Instrumentation

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Instrumentation

• 9 February 2005

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Instrumentation

• Pitot-static system
• Altimiter
• Vertical Speed Indicator
• Airspeed Indicator
• Gyroscopic Instruments
• Turn coordinator
• Artificial horizon
• Heading indicator
• Magnetic Compass
• OAT Gauge

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Pitot-static system

• Operates in response to air pressure
• Two air pressures
• Static pressure
• Taken from static vents, powers all three
  pitot-static system instruments (ASI, VSI,
  Altimeter)
• Impact pressure
• Powers airspeedindicator only

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Static System Altimetry

• Static system powers altimeter
• Altimeter operates as a barometer
• Set altimeter on the ground to local settings
• Air pressure decreases at a constant rate per
  foot increased in lower atmosphere (approximately
  1000 per 1 Hg)
• Nonstandard temperature and pressure affect
  altimeter

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Altimeter

• As static pressure decreases, indicated altitude
  increases
• Altimeter setting is adjustable in Kohlsman
  Window, aka Altimeter Setting Window
• Local altimeter setting willcause the instrument
  to readthe approximate fieldelevation when
  located onthe ground at the airport
• Reset altimeter to 29.92when climbing
  through18,000 feet.

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Altitude Terminology

• Indicated Altitude
• Altitude read on the altimeter when it is set to
  the current local altimeter setting
• Absolute altitude
• Height above the surface
• True altitude
• True height above Mean Sea Level (MSL)
• Pressure altitude
• Altitude indicated whenever the altimeter setting
  dial is set to 29.92 (Standard Datum Plane)
• Density altitude
• Pressure altitude corrected for non-standard
  temperature and/or pressure.

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Altimetry

• Standard day
• 29.92 Hg and 15 deg. C
• On a standard day at sea level, pressure
  altitude, true altitude, indicated altitude, and
  density altitude are all equal.

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High to lowlook out below

• When flying from an area of low pressure/low
  temperature to an area of higher pressure/higher
  temperature without adjusting the altimeter
  setting, the altimeter will indicate lower than
  the true altitude settingand vice versa.

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Density vs. True Altitude
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Vertical Speed Indicator (VSI)

• Operates only on static pressure, but is a
  differential pressure instrument
• Operates on the principle of a calibrated leak
• Face of VSI outputs change in pressure over time
  displayed in feet per minute.

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Airspeeds and Airspeed Indicator

• Airspeed Indicator
• Displays difference between pitot (impact)
  pressure and static pressure
• Pressures are equal when airplane is parked on
  ground in calm air.

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Airspeeds

• Indicated airspeed (IAS)
• Uncorrected reading from the airspeed indicator
• Calibrated airspeed (CAS)
• Indicated airspeed corrected for installation and
  instrument error.
• True airspeed (TAS)
• Calibrated airspeed corrected for temperature and
  pressure variations.
• Groundspeed (GS)
• Actual speed of the airplane over the ground
  this is the TAS adjusted for wind.

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Airspeeds color coded

• VSO stall speed / minimum steady flight in
  landing configuration (lower limit of white arc)
• VFE max. flap-extended speed (upper limit of
  white arc)
• VS1 stall speed in specified configuration
  (lower limit of green arc)
• VNO max. structural cruising speed (top of
  green arc, bottom of yellow arc)
• VNE never exceed speed (upper limit of yellow
  arc, marked in red)

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Airspeeds, others

• VLE max. landing gear-extended speed.
• VA design maneuvering speed (flown in rough air
  or turbulence to prevent overstressing airframe)
• VY Best rate-of-climb airspeed (creates most
  altitude in a given period of time)
• VX Best angle-of-climb speed (airspeed
  resulting in most altitude in a given distance.)

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Gyroscopic Principles

• Rigidity in space
• Axis of rotation points in a constant direction
  regardless of the position of its base.

• Precession
• Tilting or turning of a gyro in response to a
  deflective force.

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The Attitude Indicator

• Relies on rigidity in space
• Direction of bank determined by relationship of
  miniature airplane to the horizon bar.
• Miniature airplane remains stationary horizon
  moves

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Turn Coordinator

• Relies on precession
• As an airplane enters a turn, the TC indicates
  rate of roll. When bank is held constant, TC
  indicates rate of turn.
• Most TCs display an index on the Standard-rate
  turn, wherein the airplane takes 2 minutes to
  turn 360 degreers.
• The ball or inclinometer indicates quality of
  turn (skid/slip status).

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Heading indicator

• Gyroscopic compass
• Magnetic compasses are difficult to read and
  suffer from errors the heading indicator (also
  known as a directional gyro or DG)
• DGs suffer from precession due to bearing
  friction the indicator must be realigned with
  the magnetic compass during straight-and-level,
  unaccelerated flight.

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Magnetic Compass

• Compass points to magnetic north
• Susceptible to several errors

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Compass Errors

• Variation
• Deviation
• Magnetic Dip

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Dip errors

• Magnetic dip
• When turning north from an easterly or westerly
  heading, the compass lags behind the actual
  aircraft heading. When a turn is initiated while
  on a northerly heading, the compass first
  indicates a turn in the opposite direction.
• When turning south from an easterly or westerly
  heading, the compass leads the actual heading.
  When a turn is initiated on a southerly heading,
  the compass immediately leads ahead.
• Mnemonic UNOS undershoot north, overshoot south

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Dip errors continued

• Accelerating or decelerating while heading either
  east or west will also cause compass errors.
• When accelerating on an east or west heading, the
  compass indicates a turn to the north.
• When decelerating on an east or west heading, the
  compass indicates a turn to the south.
• Mnemonic ANDS accelerate north, decelerate
  south.
• Compass accurate only in SL, unaccelerated
  flight.

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Variation Errors

• Magnetic poles do not coincide with geographic
  poles.
• Most places on Earth, thecompass needle does
  notpoint to True North. Angulardifferences
  betweenmagnetic north and truenorth are called
  variationsand are displayed onaeronautical
  charts.

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Deviation Errors

• The metal, electrical systems, and operating
  engine all create magnetic fields from the
  aircraft.
• Aircraft manufacturers install compensatory
  magnets to prevent most errors. Remaining errors
  are called deviation.
• A card in the aircraft will list the deviation at
  various different compass points.

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Next Week

• Regulations
• (FAR/AIM Test Prep)