errors almost always indicate blockage
of the pitot tube the static ports or
both blockage may be caused by moisture
including ice dirt or even insects a
blocked pitot tube affects the accuracy
of the ASI but a blockage of the static
port not only affects the ASI but also
causes errors in the altimeter and vsi
if the pitot tube becomes blocked and
its associated drain hole remains clear
ram air is no longer able to enter the
pedo system air already in the system
vents through the drain hole and the
remaining pressure drops to ambient
outside air pressure under these
circumstances the ASI reading decreases
to zero as shown above because the ASI
senses no difference between RAM and
static air pressure the apparent loss of
airspeed is not usually instantaneous
but happens very quickly
if both the pitot tube opening and the
drain hole should become clogged
simultaneously then the pressure in the
pitot tube is trapped no change is noted
on the airspeed indication should the
airspeed increase or decrease if the
static port is unblocked and the
aircraft should change altitude then a
change is noted on the ASI the change is
not related to a change in airspeed but
a change in static pressure because
airspeed indications rely upon both
static and dynamic pressure together the
blockage of either of these systems
affects the ASI reading remember that
the ASI has a diaphragm in which dynamic
air pressure is entered behind this
diaphragm is a reference pressure called
static pressure that comes from the
static ports the diaphragm pressure
rises against this static pressure and
as a result changes the air speed
indication via levers and indicators as
seen above the pitot tube may become
blocked during flight due to visible
moisture some aircraft may be equipped
with pitot heat for flight invisible
moisture consult the afm poh for
specific procedures regarding the use of
pitot heat if the static system becomes
blocked but the pitot tube remains clear
the ASI continues to operate however it
is inaccurate if the aircraft descends
the static pressure increases on the
pitot side showing an increase on the
ASI this assumes that the aircraft does
not actually increase its speed the
increase in static pressure on the pedo
side is equivalent to an increase in
dynamic pressure since the pressure
cannot change on the static side if an
aircraft begins to climb after a static
port becomes blocked the airspeed begins
to show a decrease as the aircraft
continues to climb this is due to the
decrease in static pressure on the pedo
side while the pressure on the static
side is held constant shown above a
blockage of the static system also
affects the altimeter and vs I trapped
static pressure causes the altimeter to
freeze at the altitude where the
blockage
heard in the case of the vsi a blocked
static system produces a continuous zero
indication some aircraft are equipped
with an alternate static source in the
flight deck flight deck static pressure
is lower than outside static pressure
check the aircraft AOM Poh for airspeed
corrections when utilizing alternate
static pressure advances in digital
displays and solid-state electronic
components have been introduced into the
flight decks of general aviation GA
aircraft in addition to the improvement
in system reliability which increases
overall safety electronic flight
displays efd have decreased the overall
cost of equipping aircraft with
state-of-the-art instrumentation the
multi-panel digital flight displays
combined all flight instruments onto a
single screen which is called a primary
flight display or PFD the traditional
6-pack of instruments is now displayed
on one liquid crystal display LCD screen
configured similarly to traditional
panel layouts the ASI is located on the
left side of the screen and is displayed
as a vertical speed tape as the aircraft
increases in speed the larger numbers
descend from the top of the tape
airspeed markings for VX v y and
rotation speed vr are displayed for
pilot reference as on traditional analog
ASIS the electronic airspeed tape
displays the color-coded ranges for the
flap operating range normal range and
caution range one improvement over
analog instrumentation is the larger
attitude indicator on efd the artificial
horizon spans the entire width of the
PFD this expanded instrumentation offers
better reference through all phases of
flight and all flight maneuvers the
attitude indicator receives its
information from the attitude heading
and reference system a hrs the altimeter
is located on the right side of the PFD
as the altitude increases the larger
numbers descend from the top of the
display tape with the current altitude
being displayed in the black box in the
the display tape the altitude is
displayed in increments of 20 feet the
vsi is displayed to the right of the
altimeter tape and can take the form of
an arced indicator or a vertical speed
tape both are equipped with a vertical
speed bug the heading indicator is
located below the artificial horizon and
is normally modeled after a horizontal
situation indicator HSI as in the case
of the attitude indicator the heading
indicator receives its information from
the magnetometer which feeds information
to the a hrs unit and then out to the
PFD the turn indicator takes a slightly
different form than the traditional
instrumentation a sliding bar moves left
and right below the triangle to indicate
deflection from coordinated flight
reference for coordinated flight comes
from accelerometers contained in the a
hrs unit the slip skid indicator is the
horizontal line below the roll pointer
like a ball in a turnin slip indicator a
bar width off center is equal to one
ball with displacement the turn rate
indicator is typically found directly
above the rotating compass card tick
marks to the left and right of the
lubber line denote the turn standard
rate versus half standard rate typically
denoted by a trend line if the trend
vector is extended to the second tick
mark the aircraft is in a standard rate
turn
the sixth instrument normally associated
with the six pack package is the
tachometer this is the only instrument
that is not located on the PFD the
tachometer is normally located on the
multifunction display MFD in the event
of a display screen failure it is
displayed on the remaining screen with
the PFD flight instrumentation
individual panel displays are able to be
configured for a variety of aircraft
simply by installing different software
packages manufacturers are also able to
upgrade existing instrument displays in
a similar manner eliminating the need to
replace individual gauges in order to
upgrade electronic flight displays
utilize the same type of instrument
inputs as traditional analog gauges
however the processing system is
different the pitot-static inputs are
received by an air data computer ADC the
ADC computes the difference between the
total pressure and the static pressure
and generates the information necessary
to display the air speed on the PFD
outside air temperatures are also
monitored and introduced into various
components within the system as well as
being displayed on the PFD screen as
shown on the right the ADC is a separate
solid-state device which in addition to
providing data to the PFD is capable of
providing data to the autopilot control
system
trend vectors are magenta lines which
move up and down both the ASI and the
altimeter the ADC computes the rate of
change and displays the 6 second
projection of where the aircraft will be
pilots can utilize the trend vectors to
better control the aircraft's attitude
by including the trend vectors in the
instrument scan pilots are able to
precisely control airspeed and altitude
several flight instruments utilize the
properties of a gyroscope for their
operation the most common instruments
containing gyroscopes are the turn
coordinator heading indicator and the
attitude indicator to understand how
these instruments operate requires
knowledge of the instrument power
systems gyroscopic principles and the
operating principles of each instrument
two important design characteristics of
an instrument gyro are great weight for
its size or high density and rotation at
high speed with low friction bearings
there are two general types of mountings
the type used depends on which property
of the gyro is utilized a freely or
universally mounted gyroscope is free to
rotate in any direction about its center
of gravity such a wheel is said to have
three planes of freedom restricted or
semi-rigid li mounted gyroscopes are
those mounted so that one of the planes
of freedom is held fixed in relation to
the base there are two fundamental
properties of gyroscopic action rigidity
and space and procession rigidity in
space refers to the principle that a
gyroscope remains in a fixed position in
the plane in which it is spinning an
example of rigidity in space is that of
a bicycle wheel as the bicycle wheels
increase speed they become more and more
stable in their plane of rotation this
is why a bicycle is very unstable and
very maneuverable at low speeds and very
stable and less maneuverable at higher
speeds
precession is the tilting or turning of
a gyro in response to a deflective force
the reaction to this force does not
occur at the point at which it was
applied rather it occurs at a point that
is 90 degrees later in the direction of
rotation this principle allows the gyro
to determine a rate of turn by sensing
the amount of pressure created by a
change in direction using the example of
the bicycle precession acts on the
wheels in order to allow the bicycle to
turn while writing at normal speed it is
not necessary to turn the handlebars in
the direction of the desired turn a
writer simply leans in the direction
that he or she wishes to go since the
wheels are rotating in a clockwise
direction when viewed from the right
side of the bicycle if a rider leans to
the left a force is applied to the top
of the wheel to the left the force
actually acts 90 degrees in the
direction of rotation which has the
effect of applying a force to the front
of the tire causing the bicycle to move
to the left there is a need to turn the
handlebars at low speeds because of the
instability of the slowly turning gyros
and also to increase the rate of turn
precession can also create some minor
errors in some instruments precession
can cause a freely spinning gyro to
become displaced from its intended plane
of rotation through bearing friction etc
certain instruments may require
corrective realignment during flight
such as the heading indicator
in some aircraft all the gyros are
vacuum pressure or electrically operated
another aircraft vacuum or pressure
systems provide the power for the
heading and attitude indicators while
the electrical system provides the power
for the turn coordinator most aircraft
have at least two sources of power to
ensure at least one source of bank
information is available if one power
source fails the vacuum or pressure
system spins the gyro by drawing a
stream of air against the rotor veins to
spin the rotor at high speed much like
the operation of a water wheel or turban
a typical vacuum system consists of an
engine-driven vacuum pump relief valve
air filter gauge and tubing necessary to
complete the connections the gauge is
mounted in the aircraft's instrument
panel and indicates the amount of
pressure in the system vacuum is
measured in inches of mercury less than
ambient pressure as shown above air is
drawn into the vacuum system by the
engine-driven vacuum pump it first goes
through a filter which prevents foreign
matter from entering the vacuum or
pressure system the air then moves
through the attitude and heading
indicators where it causes the gyros to
spin a relief valve prevents the vacuum
pressure or section from exceeding
prescribed limits after that the air is
expelled overboard or used in other
systems such as for inflating pneumatic
de-icing boots it is important to
monitor vacuum pressure during flight
because the attitude and heading
indicators may not provide reliable
information when suction pressure is low
aircraft used two types of turn
indicators turn in slip indicator and
turn coordinator both instruments
indicate turn direction and quality
coordination and also serve as a backup
source of bank information in the event
and attitude indicator fails
coordination is achieved by referring to
the inclement which consists of a
liquid-filled curved tube with a ball
inside the gyro in the turn and slip
indicator rotates in the vertical plane
corresponding to the aircraft's
longitudinal axis a single gimbal limits
the planes in which the gyro can tilt
and a spring tries to return it to
Center because of precession a young
force causes the gyro to tilt left or
right as viewed from the pilot seat the
turn and slip indicator uses a pointer
called the turn needle to show the
direction and rate of turn the gimbal in
the turn coordinator is canted therefore
it's gyro can sense both rate of rule
and rate of turn when rolling into or
out of a turn the miniature aircraft
banks in the direction the aircraft is
rolled a rapid roll rate causes the
miniature aircraft to bank more steeply
than a slow roll rate the turn
coordinator can be used to establish and
maintain a standard rate turn by
aligning the wing of the miniature
aircraft with the turn index the above
image shows a picture of a turn
coordinator there are two marks on each
side left and right of the face of the
instrument the first mark is used to
reference a wings level zero rate of
turn the second mark on the left and
right sides of the instrument serve to
indicate a standard rate turn a standard
rate turn is defined as a turn rate of
three degrees per second the turn
coordinator indicates only the rate and
direction of turn it does not display a
specific angle of bank the inclined
ometer is used to depict aircraft yo
which is the side-to-side movement of
the aircraft's nose during coordinated
straight and level flight the
of gravity causes the ball to rest in
the lowest part of the tube centered
between the reference lines coordinated
flight is maintained by keeping the ball
centered if the ball is not centered it
can be centered by using the rudder to
Center the ball apply rudder pressure on
the side to which the ball is deflected
use the simple rule step on the ball to
remember which rudder pedal to press if
aileron and rudder are coordinated
during a turn the ball remains centered
in the tube if aerodynamic forces are
unbalanced the ball moves away from the
center of the tube as shown above in a
slip the rate of turn is too slow for
the angle of bank and the ball moves to
the inside of the turn in a skid the
rate of turn is too great for the angle
of bank and the ball moves to the
outside of the turn one additional tool
which may be added to the aircraft is a
yo string I yah string is simply a
string or piece of yarn attached to the
center of the windscreen when in
coordinated flight the string trail
straight back over the top of the
windscreen when the aircraft is either
slipping or skidding the yaw string
moves to the right or left depending on
the direction of slip or skid during the
pre-flight check to see that the
inclined honor is full of fluid and has
no air bubbles the ball should also be
resting at its lowest point when taxiing
the turn coordinator should indicate a
turn in the correct direction while the
ball moves opposite the direction of
turn the attitude indicator with its
miniature aircraft and horizon bar
displays a picture of the attitude of
the aircraft the relationship of the
miniature aircraft to the horizon bar is
the same as the relationship of the real
aircraft to the actual horizon the
instrument gives an instantaneous
indication of even the smallest changes
in attitude the gyro and the attitude
indicator is mounted in a horizontal
plane and depends upon rigidity and
space
for its operation the horizon bar
represents the true horizon this bar is
fixed to the gyro and remains in a
horizontal plane as the aircraft is
pitched or banked about its lateral or
longitudinal axis indicating the
attitude of the aircraft relative to the
true horizon the gyro spins in the
horizontal plane and resists deflection
of the rotational path since the gyro
relies on rigidity and space the
aircraft actually rotates around the
spinning gyro an adjustment knob is
provided with which the pilot may move
the miniature aircraft up or down to
align the miniature aircraft with a
horizon bar to suit the pilots line of
vision normally the miniature aircraft
is adjusted so that the wings overlap
the horizon bar when the aircraft is in
straight and level cruising flight every
pilot should be able to interpret the
banking scale illustrated above most
banking scale indicators on the top of
the instrument move in the same
direction from that in which the
aircraft is actually banked the
relationship of the miniature aircraft
to the horizon bar should be used for an
indication of the direction of bank the
attitude indicator is reliable and the
most realistic flight instrument on the
instrument panel it's indications are
very close approximations of the actual
attitude of the aircraft we hope you
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