can exercise full functionality, command actuators to travel limits, interrupt normal operation Continuous BIT Throughout the flight Monitoring only — cannot interrupt normal operation; flags degraded units for post-flight action Maintenance BIT Initiated by a certified technician Intermediate depth — exercises specific sub-functions to isolate a suspected fault The distinction between power-on BIT and continuous BIT is fundamental: on the ground, it is acceptable to command a flight control surface to its physical stop to verify actuator travel; in flight, this is obviously not permitted. The continuous BIT therefore performs a more conservative, non-intrusive monitoring function. 5.3 Federated vs. Integrated Architecture The federated (LRU-based) architecture has practical costs: ● Each unit has its own power supply, connectors, and wiring harness ● Weight is not optimised (redundant infrastructure per unit) Page 24 ● Volume is not optimised Advantages that justify these costs: ● Modularity : a single LRU can be upgraded (e.g., a more capable inertial navigation unit) without affecting the rest of the system ● Fault isolation : a failed unit does not propagate its failure to others ● Maintainability : LRU replacement is rapid and requires no specialised tooling Integrated architecture (fewer, more powerful computers handling multiple functions) exists but is less common in commercial transport aircraft. It is used in some very large platforms and in small UAVs , where the weight overhead of a federated architecture is unacceptable. Advanced Air Mobility (AAM) / Urban Air Mobility (UAM) : new categories of aircraft (electric VTOL, personal air vehicles) do not use a traditional LRU-based avionics architecture, because they are small enough that the overhead is prohibitive and their certification basis differs. End of Lecture 2 notes. Aeronautical Instrumentation — Lecture 3 The Standard Atmosphere, Altimetry, and Air Data Systems 1. The International Standard Atmosphere (ISA) 1.1 Why a Standard Atmosphere is Needed Since the earliest days of aviation, designers and operators have needed a common reference atmosphere for design calculations, performance comparisons, and operational procedures. The International Standard Atmosphere (ISA) was developed in the 1950s, primarily from North Ameri