Basic Flight Instruments and Atmosphere

Study Notes

6 basic flight instruments are divided into 2 groups: Pitot-Static and Gyroscopic.
Pitot-Static instruments include: Airspeed Indicator, Vertical Speed Indicator, and Altimeter.
Gyroscopic instruments include: Heading Indicator, Turn Indicator or Turn & Slip Indicator, and Attitude Indicator.
James Harold Doolittle was the first pilot to successfully fly an airplane using instruments alone.

Atmosphere is the gaseous envelope surrounding a heavenly body.
Composition of the Earth's atmosphere:
- 78% Nitrogen
- 21% Oxygen
- 1% Other gases
Properties of the Earth's atmosphere:
- Pressure: force exerted in every direction by the weight of the atmosphere.
- Atmospheric Pressure: weight exerted on any object by the column of air above it. Decreases with altitude.
- Temperature: degree of hotness or coldness. Cold temperature slows down molecule movement, while hot temperature speeds it up.
- Terrestrial Radiation: The Earth's ground absorbs heat from the sun and releases it, making lower altitudes hotter. Air molecules are bad conductors of heat.
- Density: mass per unit volume, or the amount of matter in a given space. Higher pressure means more air is present.

A standard model of the Earth's atmosphere used for aviation calculations.
Mean Sea Level (MSL): The level of the sea's surface at its average position.
Pressure and temperature changes with Altitude:
- For every 1000ft increase in altitude, pressure decreases by 1 inch of mercury (Hg) and temperature decreases by 2 degrees Celsius.
- For example:
  - At 0ft (MSL): Pressure = 29.92 in Hg, Temperature = 15 degrees Celsius
  - At 1000 ft: Pressure = 28.92 in Hg, Temperature = 13 degrees Celsius
  - At 500 ft: Pressure = 28.42 in Hg, Temperature = 14 degrees Celsius

Air Pressure Types:
- Static Pressure (SP): Atmospheric pressure exerted equally in all directions regardless of motion.
- Dynamic Pressure (DP): Pressure exerted by the air as an object moves through it. Proportional to airspeed and air density.
- Total Pressure (TP): Sum of Static Pressure and Dynamic Pressure.
Airspeed Indicator: Indicates the speed of the aircraft relative to the air by measuring Dynamic Pressure.
- Higher speed = More dynamic pressure
- Lower speed = Less dynamic pressure.

Total pressure enters the pitot tube and pushes a diaphragm outward.
Static pressure from the static port enters the case, pushing the diaphragm inward.
The opposing forces of static pressure cancel out, leaving dynamic pressure to expand the diaphragm.
The diaphragm movement is transferred through gears to a needle dial on the airspeed indicator.
Decreasing speed causes the diaphragm to contract.

Position Error: Occurs at high angles of attack when airflow is not parallel to the aircraft's longitudinal axis.
Density Error: Dynamic pressure is affected by air density (temperature and pressure), not just airspeed.
Wind Error: Impacts airspeed readings.

Indicated Airspeed (IAS): Speed displayed on the instrument without any corrections for errors.
Calibrated Airspeed (CAS): IAS corrected for instrument and position errors.
True Airspeed (TAS): CAS corrected for density errors (temperature and pressure), representing the actual speed of the aircraft relative to the air.
TAS Calculation: Can be done using a flight computer or an airspeed indicator's TAS scale.
Rule of Thumb for TAS: TAS increases by 2% for every 1000ft increase in altitude.