1. Theory of Flight Flight Instruments

2. Reference
From the Ground Up Chapter 2.2: Flight Instruments Pages

3. Introduction
Flight instruments give the pilot all the information they need to properly operate the plane.
Pilots need to know what all the instruments do and how they operate.

4. Outline Pitot-Static Instruments Gyroscopic Instruments
Magnetic Compass

5. Pitot-Static System Pitot Tube
Measures dynamic and static pressures (AKA pitot pressure)
Positioned clear of slipstream and facing line of flight.

6. Pitot-Static System Static Port (AKA Static Vent)
Measures static pressure
Allows instrument cases to be same pressure as outside
Normally on side of aircraft and parallel to airflow

7. Pitot-Static System Static Port Pitot Tube Vertical Speed
Indicator (VSI)
Airspeed
Indicator (ASI)
Altimeter (Alt)
Static Port
Pitot Tube

8. Altimeter (Alt) Shows height in feet
Measures pressure of outside air (drops with altitude)
Aneroid capsules (like balloons) inside are set to standard pressure. As altitude changes, capsules expand and contract, moving needle on dial.

9. Altimeter Errors
Pressure Error - Atmospheric pressure changes from place to place
Temperature Error - Pressure changes with temperature
Mountain Effect - Near mountains, gusty winds can drop local pressure

10. Altitudes
Indicated Altitude – What is read off altimeter when set to current pressure
Pressure Altitude – What is read off altimeter when set to standard pressure (29.92” Hg)
Density Altitude – Pressure altitude corrected for temperature
True Altitude – Exact height above Mean Sea Level (MSL)
Absolute Altitude – Actual height above ground

11. Airspeed Indicator (ASI)
Shows speed through air (not over ground)
Shows Indicated Airspeed (IAS) in Knots or Miles Per Hour (MPH, older system)
Aneroid capsule connected to pitot pressure. Case connected to static pressure. Aneroid capsule inflates with more airspeed, moving dial clockwise. Static pressure in case corrects for altitude.

12. Airspeed Errors
Density – Atmospheric density Varies and thus changes accuracy of ASI
Position – Eddies/turbulence created by aircraft disrupt normal airflow into pitot tube or around static port.
Lag - Slowness of mechanical parts due to friction
Icing/Water – Can block pitot tube or static port

13. Airspeeds Indicated Airspeed (IAS) – What is read off dial
Calibrated Airspeed (CAS) - IAS corrected for instrument and installation errors
True airspeed (TAS) - CAS corrected for density and temperature (actual speed through the air)

14. Airspeed Markings Red Line Yellow Arc Green Arc White Arc
Never Exceed Speed (VNE)
Yellow Arc
Caution Range (Lower limit = VNO)
Green Arc
Normal Range (Lower limit = VS)
White Arc
Flaps Range
(Upper Limit = VFE, Lower limit = VSO)

15. Vertical Speed Indicator
VSI Indicates speed up or down (rate of climb/descent) in feet per minute (FPM)
Static pressure enters aneroid capsule and case. But pressure in case delayed. Capsule registers difference in pressure (as descent or climb) on dial.
Tends to lag. Will only show accurate rate after several seconds

16. Variometer
Very sensitive VSI used in gliders to find thermals

17. Gyroscope
Wheel (rotor) spinning fast in universal mounting (gimbal), so axis can point any direction
Gyroscopic Inertia - Tendency of rotating body to maintain it’s plane of motion
Precession – When force applied to point on rotating body, body acts as if force applied 90 degrees in direction of spin

18. Gyroscope

19. Heading Indicator (HI)
HI (AKA directional gyro) indicates heading without errors associated with compass
Operates on principle of gyroscopic inertia
Drifts off and must be corrected approx every 15 min (set to compass)

20. Attitude Indicator (AI)
AI (AKA Artificial Horizon) represents horizon as seen by pilot
Used when horizon is obscured by weather
Operates on Gyroscopic Inertia

21. Turn and Slip Indicator
Needle shows direction and rate of turn
Ball shows slipping or skidding, in turn:
If ball opposite needle = skidding
If ball on same side as needle = slipping
Operates on Gyroscopic Precession

22. Turn Co-Ordinator (TC)
Replaces Turn and Slip Indicator on newer planes
Similar to turn and slip indicator, but responds to roll and yaw in a turn
Operates on Gyroscopic Precession

23. Magnetic Compass
Dual magnet system mounted on pivot and able to freely rotate in compass bowl filled with alcohol (lower freezing point and more lubricating than water)
Lubber line is parallel with longitudinal axis, indicates aircraft heading
Only accurate when aircraft flying straight and level at constant airspeed

24. Next Lesson
4.1 – Air Law Aerodromes From the Ground Up Chapter 4.1: Pages