Sep 2012Lesson 2.5

Theory of Flight

Flight Instruments

Reference

From the Ground UpChapter 2.2:Flight InstrumentsPages 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 SpeedIndicator (VSI)Altimeter (Alt)

AirspeedIndicator (ASI)

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

Green ArcNormal Range (Lower limit = VS)

Yellow ArcCaution Range (Lower limit = VNO)

Red LineNever Exceed Speed (VNE)

White ArcFlaps 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 LawAerodromes

From the Ground UpChapter 4.1:AerodromesPages 89 - 98