Introduction
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. This chapter addresses the
pitot-static system and associated instruments, the vacuum
system and related instruments, gyroscopic instruments, and
the magnetic compass. When a pilot understands how each
instrument works and recognizes when an instrument is
malfunctioning, he or she can safely utilize the instruments
to their fullest potential.
Pitot-Static Flight 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). [Figure 8-1]
Flight
Chapter 8
Instruments
Figure 8-1. Pitot-static system and instruments.
30.0
29.929.8
Altimeter
Heater (35 watts)
Static port
Vertical speed indicator (VSI)Airspeed indicator (ASI)
Pitot heater switch
Drain hole
Pressure chamber
Alternate static source
Static hole
Heater (100 watts)
Pitot tube
Baffle plate
Static chamber
Ram air
Impact Pressure Chamber and Lines
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.
Wind 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.
When the wind blows from an angle less than 90° off the
nose of the aircraft, dynamic pressure can be depicted on the
ASI. The wind moving across the airfoil at 20 knots is the
same as the aircraft moving through calm air at 20 knots.
The pitot tube captures the dynamic pressure, as well as the
static pressure that is always present.
The pitot tube has a small opening at the front that allows
the total pressure to enter the pressure chamber. The total
pressure is made up of dynamic pressure plus static pressure.
In addition to the larger hole in the front of the pitot tube,
there is a small hole in the back of the chamber that allows
moisture to drain from the system should the aircraft enter
precipitation. Both openings in the pitot tube must be checked
prior to flight to ensure that neither is blocked. Many aircraft
have pitot tube covers installed when they sit for extended
periods of time. This helps to keep bugs and other objects
from becoming lodged in the opening of the pitot tube.
The one instrument that utilizes the pitot tube is the ASI. The
total pressure is transmitted to the ASI from the pitot tube’s
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 VSI) utilize only
the static pressure that is derived from the static port.
Static Pressure Chamber and Lines
The static chamber is vented through small holes to the
free undisturbed air on the side(s) 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
that connect the instruments to 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
Figure 8-2. Altimeter.
Aneroid wafers
Static port
100 ft. pointer
A crosshatched area appears on
some altimeters when displaying
an altitude below 10,000 feet MSL.
Crosshatch flag
Barometric scale adjustment knob
10,000 ft. pointer
1,000 ft. pointer
Altimeter setting window
deck. Due to the venturi effect of the air flowing around the
fuselage, the air pressure inside the flight deck is lower than
the exterior pressure.
When the alternate static source pressure is used, the
following instrument indications are observed:
1. The altimeter indicates a slightly higher altitude than
actual.
2. The ASI indicates an airspeed greater than the actual
airspeed.
3. The VSI shows a momentary climb and then stabilizes
if the altitude is held constant.
Each pilot is responsible for consulting the Aircraft Flight
Manual (AFM) or the Pilot’s Operating Handbook (POH)
to determine the amount of error that is introduced into the
system when utilizing the alternate static source. In an aircraft
not equipped with an alternate static source, an alternate
method of introducing static pressure into the system should
a blockage occur is to break the glass face of the VSI. This
most likely renders the VSI inoperative. The reason for
choosing the VSI as the instrument to break is that it is the
least important static source instrument for flight.
Altimeter
The altimeter is an instrument that measures the height of
an aircraft above a given pressure level. Pressure levels
are discussed later in detail. 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 aneroid wafer is a sealed
wafer that is evacuated to an internal pressure of 29.92
inches of mercury ("Hg). 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 (less than 29.92 "Hg)
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. [Figure 8-2]
Notice how the static pressure is introduced into the rear of the
sealed altimeter case. The altimeter’s outer chamber is sealed,
which allows the static pressure to surround the aneroid
wafers. If the static pressure is higher than the pressure in the
aneroid wafers (29.92 "Hg), then the wafers are compressed
until the pressure inside the wafers is equal to the surrounding
static pressure. Conversely, if the static pressure is less than
the pressure inside of the wafers, the wafers are able to expand
which increases the volume. The expansion and contraction
of the wafers moves the mechanical linkage which drives the
needles on the face of the altimeter.
Principle of Operation
The pressure altimeter is an aneroid barometer that measures
the pressure of the atmosphere at the level where the altimeter is
located and presents an altitude indication in feet. The altimeter
uses static pressure as its source of operation. Air is denser
at sea level than aloft—as altitude increases, atmospheric
pressure decreases. This difference in pressure at various levels
causes the altimeter to indicate changes in altitude.
The presentation of altitude varies considerably between
different types of altimeters. Some have one pointer while
others have two or more. Only the multipointer type is
discussed in this handbook. The dial of a typical altimeter
is graduated with numerals arranged clockwise from zero
to nine. Movement of the aneroid element is transmitted
through gears to the three hands that indicate altitude. In
Figure 8-2, the long, thin needle with the inverted triangle
at the end indicates tens of thousands of feet; the short, wide
needle indicates thousands of feet; and the long needle on
top indicates hundreds of feet.
Figure 8-3. Effects of nonstandard temperature on an altimeter.
5,000 foot pressure level
4,000 foot pressure level
3,000 foot pressure level
2,000 foot pressure level
1,000 foot pressure level
Sea level
30°C 15°C 0°C
This indicated altitude is correct, however, only when the
sea level barometric pressure is standard (29.92 "Hg), the sea
level free air temperature is standard (+15 degrees Celsius
(°C) or 59 degrees Fahrenheit (°F)), and the pressure and
temperature decrease at a standard rate with an increase
in altitude. Adjustments for nonstandard pressures are
accomplished by setting the corrected pressure into a
barometric scale located on the face of the altimeter. The
barometric pressure window is sometimes referred to as
the Kollsman window; only after the altimeter is set does it
indicate the correct altitude. The word “correct” will need
to be better explained when referring to types of altitudes,
but is commonly used in this case to denote the approximate
altitude above sea level. In other words, the indicated
altitude refers to the altitude read off of the altitude which is
uncorrected, after the barometric pressure setting is dialed
into the Kollsman window. The additional types of altitudes
are further explained later.
Effect of Nonstandard Pressure and Temperature
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 and temperature can
change between takeoff and landing even on a local flight.
If these changes are not taken into consideration, flight
becomes dangerous.
If altimeters could not be adjusted for nonstandard 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 then 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 en route to ensure
terrain and obstruction clearance.
Many altimeters do not have an accurate means of being
adjusted for barometric pressures in excess of 31.00
"Hg. When the altimeter cannot be set to the higher
pressure setting, the aircraft actual altitude is higher than
the altimeter indicates. When low barometric pressure
conditions occur (below 28.00), flight operations by
aircraft unable to set the actual altimeter setting are
not recommended.
Adjustments to compensate for nonstandard pressure do
not compensate for nonstandard 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. [Figure 8-3] It is the magnitude of this
“difference” that determines the magnitude of the error. It is
the difference due to colder temperatures that concerns the
pilot. When flying into a cooler air mass while maintaining a
constant indicated altitude, true altitude is lower. If terrain or
obstacle clearance is a factor in selecting a cruising altitude,
particularly in mountainous terrain, remember to anticipate
that a colder-than-standard temperature places the aircraft
lower than the altimeter indicates. Therefore, a higher
indicated altitude may be required to provide adequate terrain
clearance. A variation of the memory aid used for pressure
Figure 8-4. Look at the chart using a temperature of –10 °C and
an aircraft altitude of 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.
Height Above Airport in Feet
+10
0
-10
-20
-30
-40
-50
10
20
20
30
40
50
60
10
20
30
50
60
80
90
10
30
40
60
80
100
120
10
30
50
70
100
120
150
20
40
60
90
120
150
180
20
40
70
100
140
170
210
20
50
80
120
150
190
240
20
50
90
130
170
220
270
20
60
100
140
190
240
300
30
90
150
210
280
360
450
40
120
200
280
380
480
590
60
170
290
420
570
720
890
80
230
390
570
760
970
1,190
90
280
490
710
950
1,210
1,500
Reported
Temp 0 °C200 300 400 500 600 700 800 900 1,0001,5002,0003,0004,0005,000
can be employed: “FROM HOT TO COLD, LOOK OUT
BELOW.” When the air is warmer than standard, the aircraft
is higher than the altimeter indicates. Altitude corrections for
temperature can be computed on the navigation computer.
Extremely cold temperatures also affect altimeter indications.
Figure 8-4, which was derived from ICAO formulas,
indicates how much error can exist when the temperature is
extremely cold.
Setting the Altimeter
Most altimeters are equipped with a barometric pressure
setting window (or Kollsman 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. Air traffic
control (ATC) will advise when updated altimeter settings
are available. If a pilot is not utilizing ATC assistance,
local altimeter settings can be obtained by monitoring local
automated weather observing system/automated surface
observation system (AWOS/ASOS) or automatic terminal
information service (ATIS) broadcasts.
Many pilots confidently expect the current altimeter setting
will compensate for irregularities in atmospheric pressure at
all altitudes, but this is not always true. The altimeter setting
broadcast by ground stations is the station pressure corrected
to mean sea level. It does not account for the irregularities
at higher levels, particularly the effect of nonstandard
temperature. If each pilot in a given area is using the same
altimeter setting, each altimeter should be equally affected
by temperature and pressure variation errors, making it
possible to maintain the desired vertical separation between
aircraft. This does not guarantee vertical separation though.
It is still imperative to maintain a regimented visual scan for
intruding air traffic.
When flying over high, mountainous terrain, certain
atmospheric conditions cause the altimeter to indicate an
altitude of 1,000 feet or more higher than the actual altitude.
For this reason, a generous margin of altitude should be
allowed—not only for possible altimeter error, but also for
possible downdrafts that might be associated with high winds.
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
29.85 "Hg from the control tower or ATIS and sets this value
in the altimeter setting window. The altimeter indication
should then be compared with the known airport elevation of
487 feet. Since most altimeters are not perfectly calibrated,
an error may exist.
When over Mineral Wells, assume the pilot receives a current
altimeter setting of 29.94 "Hg and sets this in the altimeter
window. Before entering the traffic pattern at Abilene
Municipal Airport, a new altimeter setting of 29.69 "Hg
is received from the Abilene Control Tower and set in
the altimeter setting window. If the pilot desires to fly the
traffic pattern at approximately 800 feet above the terrain,
and the field elevation of Abilene is 1,791 feet, an indicated
altitude of 2,600 feet should be maintained (1,791 feet +
800 feet = 2,591 feet, rounded to 2,600 feet).
The importance of properly setting the altimeter cannot
be overemphasized. Assume the pilot did not adjust the
altimeter at Abilene to the current setting and continued using
the Mineral Wells setting of 29.94 "Hg. 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.
Mineral Wells altimeter setting 29.94
Abilene altimeter setting 29.69
Difference 0.25
(Since 1 inch of pressure is equal to approximately 1,000 feet
of altitude, 0.25 × 1,000 feet = 250 feet.)
When determining whether to add or subtract the amount of
altimeter error, remember that when the actual pressure is lower
than what is set in the altimeter window, the actual altitude
of the aircraft is lower than what is indicated on the altimeter.
The following is another method of computing the altitude
deviation. Start by subtracting the current altimeter setting
from 29.94 "Hg. Always remember to place the original setting
as the top number. Then subtract the current altimeter setting.
Mineral Wells altimeter setting 29.94
Abilene altimeter setting 29.69
29.94 – 29.69 = Difference 0.25
(Since 1 inch of pressure is equal to approximately 1,000 feet
of altitude, 0.25 × 1,000 feet = 250 feet.) Always subtract
the number from the indicated altitude.
2,600 – 250 = 2,350
Now, try a lower pressure setting. Adjust from altimeter
setting 29.94 to 30.56 "Hg.
Mineral Wells altimeter setting 29.94
Altimeter setting 30.56
29.94 – 30.56 = Difference –0.62
(Since 1 inch of pressure is equal to approximately 1,000 feet
of altitude, 0.62 × 1,000 feet = 620 feet.) Always subtract
the number from the indicated altitude.
2,600 – (–620) = 3,220
The pilot will be 620 feet high.
Notice the difference is a negative number. Starting with the
current indicated altitude of 2,600 feet, subtracting a negative
number is the same as adding the two numbers. By utilizing
this method, a pilot will better understand the importance of
using the current altimeter setting (miscalculation of where
and in what direction an error lies can affect safety; if altitude
is lower than indicated altitude, an aircraft could be in danger
of colliding with an obstacle).
Altimeter Operation
There are two means by which the altimeter pointers can
be moved. The first is a change in air pressure, while the
other is an adjustment to the barometric scale. When the
aircraft climbs or descends, changing pressure within the
altimeter case expands or contracts the aneroid barometer.
This movement is transmitted through mechanical linkage
to rotate the pointers.
A decrease in pressure causes the altimeter to indicate an
increase in altitude, and an increase in pressure causes the
altimeter to indicate a decrease in altitude. Accordingly, if
the aircraft is sitting on the ground with a pressure level of
29.98 "Hg and the pressure level changes to 29.68 "Hg, the
altimeter would show an increase of approximately 300 feet
in altitude. This pressure change is most noticeable when the
aircraft is left parked over night. As the pressure falls, the
altimeter interprets this as a climb. The altimeter indicates
an altitude above the actual field elevation. If the barometric
pressure setting is reset to the current altimeter setting of 29.68
"Hg, then the field elevation is again indicated on the altimeter.
This pressure change is not as easily noticed in flight since
aircraft fly at specific altitudes. The aircraft steadily decreases
true altitude while the altimeter is held constant through pilot
action as discussed in the previous section.
Knowing the aircraft’s altitude is vitally important to a
pilot. The pilot must be sure that the aircraft is flying high
enough to clear the highest terrain or obstruction along the
intended route. It is especially important to have accurate
altitude information when visibility is restricted. To clear
obstructions, the pilot must constantly be aware of the altitude
of the aircraft and the elevation of the surrounding terrain. To
reduce the possibility of a midair collision, it is essential to
maintain altitude in accordance with air traffic rules.
Types of Altitude
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 to 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:
1. Indicated altitude—read directly from the altimeter
(uncorrected) when it is set to the current altimeter
setting.
2. 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.