4.1 Electronic Flight Displays (PFD/MFD) & System Architecture

Electronic Flight Displays (PFD/MFD) & System Architecture

Quick Answer: Modern glass cockpit aircraft replace traditional vacuum and pneumatic flight gauges with integrated Electronic Flight Instrument Systems (EFIS). An EFIS suite typically centers on a Primary Flight Display (PFD) consolidating the classic flight "six-pack" into dynamic vertical tapes, attitude cues, and a full-featured Horizontal Situation Indicator (HSI), paired with a Multi-Function Display (MFD) presenting moving maps, terrain, weather, and engine instrumentation. Advanced features like 6-second predictive trend vectors provide immediate awareness of acceleration and climb performance, while reversionary mode ensures complete flight and engine data redundancy during screen failures.

Over the past two decades, general aviation cockpits have undergone a technological revolution. Mechanical gyros, aneroid wafers, and bourdon tubes have given way to solid-state digital avionics suites such as the Garmin G1000/NXi, Avidyne Entegra/Vantage, and Aspen Evolution. Operating under Instrument Flight Rules (IFR) in high-workload glass cockpit environments requires mastering electronic display symbology, data bus architectures, and reversionary protocols.

---

1. Integrated Avionics Architecture & Data Buses

An integrated electronic flight display system is not merely a collection of computer monitors; it is a distributed network of microprocessors, digital sensors, and high-speed data buses. Understanding how data travels from external physical sensors to the cockpit glass is essential for diagnosing anomalous indications and system failures.

+-----------------------------------------------------------------------------+
|                   INTEGRATED GLASS COCKPIT SYSTEM ARCHITECTURE              |
|                                                                             |
|   +-----------------------+                    +------------------------+   |
|   |  Air Data Computer    |                    |   AHRS (Solid-State    |   |
|   |  (ADC - Pitot/Static) |                    |   Gyros/Accelerom.)    |   |
|   +-----------+-----------+                    +------------+-----------+   |
|               | (ARINC 429 Data)                            | (ARINC 429)   |
|               |                                             |               |
|               +----------------------+----------------------+               |
|                                      |                                      |
|                                      v                                      |
|                   +------------------------------------+                    |
|                   | Integrated Avionics Units (IAU)    |                    |
|                   | (e.g., Garmin GIA 63/64 Nav/Com/GPS|                    |
|                   +------------------+-----------------+                    |
|                                      | (High-Speed Ethernet Data Bus)       |
|               +----------------------+----------------------+               |
|               |                                             |               |
|               v                                             v               |
|   +-----------------------+                    +------------------------+   |
|   | Primary Flight Display| <=====[HSDB]=====> | Multi-Function Display |   |
|   |        (PFD)          |  Reversionary Bus  |        (MFD)           |   |
|   +-----------------------+                    +------------------------+   |
+-----------------------------------------------------------------------------+

System Components & Line Replaceable Units (LRUs)

• Line Replaceable Units (LRUs): Modular avionics sub-assemblies installed in remote avionics racks or behind the instrument panel. Each LRU handles a dedicated task (e.g., GPS navigation, VHF communication, air data processing, transponder encoding).
• High-Speed Data Buses (ARINC 429 & Ethernet): Specialized digital communication protocols transmitting binary data packets at high refresh rates. ARINC 429 provides robust point-to-point sensor communication, while High-Speed Data Buses (HSDB / Ethernet) handle heavy data loads like moving map graphics, synthetic vision terrain rendering, and inter-display synchronization.
• Integrated Avionics Units (IAUs): Central processing hubs (such as the Garmin GIA 63/64 series) containing the VHF communications receiver, VOR/ILS localizer and glideslope receivers, GPS/WAAS engine, and system interface processors.

---

2. Primary Flight Display (PFD) Anatomy & Symbology

The Primary Flight Display (PFD) organizes all critical flight parameters directly in the pilot's primary field of view. Instead of scanning six separate instruments spread across the panel, the pilot scans an integrated visual presentation structured around a central artificial horizon.

+-----------------------------------------------------------------------------+
|                        PRIMARY FLIGHT DISPLAY (PFD) LAYOUT                  |
|                                                                             |
|   [AIRSPEED TAPE]        [ATTITUDE INDICATOR / SVS]       [ALTIMETER TAPE]  |
|   +-------------+       +--------------------------+      +-------------+   |
|   |    130      |       |        /\ Sky Pointer    |      |    8500     |   |
|   |    120      |       |       /  \ Roll Scale    |      |    8400     |   |
|   |   |---| <== |=====> | ------------------------ | <==> | [ 8320 ]    |   |
|   | [ 114 ]     | (Trend| ---------+--+----------- |(Trend|    8200     |   |
|   |    110  (6s)| Vector|          |--| Pitch      |Vector|    8100     |   |
|   |    100      |       |          \__/ Ladder     | (6s) | [29.92" Hg] |   |
|   | [TAS 122KT] |       |      Synthetic Vision    |      |  [SEL 8000] |   |
|   +-------------+       +--------------------------+      +-------------+   |
|                                                                  ^          |
|                                                           [VSI TAPE/NEEDLE] |
|                                                           | +1000 FPM |     |
|                                                                             |
|   [HORIZONTAL SITUATION INDICATOR (HSI)]                                    |
|   - 360° Compass Rose / Arc Mode                                            |
|   - Course Deviation Indicator (CDI) & Glideslope / Glidepath Diamond       |
|   - Heading Bug, Bearing Pointers (NAV1/NAV2/GPS), Wind Vector Arrow        |
+-----------------------------------------------------------------------------+

Airspeed Tape Symbology

• Vertical Tape Format: Airspeed is displayed on a moving vertical tape on the left side of the PFD. Lower speeds are at the bottom, higher speeds at the top.
• Airspeed Readout Pointer: A central black pointer box with a rolling numerical drum displaying the current Indicated Airspeed (IAS) in 1-knot increments.
• Color-Coded Speed Ranges (14 CFR § 23):
  • White Band: Flap operating range ($V_{SO}$ to $V_{FE}$).
  • Green Band: Normal operating range ($V_{S1}$ to $V_{NO}$).
  • Yellow Band: Caution range (smooth air only, $V_{NO}$ to $V_{NE}$).
  • Red Radial Line / Red-and-White Barber Pole: Never Exceed Speed ($V_{NE}$) or Maximum Operating Limit ($V_{MO}$).
• True Airspeed (TAS) Readout: Computed digitally by the Air Data Computer and displayed in real time at the bottom of the airspeed tape.
• Airspeed Reference Bugs: Pilot-selectable or automated takeoff/approach reference speeds ($V_R, V_X, V_Y, V_{REF}, V_{APP}$) displayed alongside the tape.

Altimeter Tape & Vertical Speed Indicator

• Altimeter Tape: Located on the right side of the PFD. Displays altitude on a vertical moving tape in 20-foot increments, with a central rolling drum pointer.
• Barometric Setting Window: Located directly beneath the altimeter tape, displaying the current altimeter setting in inches of mercury ($"\text{Hg}$) or hectopascals ($\text{hPa}$). Adjusting the barometric setting knob updates the tape and digital readout instantaneously.
• Target Altitude Bug & Readout: Selected altitude bug appears on the tape, with a digital window above the tape showing the preselected altitude. Visual flashing and aural alerts activate when approaching within $1,000\text{ ft}$ of the selected altitude or deviating by more than $200\text{ ft}$.
• Vertical Speed Indicator (VSI): Located adjacent to the altimeter tape. Features a vertical scale with an analog pointer indicating climb or descent rate in feet per minute (FPM). When vertical speed exceeds $\pm 100\text{ FPM}$, a digital numerical readout appears over the pointer.

Attitude Indicator & Roll Conventions

• Horizon Line & Pitch Ladder: The central display represents the artificial horizon, with blue sky above and brown ground below. The pitch ladder provides $2.5^\circ$ and $5^\circ$ pitch reference marks.
• Roll Pointer Conventions (Sky Pointer vs. Ground Pointer):
  • Sky Pointer (Roll Pointer points to Sky): The roll index pointer is fixed to the aircraft symbol and points upward to the sky index. When the aircraft banks right, the roll scale rotates left, and the triangle pointer tilts right against the sky index. This is the standard FAA/general aviation glass cockpit convention (Garmin G1000).
  • Ground Pointer (Fixed Zero Index): Common in military and certain transport-category aircraft, where the pointer represents the aircraft bank angle against a stationary arc at the top of the display.
• Synthetic Vision System (SVS): Projects a 3D topographic terrain map, runway outlines, flight path markers, and obstacle warnings directly onto the attitude indicator background based on GPS position and internal terrain databases.

---

3. Predictive Trend Vectors & Dynamic Cues

One of the greatest safety advancements of electronic flight displays is the integration of dynamic predictive trend vectors. Utilizing real-time rate calculations from the Air Data Computer (ADC) and Attitude and Heading Reference System (AHRS), the PFD predicts aircraft state 6 seconds into the future.

+-----------------------------------------------------------------------------+
|                          6-SECOND TREND VECTOR MECHANICS                    |
|                                                                             |
|   AIRSPEED TAPE TREND VECTOR:                                               |
|   - Magenta line extending vertically from current IAS pointer.             |
|   - Shows projected airspeed in 6 SECONDS based on current acceleration.    |
|   - Example: IAS = 110 KIAS, accelerating at 2.5 kt/sec                     |
|     --> Magenta vector tip rests at 125 KIAS.                               |
|                                                                             |
|   ALTITUDE TAPE TREND VECTOR:                                               |
|   - Magenta line extending vertically from current altitude drum.           |
|   - Shows projected altitude in 6 SECONDS based on current vertical speed.  |
|   - Example: Alt = 5,000 ft, climb rate = 1,000 FPM (16.67 ft/sec)          |
|     --> Magenta vector tip rests at 5,100 ft.                               |
|                                                                             |
|   HSI TURN RATE TREND VECTOR:                                               |
|   - Curved magenta line extending along the HSI compass arc.                |
|   - Shows projected heading in 6 SECONDS based on rate of turn.             |
|   - Standard Rate Turn (3°/sec): Vector length equals exactly 18° of arc.   |
|   - Half-Standard Rate Turn (1.5°/sec): Vector length equals 9° of arc.     |
+-----------------------------------------------------------------------------+

Practical IFR Application of Trend Vectors

• Level-Off Anticipation: During rapid climbs or descents ($>2,000\text{ FPM}$), pilots use the 6-second altitude trend vector to anticipate level-off, smoothly pitching down as the tip of the magenta trend vector touches the target altitude.
• Airspeed Stability on Approach: On final approach in turbulent conditions, monitoring the airspeed trend vector allows the pilot to correct engine power immediately before an airspeed deviation manifests on the numerical drum.
• Standard Rate Turn Verification: On the HSI, maintaining the turn rate trend vector tip on the second tick mark establishes an exact standard rate turn ($3^\circ / \text{sec}$), ensuring precision during holding pattern turns and procedure turns.

---

4. Multi-Function Display (MFD) & Engine Indication System (EIS)

The Multi-Function Display (MFD) supplements the PFD by presenting large-format situational awareness displays and engine telemetry:

---

5. Reversionary / Display Backup Mode Operations

Federal aviation certification regulations require absolute redundancy for primary flight instrumentation. If one of the cockpit screens suffers a hardware failure (e.g., backlight burnout, processor crash, internal power supply failure), the system enters Reversionary Mode (also called Display Backup Mode).

+-----------------------------------------------------------------------------+
|                   REVERSIONARY / DISPLAY BACKUP MODE                        |
|                                                                             |
|   NORMAL OPERATION (DUAL SCREEN):                                           |
|   +--------------------------+          +--------------------------+        |
|   |       PFD SCREEN         |          |       MFD SCREEN         |        |
|   | - Flight Instruments     |          | - Moving Map / Weather   |        |
|   | - Attitude / Airspeed    |          | - Traffic Display        |        |
|   | - Altimeter / HSI        |          | - Engine System (EIS)    |        |
|   +--------------------------+          +--------------------------+        |
|                                                                             |
|   SCREEN FAILURE (e.g., PFD Screen Fails / Goes Black):                     |
|   +--------------------------+          +--------------------------+        |
|   |        PFD SCREEN        |          |   MFD IN REVERSION MODE  |        |
|   |                          |          | - Complete EIS Engine Bar|        |
|   |     [ BLACK SCREEN ]     | =======> | - Full PFD Instruments   |        |
|   |       Hardware Dead      |          | - Airspeed/Alt Tapes     |        |
|   |                          |          | - Attitude & HSI Display |        |
|   +--------------------------+          +--------------------------+        |
+-----------------------------------------------------------------------------+

Reversionary Operating Principles

1. Automatic vs. Manual Activation: Most modern glass cockpits automatically detect display communication loss and switch the surviving screen to reversionary mode within milliseconds. In all systems, a physical Display Backup / Reversionary Mode switch (red button on the audio panel or sub-panel) allows the pilot to manually force reversion.
2. Consolidated Symbology: In reversionary mode on the surviving display:
   • A vertical strip on the left side displays essential Engine Indication System (EIS) parameters (RPM, manifold pressure, fuel flow, oil pressure/temp).
   • The remainder of the screen displays the complete PFD flight presentation (attitude indicator, airspeed tape, altimeter tape, VSI, HSI).
   • The moving map is typically minimized or replaced by a small inset map on the HSI.
3. Sensor Continuity: A screen failure is purely a display presentation failure. The underlying AHRS and ADC computers continue functioning normally and deliver uninterrupted sensor data to the surviving screen.

---

6. Electronic Flight Bag (EFB) Integration & AC 120-76 Guidelines

An Electronic Flight Bag (EFB) is any portable or installed electronic computing device designed to perform flight planning, performance calculations, and display aeronautical charts, replacing physical paper chart books.

FAA Advisory Circular AC 120-76 Classifications

FAA AC 120-76 (Guidelines for the Certification, Airworthiness, and Operational Use of Electronic Flight Bags) outlines hardware and software categories for EFBs:

+-----------------------------------------------------------------------------+
|                        AC 120-76 EFB CLASSIFICATIONS                        |
|                                                                             |
|   HARDWARE CATEGORIES:                                                      |
|   - Portable (PEDs / Tablets): iPads, tablets, portable GPS. Not part of    |
|     certified aircraft type design; may be secured in a kneeboard or mount. |
|   - Installed: Certified equipment permanently wired to aircraft power and   |
|     data systems, governed by FAA Technical Standard Orders (TSO).          |
|                                                                             |
|   SOFTWARE CATEGORIES:                                                      |
|   - Type A Software: Static aviation documents (AFM/POH, weight and balance |
|     manuals, airport directories, regulatory texts).                        |
|   - Type B Software: Dynamic interactive aeronautical applications          |
|     (georeferenced IFR en route charts, terminal approach plates, live      |
|     weather overlays, weight & balance performance calculation engines).    |
+-----------------------------------------------------------------------------+

Operational Rules for EFB Own-Ship Position Display

• Situational Awareness vs. Primary Navigation: While AC 120-76 allows Type B software to display GPS "own-ship" position directly on georeferenced instrument approach plates and taxi diagrams, the portable EFB is legally classified as a supplemental situational awareness tool. It cannot be used as the primary navigational reference to execute an instrument approach in lieu of certified panel avionics.
• Database Currency Verification: Prior to every IFR flight, the pilot in command must verify that all EFB aeronautical databases (IFR low charts, terminal procedures, obstacle databases) are active and within the current 28-day AIRAC cycle.
• Backup Power & Redundancy: To legally replace paper charts under Part 91 operations (AC 91-78A):
  • The EFB must have sufficient battery reserve for the duration of the flight plus reserve margins.
  • A secondary independent backup source must be available (such as a second tablet/phone, dedicated external battery pack, or current paper backup charts).
  • Pilots must have a documented plan for lithium-ion battery thermal runaway containment.