tutorial 7 flight instruments this
tutorial will cover flight instruments
in order to safely fly any aircraft a
pilot must understand how to interpret
and operate the flight instruments the
pilot also needs to be able to recognize
associated errors and malfunctions of
these instruments the pitot-static
system is a combined system that
utilizes the static air pressure and the
dynamic pressure due to the motion of
the aircraft through the air these
combined pressures are utilized for the
operation of the airspeed indicator ASI
altimeter and vertical speed indicator
vsi shown above the pitot tube is
utilized to measure the total combined
pressures that are present when an
aircraft moves through the air static
pressure also known as ambient pressure
is always present whether an aircraft is
moving or at rest it is simply the
barometric pressure in the local area
dynamic pressure is present only when an
aircraft is in motion therefore it can
be thought of as a pressure due to
motion wend also generates dynamic
pressure it does not matter if the
aircraft is moving through still air at
70 knots or if the aircraft is facing a
wind with a speed of 70 knots the same
dynamic pressure is generated the pitot
tube has a small opening at the front
which allows the total pressure to enter
the pressure chamber the total pressure
is made up of the dynamic pressure plus
static pressure the one instrument that
utilizes the pitot tube is the ASI the
total pressure is transmitted to the ASI
from the pitot tubes pressure chamber
via a small tube the static pressure is
also delivered to the opposite side of
the ASI which serves to cancel out the
two static pressures thereby leaving the
dynamic pressure to be indicated on the
instrument when the dynamic pressure
changes the ASI shows either increase or
decrease corresponding to the direction
of change the two remaining instruments
altimeter and vs I utilize only the
static pressure which is derived from
the static port the static chamber is
vented through small holes to the free
undisturbed air on the sides of the
aircraft as the atmospheric pressure
changes the pressure is able to move
freely in and out of the instruments
through the small lines which connect
the instruments into the static system
an alternate static source is provided
in some aircraft to provide static
pressure should the primary static
source become blocked the alternate
static source is normally found inside
the flight deck
the altimeter is an instrument that
measures the height of an aircraft above
a given pressure level since the
altimeter is the only instrument that is
capable of indicating altitude this is
one of the most vital instruments
installed in the aircraft to use the
altimeter effectively the pilot must
understand the operation of the
instrument as well as the errors
associated with the altimeter and how
each affect the indication a stack of
sealed aneroid wafers comprise the main
component of the altimeter an Android
wafer is a sealed wafer that is
evacuated to an internal pressure of
29.92 inches of mercury these wafers are
free to expand and contract with changes
to the static pressure a higher static
pressure presses down on the wafers and
causes them to collapse a lower static
pressure allows the wafers to expand a
mechanical linkage connects the wafer
movement to the needles on the indicator
face which translates compression of the
wafers into a decrease in altitude and
translates an expansion of the wafers
into an increase in altitude the dial of
a typical altimeter is graduated with
numbers arranged clockwise from 0 to 9
movement of the aneroid element is
transmitted through gears to the three
hands that indicate altitude the
shortest hand indicates altitude in tens
of thousands of feet the intermediate
hand in thousands of feet and the
longest hand in hundreds of feet
adjustments for non-standard pressures
are accomplished by setting the
corrected pressure into a barometric
scale located on the face of the
altimeter
it is easy to maintain a consistent
height above ground if the barometric
pressure and temperature remain constant
but this is rarely the case the pressure
temperature can change between takeoff
and landing even a local flight if these
changes are not taken into consideration
flight becomes dangerous if altimeters
could not be adjusted for non-standard
pressure a hazardous situation could
occur for example if an aircraft is
flown from a high pressure area to a low
pressure area without adjusting the
altimeter a constant altitude will be
displayed but the actual height of the
aircraft above the ground would be lower
than the indicated altitude there is an
old aviation axiom going from a high to
a low look out below conversely if an
aircraft is flown from a low pressure
area to a high pressure area without an
adjustment of the altimeter the actual
altitude of the aircraft is higher than
the indicated altitude once in flight it
is important to frequently obtain
current altimeter settings in route to
ensure terrain and obstruction clearance
adjustments to compensate for
non-standard pressure do not compensate
for non-standard temperature since cold
air is denser than warm air when
operating in temperatures that are
colder than standard the altitude is
lower than the altimeter indication is
shown above a variation of the memory
aid used for pressure can be employed
from hot to cold look out below
extremely cold temperatures will also
affect altimeter indications the above
image which was derived from ICAO
formulas indicates how much error can
exist when the temperature is extremely
cold look at the chart using a
temperature of minus 10 degrees Celsius
and the aircraft altitude is 1,000 feet
above the airport elevation the chart
shows that the reported current
altimeter setting may place the aircraft
as much as 100 feet below the altitude
indicated by the altimeter most
altimeter czar equipped with a
barometric pressure setting window or
kohls minh window providing a means to
adjust the altimeter a knob is located
at the bottom of the instrument for this
adjustment to adjust the altimeter for
variation in atmospheric pressure the
pressure scale in the altimeter setting
window calibrated in inches of mercury
hg and/or millibars mb is adjusted to
match the given altimeter setting
altimeter setting is defined as station
pressure reduced to sea level but an
altimeter setting is accurate only in
the vicinity of the reporting station
therefore the altimeter must be adjusted
as the flight progresses from one
station to the next to illustrate the
use of the altimeter setting system
follow a flight from Dallas Love Field
Texas to Abilene Municipal Airport Texas
via mineral wells before taking off from
Love Field the pilot receives a current
altimeter setting of twenty nine point
eight five inches from the control tower
or adis and sets this value in the
altimeter setting window when over
mineral wells assume the pilot receives
a current altimeter setting of 29.9 four
inches and sets this in the altimeter
window before entering the traffic
pattern at Abilene Municipal Airport a
new altimeter setting of twenty nine
point six nine inches is received from
the Abilene control tower and set in the
altimeter setting window however assume
the pilot did not adjust the altimeter
at Abilene to the current setting
continued using the mineral wells
setting of 29.9 four inches when
entering the Abilene traffic pattern at
an indicated altitude of 2,600 feet the
aircraft would be approximately 250 feet
below the proper traffic pattern
altitude upon landing the altimeter
would indicate approximately 250 feet
higher than the field elevation altitude
in itself is a relevant term only when
it is specifically stated to which type
of altitude a pilot is referring
normally when the term altitude is used
it is referring to altitude above sea
level since this is the altitude which
is used to depict obstacles and airspace
as well as two separate air traffic
altitude is vertical distance above some
point or level used as a reference there
are as many kinds of altitude as there
are reference levels from which altitude
is measured and each may be used for
specific reasons pilots are mainly
concerned with five types of altitudes
one indicated altitude read directly
from the altimeter uncorrect occurrent
altimeter setting to true altitude the
vertical distance of the aircraft above
sea level the actual altitude it is
often expressed as feet above mean sea
level MSL Airport terrain and obstacle
elevations on aeronautical charts are
true altitudes three absolute altitude
the vertical distance of an aircraft
above the terrain or above ground level
AGL for pressure altitude the altitude
indicated when the altimeter setting
window barometric scale is adjusted to
29.92 inches pressure altitude is used
to compute density altitude true
altitude true airspeed TAS and other
performance data 5 density altitude
your altitude corrected for variations
from standard temperature when
conditions are standard pressure
altitude and density altitude are the
same this is an important altitude
because it is directly related to the
aircraft's performance prior to each
flight a pilot should examine the
altimeter for proper indications in
order to verify its validity to
determine the condition of an altimeter
set the barometric scale to the current
reported altimeter setting transmitted
by the local automated flight service
station a FSS or any other reliable
source such as adis a wasps or a sauce
the altimeter pointers should indicate
the surveyed field elevation of the
airport if the indication is off more
than 75 feet from the surveyed field
elevation the instrument should be
referred to a certificated instrument
repair station for recalibration the vsi
indicates whether the aircraft is
climbing descending or in level flight
the rate of climb or descent is
indicated in feet per minute fpm if
properly calibrated the vsi indicates
zero in level flight the inside of the
diaphragm is connected directly to the
static line of the pitot-static system
the area outside the diaphragm which is
inside the instrument case is also
connected to the static line but through
a restricted orifice calibrated leak
both the diaphragm and the case receive
air from the static line at existing
atmospheric pressure the diaphragm
receives unrestricted error while the
case receives the static pressure via
the metered leak when the aircraft is on
the ground or in level flight the
pressures inside the diaphragm and the
instrument case are equal and the
pointer is at zero indication when the
aircraft climbs or descends the pressure
inside the diaphragm changes immediately
but due to the metering action of the
restricted passage the case pressure
remains higher or lower for a short time
causing the diaphragm to contract or
expand
this causes a pressure differential that
is indicated on the instrument needle as
a climb or descent as part of a
pre-flight check proper operation of the
vsi must be established make sure the
vsi indicates near zero prior to leaving
the ramp area and again just before
takeoff if the vsi indicates anything
other than zero that indication can be
referenced as the zero mark normally if
the needle is not exactly zero it is
only slightly above or below the zero
line after takeoff the vsi should trend
upward to indicate a positive rate of
climb and then once a stabilized climb
is established a rate of climb can be
referenced
the ASI shown above is a sensitive
differential pressure gauge which
measures and promptly indicates the
difference between pito impact dynamic
pressure and static pressure these two
pressures are equal when the aircraft is
parked on the ground in calm air when
the aircraft moves through the air the
pressure on the pedo line becomes
greater than the pressure in the static
lines this difference in pressure is
registered by the airspeed pointer on
the face of the instrument which is
calibrated in miles per hour knots
nautical miles per hour or both the ASI
introduces the static pressure into the
airspeed case while the pitot pressure
dynamic is introduced into the diaphragm
the dynamic pressure expands or
contracts one side of the diaphragm
which is attached to an indicating
system just as in altitudes there are
multiple types of air speeds pilots need
to be very familiar with each type one
indicated airspeed IAS the direct
instrument reading obtained from the ASI
uncorrect Adair EA shins in atmospheric
density installation error or instrument
error takeoff landing and stall speeds
listed in the AFM poh RI a s and do not
normally very with altitude or
temperature to calibrated airspeed CAS
IAS corrected for installation error and
instrument error although manufacturers
attempt to keep airspeed errors to a
minimum it is not possible to eliminate
all errors throughout the airspeed
operating range at certain air speeds
and with certain flap settings the
installation and instrument errors may
total several knots in the cruising and
higher air speed ranges IAS and cas are
approximately the same refer to the
airspeed calibration chart to correct
for possible airspeed errors three true
airspeed TAS cas corrected for altitude
and non-standard temperature up
pilot can find TAS by two methods the
most accurate method is to use a flight
computer with this method the CAS is
corrected for temperature and pressure
variation by using the airspeed
correction scale on the computer a
second method which is a rule of thumb
provides the approximate TAS simply add
two percent to the cas for each 1,000
feet of altitude the TAS is the speed
which is used for flight planning and is
used when filing a flight plan for
ground speed GS the actual speed of the
airplane over the ground it is TAS
adjusted for wind GS decreases with a
headwind and increases with a tailwind
aircraft weighing 12,500 pounds or less
manufactured after 1945 and certificated
by the FAA are required to have a sis
marked in accordance with a standard
color coded marking system this system
of color-coded markings enables a pilot
to determine at a glance certain
airspeed limitations that are important
to the safe operation of the aircraft as
shown in above ASIS on single-engine
small aircraft include the following
standard color-coded markings one white
arc commonly referred to as the flap
operating range since its lower limit
represents the full flap stall speed and
it's upper limit provides the maximum
flap speed to lower limit of the wide
arc vs oh the stalling speed or the
minimum steady flight speed in the
landing configuration three upper limit
of the white arc vfe the maximum speed
with the flats extended for green arc
the normal operating range of the
aircraft most flying occurs within this
range five lower limit of the green arc
vs one the stalling speed or the minimum
steady flight speed obtained in a
specified configuration for most
aircraft this is the power-off stall
speed at the maximum takeoff weight in
the clean configuration gear up if
retractable and flaps up six upper limit
of green arc V in oh the maximum
structural cruising speed do not exceed
this speed except in smooth air seven
yellow arc caution range fly within this
range only in smooth air and then only
with caution eight redline VN e never
exceed speed operating above this speed
is prohibited since it may result in
damage or structural failure some
important airspeed limitations are not
marked on the face of the ASI but are
found on placards and in the afm
poh these air speeds include one design
maneuvering speed VA the maximum speed
at which the structural designs limit
load can be imposed either by gusts or
full deflection of a control surface
without causing structural damage to
best angle of climb speed VX the air
speed at which an aircraft gains the
greatest amount of altitude in a given
distance three best rate of climb speed
V Y the airspeed that provides the most
altitude gain in a given period of time
prior to takeoff the ASI should read
zero however if there is a strong wind
blowing directly into the pitot tube the
ASI may read higher than zero when
beginning the takeoff make sure the air
speed is increasing at an appropriate
rate
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