Theory of Flight Flight Instruments

Reference From the Ground Up Chapter 2.2: Flight Instruments Pages 33 - 44

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.

Outline Pitot-Static Instruments Gyroscopic Instruments Magnetic Compass

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

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

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

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.

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

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

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.

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

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)

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)

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

Variometer Very sensitive VSI used in gliders to find thermals

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

Gyroscope

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)

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

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

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

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

Next Lesson 4.1 – Air Law Aerodromes From the Ground Up Chapter 4.1: Pages 89 - 98