Boeing 737-800 NG EFIS
EFIS stands for Electronic Flight Instrument System, a digital flight deck display system in aviation that replaces traditional electromechanical or analog "steam gauge" instruments with electronic screens. Very Easy relative to Steam Gauge.
Lesson 3.4 – Flight Instruments
Lesson 3.4 covers the essential mechanics, operational principles, and failure profiles of core aircraft flight systems, including pitot-static instruments, gyroscopic instruments, and the magnetic compass. It provides a comprehensive overview of the standard "six-pack" arrangement, atmospheric pressure and temperature traps affecting altitude calculations, and the modernization of flight decks via glass cockpits.
To access the complete reference material, you can review the official FAA PHAK Chapter 8 Resource for the detailed text and consult the eCFR Part 91 Equipment Requirements for federal equipment and instrument mandates.
Top row from left to right: Airspeed Indicator, Attitude Indicator, Altimeter. Bottom row from left to right: Turn Coordinator, Heading Indicator, Vertical Speed Indicator.
Graphic of traditional "6 Pack" instrument cluster
1. Cockpit Instrument Architecture: The "Six-Pack"
Traditional general aviation training cockpits systematically arrange the six primary flight instruments in a standard grid configuration known as the "Six-Pack." Developing a disciplined visual scan across this cluster is essential for spatial awareness.
Airspeed Indicator (ASI): A differential pressure gauge displaying the aircraft's forward velocity through the air mass.
Attitude Indicator (AI): A gyroscopic instrument displaying pitch and roll attitude relative to an artificial horizon bar.
Altimeter: An aneroid barometer measuring the aircraft's vertical height above a selected barometric datum plane.
Turn Coordinator: A canted gyroscopic gauge indicating the rate of roll, the stabilized rate of turn, and quality of coordination (slip/skid).
Heading Indicator (HI): A directional gyro that provides a stable heading display, bypassing common magnetic compass turning errors.
Vertical Speed Indicator (VSI): A differential pressure instrument displaying the instantaneous rate of climb or descent in feet per minute (FPM).
2. Pitot-Static Systems & Instruments
The pitot-static system operates by sensing fluid air pressure changes around the airframe during flight. It relies on two separate inputs: Ram (Dynamic) Pressure and Ambient (Static) Pressure.
Core System Inputs:
Pitot Tube: Mounted externally under the wing (e.g., the left wing on a Cessna 172). It features an open impact hole at the front to capture dynamic ram air pressure, and a small drain hole at the back to expel moisture.
Static Ports: Flush-mounted openings located on the side of the fuselage. They measure undisturbed, static ambient atmospheric pressure.
The Pitot-Static Instrument Infrastructure:
Airspeed Indicator (ASI): The only instrument that utilizes both Pitot (Ram) and Static pressure. The ram line feeds directly inside an internal diaphragm, while the static line vents into the surrounding instrument case. The gauge measures the mechanical pressure differential between the two, translating the expansion of the diaphragm into knots or miles per hour.
Altimeter: Utilizes Static pressure only. It houses a sealed stack of aneroid wafers evacuated to a standard internal sea-level pressure of 29.92 "Hg. As the aircraft climbs and atmospheric pressure decreases, the air inside the case thins out, allowing the sealed wafers to expand. Mechanical linkages translate this expansion into a clockwise needle movement indicating a rise in altitude.
Vertical Speed Indicator (VSI): Utilizes Static pressure only. It operates via a highly sensitive diaphragm inside an airtight instrument case. The inside of the diaphragm receives direct, unrestricted static air. The surrounding case receives static air restricted by a microscopic orifice called a "calibrated leak". In level flight, pressure inside and outside the diaphragm is equalized (reads zero). During a climb or descent, the case pressure lags behind the changing diaphragm pressure; this structural pressure differential moves the needle to reflect the rate of altitude change.
⚖️ Atmospheric Temperature & Pressure Traps
The barometric scale adjustment knob (Kollsman window) allows pilots to correct for variations in local atmospheric pressure. However, these adjustments do not automatically compensate for nonstandard temperatures.
Cold air is significantly denser than warm air, causing atmospheric pressure levels to compress closer to the ground. If a pilot flies into a colder air mass or an area of lower barometric pressure without adjusting the altimeter setting, the instrument will interpret the denser air as a higher altitude.
You must master the classic FAA checkride rules of thumb:
"From Hot to Cold, Look Out Below!"
"From High to Low, Look Out Below!"
When operating in colder-than-standard atmospheric columns or moving toward low-pressure areas while maintaining a constant indicated altitude, the aircraft's True Altitude is lower than its Indicated Altitude, drastically reducing critical terrain and obstacle clearance margins.
The 5 Types of Altitude:
To pass the written exam and accurately communicate vertical parameters, pilots must identify the five reference levels:
Indicated Altitude: The vertical height read directly off the face of the altimeter when dialed to the current local barometric setting.
True Altitude: The actual vertical distance of the aircraft above Mean Sea Level (MSL). Airport elevations, mountain peaks, and chart obstructions are always published as true altitudes.
Absolute Altitude: The actual vertical distance of the aircraft above the immediate surface terrain (Above Ground Level, or AGL).
Pressure Altitude: The altitude indicated when the Kollsman window scale is adjusted to standard datum plane parameters (29.92 "Hg). This is used to compute performance charts.
Density Altitude: Pressure altitude corrected for nonstandard temperature variations. On a hot day, air molecules thin out, causing the aircraft to perform as if it were flying at a significantly higher altitude. This metric is a direct indicator of aircraft takeoff and climb performance.
The 4 Types of Airspeed:
Airspeed metrics diverge based on tracking systems, installation physics, and air density variables:
Indicated Airspeed (IAS): The raw instrument reading obtained directly from the Airspeed Indicator face, uncorrected for density or mechanical errors. Aircraft performance speeds (like stall speed \(V_{S0}\) or flap speed \(V_{FE}\)) are published in the POH as IAS and remain unchanged across varying altitudes.
Calibrated Airspeed (CAS): Indicated airspeed corrected for installation and instrument configuration errors. This error is generally greatest at low airspeeds near stall parameters; in cruise ranges, IAS and CAS are nearly identical.
True Airspeed (TAS): Calibrated airspeed corrected for altitude and nonstandard temperature profiles. Because air thins out at higher altitudes, an aircraft must travel physically faster through a less dense air mass to register the same dynamic impact pressure on the pitot tube. True airspeed is utilized for flight planning and calculating track timelines.
Groundspeed (GS): The actual speed of the aircraft across the ground. It is calculated by taking True Airspeed (TAS) and adjusting for wind vectors (subtracting a headwind component or adding a tailwind component).
⚠️ Critical Pitot-Static System Failure Profiles:
Blocked Pitot Entry (Drain Hole Open): If the front impact opening hits an insect or ice, but the rear moisture drain hole remains clear, ram air vents out completely. The Airspeed Indicator needle will drop immediately to zero knots.
Blocked Pitot Entry & Blocked Drain Hole (Static Open): If both openings on the pitot tube are clogged, dynamic pressure becomes completely trapped inside the diaphragm. If the aircraft climbs, the static case pressure thins out, allowing the trapped ram pressure to expand the diaphragm. The Airspeed Indicator will act like an Altimeter (climbing needle during climbs, dropping needle during descents), presenting a highly dangerous illusion to the pilot.
Blocked Static Ports (Pitot Open): The Altimeter will freeze solid at the exact altitude where the blockage occurred. The VSI will slowly vent until its pressure equalizes, locking completely at zero. The Airspeed Indicator will continue to react to speed changes but will be highly inaccurate (reading falsely low at altitudes above the blockage point and falsely high below it).
3. Gyroscopic Instruments
⚠️ The Legacy Venturi Vacuum Hazard
Many vintage light aircraft do not feature an internal engine-driven vacuum pump; instead, they rely on external venturi tubes mounted directly to the outer fuselage skin to generate system suction from passing airflow.
The Trap: A venturi vacuum architecture is highly reliable because it has no moving pump parts, but it is 100% dependent on forward airspeed to generate gyro suction. If you are operating a vintage aircraft at night or under low-visibility conditions, your gyroscopic instruments will be completely unstable and inaccurate while on the ramp or during slow taxi maneuvers. Failing to plan for this spool-up lag after takeoff can instantly strip a pilot of a reliable attitude reference when transitioning into a pitch-black sky or low cloud deck.
Core System Instruments
Attitude Indicator (AI): Operates on the principle of rigidity in space. It utilizes a gimbal-mounted gyro spinning on a vertical axis to provide simultaneous, instantaneous indications of pitch and bank. This is the primary instrument for attitude control in instrument conditions.
Heading Indicator (HI): Also operates on rigidity in space, but the gyro spins on a horizontal axis. Because mechanical friction causes the gyro to drift over time (mechanical precession), pilots must cross-check and reset the Heading Indicator to match the magnetic compass every 15 minutes during straight-and-level, unaccelerated flight.
Turn Coordinator: Operates on the principle of precession. The internal gimbal is canted (angled upward), allowing the instrument to sense both the initial rate of roll when entering a turn and the stabilized rate of turn. The inclinometer (the fluid-filled tube containing the ball) indicates aerodynamic coordination:
Slip: The ball falls to the inside of the turn (insufficient rudder for bank angle).
Skid: The ball slides to the outside of the turn (excessive rudder for bank angle)
Links:
Intro to Aviation & Airplanes
A Brief History of Flight Training
Airfoils, Lift & Angle of Attack
Stalls, Critical Angle of Attack & Spin Awareness
Primary Flight Controls & the Three Axis
Stability, Controllability & Adverse Yaw
Engine Instruments & System Monitoring
Pilot Certification & Privileges
Medical Certificates & BasicMed
Flight Rules (Part 91 Highlights)
Airport Marking, Signs, & Lighting
Traffic Patterns & Airport Operations
Special Use Airspace & Other Airspace Areas
Chart Supplements & Visual Aids
The Atmosphere & Atmospheric Pressure
Temperature,Moisture, & Cloud Formation
Airmasses, Fronts & Weather Patterns
Thunderstorms, Turbulence, Icing & Other Hazards
Weather Reports, Forecasts & Charts
VFR Navigation Charts & Flight Logs
Weight & Balance Computations and Performance Charts
Aeronautical Decision Making (ADM) & Risk Management
Multi-Engine & Advanced Transition Basics
Preparing for the Knowledge & Practical Test
Practical Test Focus Areas
Common Weak Areas & How to Fix Them
Night Operations & Post-Checkride Privileges
Night Flying Physics, Equipment, and Regulations
Lesson 11.2
Currency Requirements
Medical Certification & Basic Med After the Check ride
Continuing Education & Proficiency
Engine Failure & Forced Landings
In-Flight Fires & Electrical Problems
System and Instrument Failures
Emergency Communication and Declaring an Emergency
Pitot-Static System Failures
Blocked Pitot tube-Clear Drain Hole
(Pictured Below)
Blocked Pitot Tube-Clear Static Port (Below)
Blocked Static System (Below)
Attitude Indicator
Heading Indicator
Turn Coordinator
4. Magnetic Compass Operation & Anomalies
The magnetic compass is the ultimate backup instrument. It is entirely self-contained, requiring no electrical power or vacuum suction to operate. However, because it reacts directly to the Earth's magnetic flux lines, it is prone to several specific errors that are heavily tested on the FAA knowledge exam.
Magnetic Variation: The angular difference between geographic True North and Magnetic North. This varies by location and is corrected by reading the magenta isogonic lines printed on VFR sectional charts.
Magnetic Deviation: Deflection caused by local electromagnetic interference from the aircraft's internal radios, engine components, and metallic structure. This is corrected using the mechanical Compass Correction Card mounted adjacent to the instrument face.
Acceleration Errors (ANDS): When flying on an East or West heading, changes in airspeed cause the compass to dip due to inertia. Remember the acronym ANDS: Accelerate North, Decelerate South. (When accelerating, the compass turns toward the North; when decelerating, it turns toward the South).
Turning Errors (UNOS): When initiating a turn from a Northerly or Southerly heading, the vertical pull of the Earth's magnetic field causes the compass card to misbehave. Remember the acronym UNOS: Undershoot North, Overshoot South.
When turning from a North heading, the compass initially lags or turns briefly in the opposite direction (Undershoot).
When turning from a South heading, the compass turns much faster than the actual rate of turn, rushing ahead of your actual heading (Overshoot).
Acceleration Errors
Northerly and Southerly Turning Errors
5. Primary Flight Displays (PFD) & Electronic Deck Modernization
Modern flight instruction requires students to understand both traditional round dials and integrated Electronic Flight Instrument Systems (EFIS), commonly referred to as glass cockpits.
Primary Flight Display (PFD): Replaces the traditional "six-pack" with a single, highly integrated liquid crystal display (LCD) screen. Airspeed, attitude, altitude, vertical speed, slip/skid, and heading are consolidated into one unified display to streamline the pilot's visual scan.
The ADAHRS Infrastructure: Electronic glass cockpits rely on solid-state sensors rather than mechanical gyros and gears:
Air Data Computer (ADC): Consumes raw pitot and static pressure inputs electronically to calculate exact indicated airspeed, true airspeed, altitude, and vertical speed parameters.
Attitude Heading Reference System (AHRS): Employs solid-state tilt sensors, accelerometers, and magnetometers to calculate precise real-time pitch, roll, and directional tracking data.
This quiz serves as an example as to what to expect on the FAA Private Pilot Written Exam. I have provided links to official practice tests in the reference section
