Operation of Systems

Airspeed Indicator: The ASI measures how fast you’re moving through the air. It compares pressure from the pitot tube (facing forward into the wind) with static pressure. The difference moves a diaphragm connected to the needle. It shows speed in knots or miles per hour and is the only instrument that uses both pitot and static pressure.

• Power Source: Pitot-static (uses both pitot and static ports)

• Indication: Airspeed in knots

• Markings: Color-coded arcs (white = flap operating, green = normal ops, yellow = caution, red = never exceed)

• Principle of Operation: Measures dynamic pressure (pitot - static)

• Limitations: Requires correct pitot and static input; ice or blockage can render it inaccurate or inoperative

• Errors: Position error, density altitude effects, pitot tube/drain hole blockage

Altimeter: The altimeter has sealed, flexible metal aneroid wafers inside that expand or contract based on outside air pressure. As you climb, outside pressure drops, and the wafers expand. As you descend, pressure increases, and they compress. This movement turns gears that move the needles to show your altitude. 

• Power Source: Static system only

• Indication: Altitude above the selected pressure level

• Markings: Numbered face with 20-ft intervals, barometric setting window (Kollsman window)

• Principle of Operation: Expanding/contracting aneroid wafers as pressure changes

• Limitations: Must be set to current pressure; tolerance ±75 feet for IFR

• Errors: Incorrect setting, pressure changes, static port blockage

Vertical Speed Indicator: The VSI compares fast and slow changes in air pressure. A diaphragm inside gets pressure right away, but the rest of the instrument fills more slowly. When pressure changes quickly (like during a climb or descent), the difference causes the needle to move up or down. It has a small delay due to the restricted orifice (calibrated leak).

• Power Source: Static system only

• Indication: Rate of climb or descent in feet per minute (FPM)

• Markings: Scale with increments typically in 500 or 1000 FPM

• Principle of Operation: Measures rate of pressure change through a calibrated leak

• Limitations: Has lag (~6–9 seconds), limited accuracy in rapid altitude changes

• Errors: Static blockage, lag in reading

(Note: Some aircraft are equipped with an instantaneous vertical speed indicator (IVSI), which incorporates accelerometers to compensate for the lag in the typical VSI. [Figure 8-6])

Spinning Gyro (Mechanical Gyro)

• Used in: Standby Attitude Indicator

• Powered by: Engine-driven vacuum system

• How it works: A physical spinning mass maintains orientation in space. As the aircraft moves around it, the gyro stays fixed, providing pitch and roll reference.

• Why it matters: Functions as a backup if the G1000 system or electrical power fails.

Solid-State Gyro

• Used in: AHRS (Attitude Heading Reference System)

• Powered by: Aircraft electrical system

• No moving parts: Uses microelectromechanical sensors (MEMS) to detect angular movement (pitch, roll, yaw).

• Feeds data to: Attitude Indicator, Turn Rate Indicator, and Heading computation via AHRS.

• Why it matters: Provides stable, accurate reference without the limitations of mechanical gyros (e.g., drift, precession).

• Power Source: Vacuum or electric, depending on aircraft

• Indication: Pitch and bank relative to the horizon

• Markings: Artificial horizon with pitch ladder and roll indicators

• Principle of Operation: Gyroscopic rigidity in space

• Limitations: Precession over time, may tumble beyond pitch/bank limits

• Errors: Acceleration/deceleration errors, vacuum failure

• Power Source: Vacuum or electric, depending on aircraft

• Indication: Magnetic heading (must be synced with magnetic compass)

• Markings: 360° compass rose with heading bug (if HSI)

• Principle of Operation: Gyroscopic rigidity (rotating in vertical plane)

• Limitations: Subject to precession; needs regular realignment

• Errors: Drift, failure of power source (vacuum/electric)

• Power Source: Electric (most commonly)

• Indication: Rate of turn and coordination (slip/skid)

• Markings: Airplane symbol with standard rate turn tick marks; ball inclinometer for coordination

• Principle of Operation: Gyro precession (rate of turn) + inclinometer (ball)

• Limitations: Only accurate for standard rate turns; not good for exact bank angle

• Errors: Electric failure, slow response, instrument lag

Magnetometer

• Used in: Heading Information via AHRS

• Powered by: Electrical system

• Located remotely: Typically in a wing to reduce interference.

• What it does: Measures Earth’s magnetic field to determine magnetic heading.

• How it works with others: AHRS blends magnetometer input with gyro data to generate a smooth, drift-free heading.

• Why it matters: Replaces the function of a magnetic compass within the G1000 environment.

• What it provides: Gives your heading indicator on the PFD information through AHRS

Accelerometer: a solid state gyro

• Used in: AHRS for motion refinement

• Powered by: Electrical system

• What it senses: Linear acceleration in all directions (e.g., nose-up, descending, turning).

• Purpose: Helps the AHRS validate and refine the aircraft’s attitude and movement—especially during rapid maneuvers.

• Why it matters: Ensures the Attitude Indicator remains accurate under dynamic flight conditions.

GPS Basics

• Global Positioning System (GPS) is a U.S.-operated GNSS (Global Navigation Satellite System).

• Constellation: At least 24 satellites orbiting ~10,900 NM above Earth. At least 5 satellites are visible from any point at any time.

• • • 3 Satellites: 2D Position (Latitude + Longitude)

    • 4 Satellites: 3D Position (Latitude + Longitude + Altitude)

    • 5 Satellites: RAIM – Fault Detection (checks signal integrity)

    • 6 Satellites: RAIM – Fault Exclusion (identifies and removes bad signal)

How Position is Determined

• Each satellite creates a “pseudo-range” sphere.

• The aircraft is located at the intersection of multiple spheres from several satellites.

RAIM – Receiver Autonomous Integrity Monitoring

• Ensures GPS signal integrity by checking for faults.

• 5 satellites required for fault detection (RAIM).

• 6 satellites needed for fault exclusion.

Substitution Rules

• GPS can substitute:

  • DME

  • ADF (except for NDB approaches with no GPS overlay)

• Check GPS NOTAMs and use RAIM prediction preflight.

GPS Augmentation Systems (DGPS)

SBAS – Satellite Based Augmentation System

• In the U.S., this is called WAAS.

• Uses ground stations to monitor GPS errors and send corrections through satellites.

• Improves:

  • Accuracy

  • Integrity

  • Availability

• Enables APV approaches like:

  • LPV

  • LNAV/VNAV

  • LP

GBAS – Ground Based Augmentation System

• Previously called LAAS

• Sends error corrections via VHF, not satellites

• More accurate than WAAS but covers a smaller area

• Supports GLS approaches with Category I or better minima

Broadcast on 108.0 to 117.95 MHz, excluding 108.1-111.95 and odd tenths.

• Full scale deflection: 10º

• Must verify correct and functional VOR station

• The VOR MON (VOR Minimum Operational Network) program ensures that as old VORs are decommissioned, a MON airport (equipped with ILS or VOR approach) is available within 100 nautical miles

Reference and Variable Signals:

• The reference signal goes out in all directions.

• The variable signal rotates like a lighthouse beam—starting from 0° (North) and sweeping clockwise.

• If you’re east (090° radial) of the VOR:

  • The rotating variable signal reaches you shortly after the reference signal.
    → Small timing difference.

• If you’re west (270° radial):

  • The variable signal takes longer to sweep around and reach you.
    → Larger timing difference.

Receiver Checks:

• VOT: ±4º

• Ground: ±4º

• Airborne: ±6º

• Dual: ±4º

• Repair Station: ± 4º

• Prominent landmark: ±6º 

VOR Errors:

• Cone of confusion

• Area of ambiguity

• Reverse sensing

• Line of sight

• Propeller rotation: spinning at such an RPM that it will disrupt the VOR frequencies.

• 962-1213 UHF

• Normally tuned automatically with a paired VHF station (VOR/LOC)

• The airborne DME unit transmits and interrogation signal

• The ground DME facility receives and replies to the interrogation

• Airborne unit calculates the slant range distance to the station based on the reply time

• Due to slant range error, when flying overhead the station, DME indicates greater than zero

• Slant range error is negligible at 1 nautical mile per every 1000'

• Isobaric: The most common system, where cabin pressure is maintained at a constant value regardless of the outside air pressure.

• Isobaric Differential: Military fighter aircraft begin pressurization on ascent until the cabin reaches a pre-set altitude. If the altitude is reached, a constant pressure differential is maintained in correlation with the ambient pressure outside the airplane.

• Sealed Cabin: Used in spacecraft only, where the vessel carries its own supply of gases.