Instrumentation
9 February 2005
Instrumentation
 Pitot-static system
 Altimiter
 Vertical Speed Indicator
 Airspeed Indicator
 Gyroscopic Instruments
 Turn coordinator
 Artificial horizon
 Heading indicator
 Magnetic Compass
 OAT Gauge
Pitot-static system
 Operates in response to air pressure
 Two air pressures:
 Static pressure
 Taken from static vents, powers all three pitotstatic system instruments (ASI, VSI, Altimeter)
 Impact pressure
 Powers airspeed
indicator only
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 will
cause the instrument to read
the approximate field
elevation when located on
the ground at the airport
 Reset altimeter to 29.92
when climbing through
18,000 feet.
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.
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.
“High to low…look 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 setting…and vice versa.
Density vs. True Altitude
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.
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.
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.
Airspeeds – color coded

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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)
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.)
Gyroscopic Principles
 Rigidity in space
 Precession
Axis of rotation points in a
constant direction regardless
of the position of its base.
Tilting or turning of a gyro in
response to a deflective
force.
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
Turn Coordinator
 Relies on precession
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
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).
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.
Magnetic Compass
 Compass points to magnetic north
 Susceptible to several errors
Compass Errors
 Variation
 Deviation
 Magnetic Dip
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
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 S&L, unaccelerated
flight.
Variation Errors
 Magnetic poles do not coincide with geographic
poles.
 Most places on Earth, the
compass needle does not
point to True North. Angular
differences between
magnetic north and true
north are called variations
and are displayed on
aeronautical charts.
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.
Next Week…
-
Regulations
-
(FAR/AIM & Test Prep)