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Flight Instruments

After this ~40 minute briefing, the student should have a working knowledge of the pitot-static and gyroscopic flight instruments. In order to better understand the operation and limitations (and to keep the briefing interesting) the discussion should include instruments’ failure modes as well.

Elements

  • Pitot Static
    • Altimeter
    • Airspeed indicator
    • Vertical speed indicator
  • Gyroscopic
    • Attitude indicator
    • Heading indicator
    • Horizontal situation indicator
    • Turn-and-slip indicator / turn coordinator and inclinometer
  • Magnetic compass
  • Clock

Equipment

Instructor actions

  • Discuss the lesson objective
  • Introduce by discussing earlier private-level instrument experience
  • Describe and introduce the basic flight instruments
  • With illustrations from ASA’s Instrument Flying, describe instrument construction
  • Describe operation using the Warrior systems trainer
  • Explain and review flight instrument use, using flight simulator to show relationships
  • Evaluate student’s learning by posing review questions throughout and correcting to 100%

Student actions

  • Prepare for the briefing by reading Instrument Flying chapter 2 or Jeppesen chapter
  • Participate with discussion, taking notes throughout
  • Answer questions and leave with a general understanding

Completion Standards

The lesson is complete when the student can demonstrate an adequate level of understanding of the flight instruments, their construction, and their operation. A private pilot level of knowledge is expected in the interpretation of flight instrument indications. Throughout the lesson they should be able to correctly answer a majority of the questions without significant instructor prompting.

Teaching outline

Throughout, the Warrior systems trainer should be used to illustrate the construction and operation of the flight instruments and their systems.

Pitot-static system

  • Airspeed, altimeter, and VSI pitot+static probe
  • Static port(s) provide ‘ambient’ air to the system
    • alternate air is available in some systems, causing a rise in airspeed, altitude, and vertical trend
    • if alternate air is unavailable, breaking the glass of the VSI will create one
  • Pitot tube provides ‘ram’ air to the airspeed indicator
    • heated to prevent blockage
    • drain hole to allow impacted rain or other potential blockages to leave the lines

Airspeed indicator, IF 36

airspeed indicator

  • Displays indicated airspeed only; Kollsmann window allows for correction to true airspeed with density altitude
  • Limitations and markings:
    • White arc – flap range
    • Green arc – normal operation
    • Yellow arc – caution range
    • Red line – never exceed
  • Operated by ram air moves a diaphragm which is linked to the airspeed needle
    • Measures dynamic air pressure – the difference between static and ram

Altimeter, IF 46

sensitive altimeter

  • Displays indicated altitude
  • Operated by static system pressure entering the sealed case
    • Sealed aneroid disks expand with higher altitudes and contract with lower
    • Mechanical linkage to altitude needles
    • Kollsmann window allows sea level pressure input using the calibration knob
  • SLP must be adjusted as flight proceeds and pressure changes
    • If the flight encounters lower pressure unadjusted, the altitude will indicate lower than true; the opposite holds for higher pressure
    • High to low, look out below.

Vertical speed indicator, IF 52

vertical speed indicator

  • Displays ascent and descent rate, or vertical speed
  • Not a required IFR instrument, but commonly found in the six pack of instruments
  • Configured like the altimeter, but with a calibrated leak in the case
    • The static system is connected directly to the diaphragm, causing pressure changes to indicate a trend

System malfunctions and errors

  • Errors stem from blockages
    • pitot blocked, drain open: IAS reads zero
    • pitot and drain blocked: acts like an altimeter, and airspeed increases with altitude
    • static blocked: altimeter, VSI frozen, airspeed functions but incorrect

Gyroscopic systems

  • Vacuum or electric
    • Motor spins the gyro if electric
    • Engine-operated vacuum pump, often with electric backup pump
  • Suction gauge indicates system pressure – know the safe range

Principles of gyroscope operation

  • Precession causes errors in the heading and attitude indicators, and is used by the turn coordinator
  • Rigidity in space is used by the heading and attitude indicators

Attitude indicator or artificial horizon, IF 31

attitude indicator

  • As the name suggests, it’s a replacement for the real, outside horizon
  • Fixed airplane or wings on the face of the instrument
  • Gimballed, moving horizon reference in the back
  • Shows changes in pitch and bank
  • Self-erecting gyroscope, spinning around a vertical axis
    • double-gimballed
    • rigidity in space – the airplane moves around the gyroscope
  • Precession (when rolling out of a steep 180° turn) and acceleration errors
    • self-corrects with pendulous vanes – when not upright, vanes open due to forces and air corrects the spin
  • During the after-start checklist and taxi it should stabilize after 5 minutes and bank less than 5° during level taxi turns

Turn-and-slip indicator / turn coordinator, IF 53, and inclinometer, IF 55

turn coordinator

  • Simplified way to maintain a 3°/second turn rate
  • Electrically driven gyroscope, set at a 30° angle
    • senses roll and rate
  • Inclinometer displays the aircraft’s coordination – the quality of the turn
    • much like a carpenter’s bubble level
  • During the instrument check, there should be no flag, the aircraft should bank into the turn, and the ball should go outside, indicating a skid

Heading indicator, IF 40

heading indicator

  • Shows a top-down representation of the aircraft, with a compass card rotating around the outside
  • Uses rigidity in space; the aircraft pivots around the gyro, indicating the turn
  • Must be regularly reset to the magnetic compass, due to gyroscopic precession and mechanical inaccuracies
    • 15 minutes is the general rule

Horizontal situation indicator, IF 45

Horizontal situation indicator

  • Gets data from a remote sensing flux gate compas
    • updates the compass card with the slaving motor

Magnetic compass, IF 56

  • Points to magnetic north, subject to a variety of turn and acceleration errors

Clock, IF 57

  • Needs to be physically installed in the aircraft, set, and turning
  • It’s a clock. It tells time.