Instruments

Pedostatic Flight Instruments Study Notes

Vertical Speed Indicator (VSI)

Function: The VSI indicates whether the aircraft is climbing or descending, displaying this as a rate of climb.
- Measurement unit: Feet per Minute (FPM) or $ext{FPM}$ .
Ground Reference: The VSI should read zero or close to zero when the aircraft is on the ground.
- Reference note: If it shows a non-zero value while on the ground, this value will serve as the reference during flight.
- Example: If the VSI shows one tick above the zero mark, this is your reference for all future readings during the flight.
Simulator Indications: When the simulator is activated:
  - If climbing, VSI shows increasing values:
    - Starts from zero to 500, 1000, 1500 FPM as the climb rate increases.
  - If descending, similar behavior:
    - Values decrease back to zero.
Delayed Response: The VSI takes a few seconds to stabilize after a climb or descent begins. The altimeter, however, responds immediately.
Operational Mechanics:
  - Connected to a static port, which is the only connection for the instrument.
    - Contains a wafer that responds to changes in static pressure as the aircraft changes altitude.
    - Calibrated leak allows pressure to equalize slowly, causing the display to take time to reflect new values.
  - Mechanism Explained:
    - When the aircraft climbs, outside pressure decreases. Static pressure enters the instrument and the pressure inside the wafer decreases, causing it to contract, indicating a climb.
    - Conversely, as altitude stabilizes, pressure around the wafer equalizes, and the needle returns to zero on the VSI.

Airspeed Indicator (ASI)

Functionality: The airspeed indicator measures the difference between pressure in the pitot tube and static port.
Units of Measurement: Speeds can be expressed in Knots (KT) - a nautical mile per hour - or miles per hour (MPH).
Airspeed Types:
  - Indicated Airspeed (IAS): The speed shown on the ASI itself; used for determining aircraft performance.
  - Calibrated Airspeed (CAS): Indicated airspeed corrected for installation errors. Requires reference charts for adjustments.
  - True Airspeed (TAS): Calibrated airspeed corrected for altitude and nonstandard temperature.
  - Ground Speed: The speed of the aircraft over the ground, affected by wind conditions (headwinds vs tailwinds).
AS Display Characteristics: Marks on the airspeed indicator include different arcs:
  1. White Arc: Flap operating range, with VSO (stall speed in landing config) and VFE (max flap extended speed).
  2. Green Arc: Normal operating range, includes VS1 (stalling speed in a specified configuration) and VNO (maximum structural cruising speed).
  3. Yellow Arc: Caution range; exceeding VNO can risk structural integrity due to gusts.
  4. Red Arc: VNE (never exceed speed).
  5. Notably:
     - VA (Maneuvering speed), VLO (Landing gear operating speed), VLE (Landing gear extended speed), VMC (minimum controllable speed in multi-engine aircraft), VX (best angle of climb), VY (best rate of climb).

Gyroscopic Instruments

Overview: Instruments using gyroscopic principles include the turn coordinator, heading indicator, and attitude indicator.
Gyroscope Principles:
- Rigidity in Space: A spinning gyro remains stable while the aircraft rotates around it.
- Precession: The tilting of a gyro in response to a deflective force, resulting in a force acting at 90 degrees to the direction of the applied force.
Instrument Connections:
- Powered mainly by either a vacuum system (attitude & heading indicators) or an electric system (turn coordinator), ensuring redundancy in systems.

Attitude Indicator

Primary Function: Indicates the aircraft's orientation (pitch and roll) with respect to a reference horizon.
Displayed Information: Includes climb, descent, and turns without lag, crucial for instrument flight.
Internal Mechanics: Contains a gyro that remains rigid in space, attached to the instrument displaying the horizon.
Display Examples: Straight and level flight, climbing/descending flights, and turns indicated visually.

Heading Indicator

Functionality: Used to maintain aircraft headings; resets compared to a magnetic compass due to possible mechanical drift.
Operation: Requires periodic alignment to the compass; common errors while maneuvering lead to discrepancies in displayed direction.

Turn Coordinator

Design Type: Indicates both the rate of turn and yaw; shows standard rate of turns (3 degrees/second).
Incline Indication: Contains a ball (inclinometer) indicating coordination of turns (right or left).

Magnetic Compass

Basic Operating Principles: A simple magnet-based instrument directing the aircraft relative to magnetic north, requiring straight and level flight for accurate readings.
Errors Related to Compass:
  1. Variation: Difference between magnetic north and true north, measured in degrees.
  2. Deviation: Magnetic fields produced by aircraft components influencing compass readings.
  3. Turning Errors: Lag when nearing north or south (overshoot south, undershoot north).
  4. Acceleration Errors: Compass indicates north when accelerating and south when decelerating on east/west headings.
Key Formulas:
- $ext{True course} + ext{Variation} = ext{Magnetic course}$
- $ext{Magnetic course} + ext{Deviation} = ext{Compass course}$

Outside Air Temperature Gauge

Purpose: Indicates outside air temperature; crucial for performance calculations and assessing icing risks.

Typical FAA Questions

Magnetic Compass Indication Change: Compass will show turn towards the north when the aircraft accelerates on an east or west heading.
Turn Coordinator Indication Function: Provides an indication of the airplane's movement around the yaw and roll axis.
Heading Indicator Maintenance Requirement: Must be realigned with the magnetic compass periodically during flight.
Use of Magnetic Compass: Most effective during straight and level, unaccelerated flight.
Compass Turns: Remember OSON (Overshoot South, Undershoot North) for proper compass navigation.