Flight Instruments
Chapter 5
Introduction
Aircraft became a practical means of transportation when
accurate flight instruments freed the pilot from the necessity
of maintaining visual contact with the ground. Flight
instruments are crucial to conducting safe flight operations
and it is important that the pilot have a basic understanding
of their operation. The basic flight instruments required
for operation under visual flight rules (VFR) are airspeed
indicator (ASI), altimeter, and magnetic direction indicator.
In addition to these, operation under instrument flight rules
(IFR) requires a gyroscopic rate-of-turn indicator, slip-skid
indicator, sensitive altimeter adjustable for barometric
pressure, clock displaying hours, minutes, and seconds with
a sweep-second pointer or digital presentation, gyroscopic
pitch-and-bank indicator (artificial horizon), and gyroscopic
direction indicator (directional gyro or equivalent).
30.0
29.929.8
Ram air Pitot tube
Static port
Baffle plate
Drain hole
Heater (100 watts)
Static hole
Static hole
Static chamber
Heater (35 watts)
Pitot pressure chamber
Pitot heater switch
Alternate static source
Drain hole
Figure 3-1. A typical electrically heated poitot-static head.
Figure 5-1. A typical electrically heated pitot-static head.
Aircraft that are flown in instrument meteorological
conditions (IMC) are equipped with instruments that provide
attitude and direction reference, as well as navigation
instruments that allow precision flight from takeoff to landing
with limited or no outside visual reference.
The instruments discussed in this chapter are those required
by Title 14 of the Code of Federal Regulations (14 CFR)
part 91, and are organized into three groups: pitot-static
instruments, compass systems, and gyroscopic instruments.
The chapter concludes with a discussion of how to preflight
these systems for IFR flight. This chapter addresses additional
avionics systems such as Electronic Flight Information
Systems (EFIS), Ground Proximity Warning System
(GPWS), Terrain Awareness and Warning System (TAWS),
Traffic Alert and Collision Avoidance System (TCAS),
Head Up Display (HUD), etc., that are increasingly being
incorporated into general aviation aircraft.
Pitot/Static Systems
Pitot pressure, or impact air pressure, is sensed through an
open-end tube pointed directly into the relative wind flowing
around the aircraft. The pitot tube connects to the ASI or an
air data computer depending on your aircraft's configuration.
Static Pressure
Static pressure is also used by the ASI as well as the other
pitot static instruments for determining altitude and vertical
speed. Static pressure may be sensed at one or more locations
on an aircraft. Some may be flush mounted on the fuselage or
integrated into the electrically heated pitot tube. [Figure 5-1]
These ports are in locations proven by flight tests to be in
undisturbed air, and they may be paired, one on either side of
the aircraft. This dual location prevents lateral movement of
the aircraft from giving erroneous static pressure indications.
The areas around the static ports may be heated with electric
heater elements to prevent ice forming over the port and
blocking the entry of the static air.
Three basic pressure-operated instruments are found in
aircraft instrument panels flown under IFR. These are the
ASI, sensitive altimeter, and vertical speed indicator (VSI).
All three instruments receive static air pressure for operation
with only the ASI receiving both pitot and static pressure.
[Figure 5-2]
Blockage of the Pitot-Static System
Errors in the ASI and VSI almost always indicate a blockage
of the pitot tube, the static port(s), or both. Moisture
(including ice), dirt, or even insects can cause a blockage in
both systems. During preflight, it is very important to make
sure the pitot tube cover is removed and that static port
openings are checked for blockage and damage.
Blocked Pitot System
If the pitot tube drain hole becomes obstructed, the pitot
system can become partially or completely blocked. When
dynamic pressure cannot enter the pitot tube opening, the ASI
no longer operates. If the drain hole is open, static pressure
equalizes on both sides of the diaphram in the ASI and the
30.0 29.9 29.8
Figure 3-2. A typical pitot-static system. Figure 5-2. A typical pitot-static system.
indicated airspeed slowly drops to zero. If the pitot tube ram
pressure hole and drain hole become obstructed, the ASI
operates like an altimeter as the aircraft climbs and descends.
Refer to the Pilot’s Handbook of Aeronautical Knowledge
(FAA-H-8083-25A) for more in depth information on
blocked pitot systems along with different scenarios and
how they effect the ASI.
Blocked Static System
When a static system becomes blocked but the pitot tube
remains clear the ASI continues to operate but is inaccurate.
When the aircraft is operated above the altitude where the
static ports became blocked the airspeed indicates lower
than the actual airspeed because the trapped static pressure
is higher than normal for that altitude. The opposite holds
true for operations at lower altitudes; a faster than actual
airspeed is displayed due to the relatively low static pressure
trapped in the system.
A blockage of the static system can also affect the altimeter
and VSI. Trapped static pressure causes the altimeter to
freeze at the altitude where the blockage occurred. In the case
of the VSI, a blocked static system produces a continuous
zero indication.
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 of the flight 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.
For more information on static system blockages and how to
best react to such situations, refer to the Pilot’s Handbook of
Aeronautical Knowledge (FAA-H-8083-25A).
Effects of Flight Conditions
The static ports are located in a position where the air at
their surface is as undisturbed as possible. But under some
flight conditions, particularly at a high angle of attack with
the landing gear and flaps down, the air around the static
port may be disturbed to the extent that it can cause an error
in the indication of the altimeter and ASI. Because of the
importance of accuracy in these instruments, part of the
certification tests for an aircraft is a check of position error
in the static system.
The Pilot’s Operating Handbook (POH)/Aircraft Flight
Manual (AFM) contains any corrections that must be applied
to the airspeed for the various configurations of flaps and
landing gear.
Pitot/Static Instruments
Sensitive Altimeter
A sensitive altimeter is an aneroid barometer that measures
the absolute pressure of the ambient air and displays it in
terms of feet or meters above a selected pressure level.
Principle of Operation
The sensitive element in a sensitive altimeter is a stack of
evacuated, corrugated bronze aneroid capsules. [Figure 5-3]
The air pressure acting on these aneroids tries to compress
them against their natural springiness, which tries to expand
them. The result is that their thickness changes as the air
pressure changes. Stacking several aneroids increases the
dimension change as the pressure varies over the usable
range of the instrument.
Below 10,000 feet, a striped segment is visible. Above this
altitude, a mask begins to cover it, and above 15,000 feet,
all of the stripes are covered. [Figure 5-4]
Another configuration of the altimeter is the drum-type.
[Figure 5-5] These instruments have only one pointer that
A crosshatched area appears on some
altimeters when displaying an altitude
below 10,000 feet MSL.
10,000 ft. pointer
Altimeter setting window
Static port
Barometric scale adjustment knob
Aneroid
Crosshatch flag
Altitude indication scale
100 ft. pointer
1,000 ft. pointer
Figure 3-3. Sensitive altimeter componements.
Figure 5-3. Sensitive altimeter components.
30.0 29.9 29.8
I00 FEET
4 5 6
CALIBRATED
TO
20,000 FEET
ALT
Figure 3-4. Three-pointer altimeter.Figure 5-4. Three-pointer altimeter.
4 5 6
0 6, 5 0 0
ALT
M B
2 9 9 2
I N H G
Figure 3-5. Drum-type altimeter.Figure 5-5. Drum-type altimeter.
makes one revolution for every 1,000 feet. Each number
represents 100 feet and each mark represents 20 feet. A drum,
marked in thousands of feet, is geared to the mechanism that
drives the pointer. To read this type of altimeter, first look at
the drum to get the thousands of feet, and then at the pointer
to get the feet and hundreds of feet.
A sensitive altimeter is one with an adjustable barometric scale
allowing the pilot to set the reference pressure from which the
altitude is measured. This scale is visible in a small window
called the Kollsman window. A knob on the instrument adjusts
the scale. The range of the scale is from 28.00 to 31.00 inches
of mercury ("Hg), or 948 to 1,050 millibars.
Rotating the knob changes both the barometric scale and the
altimeter pointers in such a way that a change in the barometric
scale of 1 "Hg changes the pointer indication by 1,000 feet.
This is the standard pressure lapse rate below 5,000 feet.
When the barometric scale is adjusted to 29.92 "Hg or 1,013.2
millibars, the pointers indicate the pressure altitude. The pilot
displays indicate altitude by adjusting the barometric scale
to the local altimeter setting. The altimeter then indicates the
height above the existing sea level pressure.
Altimeter Errors
A sensitive altimeter is designed to indicate standard changes
from standard conditions, but most flying involves errors
caused by nonstandard conditions and the pilot must be able
to modify the indications to correct for these errors. There
are two types of errors: mechanical and inherent.
Mechanical Altimeter Errors
A preflight check to determine the condition of an altimeter
consists of setting the barometric scale to the local 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
Figure 3-6. Effects of nonstandard temperature on an altimeter.Figure 5-6. The loss of altitude experienced when flying into an area where the air is colder (more dense) than standard.
setting. The altimeter should indicate the surveyed elevation
of the airport. If the indication is off by more than 75 feet from
the surveyed elevation, the instrument should be referred
to a certificated instrument repair station for recalibration.
Differences between ambient temperature and/or pressure
causes an erroneous indication on the altimeter.
Inherent Altimeter Error
When the aircraft is flying in air that is warmer than standard,
the air is less dense and the pressure levels are farther apart.
When the aircraft is flying at an indicated altitude of 5,000
feet, the pressure level for that altitude is higher than it would
be in air at standard temperature, and the aircraft is higher
than it would be if the air were cooler. If the air is colder
than standard, it is denser and the pressure levels are closer
together. When the aircraft is flying at an indicated altitude
of 5,000 feet, its true altitude is lower than it would be if the
air were warmer. [Figure 5-6]
Cold Weather Altimeter Errors
A correctly calibrated pressure altimeter indicates true
altitude above mean sea level (MSL) when operating within
the International Standard Atmosphere (ISA) parameters of
pressure and temperature. Nonstandard pressure conditions are
corrected by applying the correct local area altimeter setting.
Temperature errors from ISA result in true altitude being
higher than indicated altitude whenever the temperature is
warmer than ISA and true altitude being lower than indicated
altitude whenever the temperature is colder than ISA.
True altitude variance under conditions of colder than ISA
temperatures poses the risk of inadequate obstacle clearance.
Under extremely cold conditions, pilots may need to add an
appropriate temperature correction determined from the chart
in Figure 5-7 to charted IFR altitudes to ensure terrain and
obstacle clearance with the following restrictions:
• Altitudes specifically assigned by Air Traffic Control
(ATC), such as “maintain 5,000 feet” shall not be
corrected. Assigned altitudes may be rejected if the
pilot decides that low temperatures pose a risk of
inadequate terrain or obstacle clearance.
• If temperature corrections are applied to charted
IFR altitudes (such as procedure turn altitudes, final
approach fix crossing altitudes, etc.), the pilot must
advise ATC of the applied correction.
ICAO Cold Temperature Error Table
The cold temperature induced altimeter error may be
significant when considering obstacle clearances when
temperatures are well below standard. Pilots may wish to
increase their minimum terrain clearance altitudes with a
corresponding increase in ceiling from the normal minimum
when flying in extreme cold temperature conditions. Higher
altitudes may need to be selected when flying at low terrain
clearances. Most flight management systems (FMS) with
air data computers implement a capability to compensate
for cold temperature errors. Pilots flying with these systems
should ensure they are aware of the conditions under which
the system automatically compensates. If compensation is
applied by the FMS or manually, ATC must be informed
that the aircraft is not flying the assigned altitude. Otherwise,
vertical separation from other aircraft may be reduced creating
a potentially hazardous situation. The table in Figure 5-7,
derived from International Civil Aviation Organization
Reported Temp C°
Height Above Airport in Feet
+10
-10
-20
-30
-40
-50
200 300 400 500 600 700 800 900 1,000 1,500 2,000 3,000 4,000 5,000
10 10 10 10 20 20 20 20 20 30 40 60 80 90
20 20 30 30 40 40 50 50 60 90 120 170 230 280
20 30 40 50 60 70 80 90 100 150 200 290 390 490
30 50 60 70 90 100 120 130 140 210 280 420 570 710
40 60 80 100 120 130 150 170 190 280 380 570 760 950
50 80 100 120 150 170 190 220 240 360 480 720 970 1,210
60 90 120 150 180 210 240 270 300 450 590 890 1,190 1,500
Figure 3-7. ICAO Cold Temperature Error.
Figure 5-7. International Civil Aviation Organization (ICAO) cold temperature error table.
(ICAO) standard formulas, shows how much error can exist
when the temperature is extremely cold. To use the table, find
the reported temperature in the left column, and then read
across the top row to the height above the airport/reporting
station. Subtract the airport elevation from the altitude of the
final approach fix (FAF). The intersection of the column and
row is the amount of possible error.
Example: The reported temperature is –10 degrees Celsius
(°C) and the FAF is 500 feet above the airport elevation. The
reported current altimeter setting may place the aircraft as
much as 50 feet below the altitude indicated by the altimeter.
When using the cold temperature error table, the altitude
error is proportional to both the height above the reporting
station elevation and the temperature at the reporting
station. For IFR approach procedures, the reporting station
elevation is assumed to be airport elevation. It is important
to understand that corrections are based upon the temperature
at the reporting station, not the temperature observed at the
aircraft’s current altitude and height above the reporting
station and not the charted IFR altitude.
To see how corrections are applied, note the following example:
Airport Elevation 496 feet
Airport Temperature –50 °C
A charted IFR approach to the airport provides the following
data:
Minimum Procedure Turn Altitude 1,800 feet
Minimum FAF Crossing Altitude 1,200 feet
Straight-in Minimum Descent Altitude 800 feet
Circling Minimum Descent Altitude (MDA) 1,000 feet
The Minimum Procedure Turn Altitude of 1,800 feet is used
as an example to demonstrate determination of the appropriate
temperature correction. Typically, altitude values are rounded
up to the nearest 100-foot level. The charted procedure turn
altitude of 1,800 feet minus the airport elevation of 500 feet
equals 1,300 feet. The altitude difference of 1,300 feet falls
between the correction chart elevations of 1,000 feet and 1,500
feet. At the station temperature of –50 °C, the correction falls
between 300 feet and 450 feet. Dividing the difference in
compensation values by the difference in altitude above the
airport gives the error value per foot.
In this case, 150 feet divided by 500 feet = 0.33 feet for each
additional foot of altitude above 1,000 feet. This provides a
correction of 300 feet for the first 1,000 feet and an additional
value of 0.33 times 300 feet, or 99 feet, which is rounded to
100 feet. 300 feet + 100 feet = total temperature correction
of 400 feet. For the given conditions, correcting the charted
value of 1,800 feet above MSL (equal to a height above the
reporting station of 1,300 feet) requires the addition of 400
feet. Thus, when flying at an indicated altitude of 2,200 feet,
the aircraft is actually flying a true altitude of 1,800 feet.
Minimum Procedure Turn Altitude
1,800 feet charted = 2,200 feet corrected
Minimum FAF Crossing Altitude
1,200 feet charted = 1,500 feet corrected
Straight-in MDA
800 feet charted = 900 feet corrected
Circling MDA
1,000 feet charted = 1,200 feet corrected
Nonstandard Pressure on an Altimeter
Maintaining a current altimeter setting is critical because the
atmosphere pressure is not constant. That is, in one location
the pressure might be higher than the pressure just a short
distance away. Take an aircraft whose altimeter setting is set
to 29.92" of local pressure. As the aircraft moves to an area
of lower pressure (Point A to B in Figure 5-8) and the pilot
fails to readjust the altimeter setting (essentially calibrating
it to local pressure), then as the pressure decreases, the true
altitude is lower. Adjusting the altimeter settings compensates
for this. When the altimeter shows an indicated altitude of
5,000 feet, the true altitude at Point A (the height above
28.86 "Hg 1,000 feet
27.82 "Hg 2,000 feet
26.82 "Hg 3,000 feet
25.84 "Hg 4,000 feet
24.90 "Hg 5,000 feet
28.4
28.5
28.3 28.2
30.0 29.9 29.8
Figure 3-9. Effects of nonstandard pressure on an altimeter.
29.92 "Hg
Figure 5-8. Effects of nonstandard pressure on an altimeter of an
aircraft flown into air of lower than standard pressure (air is less
dense).
mean sea level) is only 3,500 feet at Point B. The fact that
the altitude indication is not always true lends itself to the
memory aid, “When flying from hot to cold or from a high
to a low, look out below.” [Figure 5-8]
Altimeter Enhancements (Encoding)
It is not sufficient in the airspace system for only the pilot
to have an indication of the aircraft’s altitude; the air traffic
controller on the ground must also know the altitude of the
aircraft. To provide this information, the aircraft is typically
equipped with an encoding altimeter.
When the ATC transponder is set to Mode C, the encoding
altimeter supplies the transponder with a series of pulses
identifying the flight level (in increments of 100 feet) at
which the aircraft is flying. This series of pulses is transmitted
to the ground radar where they appear on the controller’s
scope as an alphanumeric display around the return for the
aircraft. The transponder allows the ground controller to
identify the aircraft and determine the pressure altitude at
which it is flying.
A computer inside the encoding altimeter measures the
pressure referenced from 29.92 "Hg and delivers this data to
the transponder. When the pilot adjusts the barometric scale
to the local altimeter setting, the data sent to the transponder
is not affected. This is to ensure that all Mode C aircraft are
transmitting data referenced to a common pressure level. ATC
equipment adjusts the displayed altitudes to compensate for
local pressure differences allowing display of targets at correct
altitudes. 14 CFR part 91 requires the altitude transmitted by
the transponder to be within 125 feet of the altitude indicated
on the instrument used to maintain flight altitude.
Reduced Vertical Separation Minimum (RVSM)
Below 31,000 feet, a 1,000 foot separation is the minimum
required between usable flight levels. Flight levels (FLs)
generally start at 18,000 feet where the local pressure is
29.92 "Hg or greater. All aircraft 18,000 feet and above use
a standard altimeter setting of 29.92 "Hg, and the altitudes
are in reference to a standard hence termed FL. Between FL
180 and FL 290, the minimum altitude separation is 1,000
feet between aircraft. However, for flight above FL 290
(primarily due to aircraft equipage and reporting capability;
potential error) ATC applied the requirement of 2,000 feet of
separation. FL 290, an altitude appropriate for an eastbound
aircraft, would be followed by FL 310 for a westbound
aircraft, and so on to FL 410, or seven FLs available for flight.
With 1,000-foot separation, or a reduction of the vertical
separation between FL 290 and FL 410, an additional six
FLs become available. This results in normal flight level and
direction management being maintained from FL 180 through
FL 410. Hence the name is Reduced Vertical Separation
Minimum (RVSM). Because it is applied domestically, it is
called United States Domestic Reduced Vertical Separation
Minimum (DRVSM).
However, there is a cost to participate in the DRVSM program
which relates to both aircraft equipage and pilot training. For
example, altimetry error must be reduced significantly and
operators using RVSM must receive authorization from the
appropriate civil aviation authority. RVSM aircraft must meet
required altitude-keeping performance standards. Additionally,
operators must operate in accordance with RVSM policies/
procedures applicable to the airspace where they are flying.
The aircraft must be equipped with at least one automatic
altitude control—
• Within a tolerance band of ±65 feet about an acquired
altitude when the aircraft is operated in straight-and-
level flight.
• Within a tolerance band of ±130 feet under no
turbulent, conditions for aircraft for which application
for type certification occurred on or before April 9,
1997 that are equipped with an automatic altitude
control system with flight management/performance
system inputs.
That aircraft must be equipped with an altitude alert system
that signals an alert when the altitude displayed to the flight
crew deviates from the selected altitude by more than (in most
cases) 200 feet. For each condition in the full RVSM flight
FL
Without RVSM With RVSM
7 Usable Flight Levels 13 Usable Flight Levels
Figure 3-9. Increase aircraft permitted between FL180 to FL410.Figure 5-9. Increase in aircraft permitted between FL 180 and FL 410.
Figure 5-10. Rate of climb or descent in thousands of feet per minute.
envelope, the largest combined absolute value for residual
static source error plus the avionics error may not exceed 200
feet. Aircraft with TCAS must have compatibility with RVSM
Operations. Figure 5-9 illustrates the increase in aircraft
permitted between FL 180 and FL 410. Most noteworthy,
however, is the economization that aircraft can take advantage
of by the higher FLs being available to more aircraft.
Vertical Speed Indicator (VSI)
The VSI in Figure 5-10 is also called a vertical velocity
indicator (VVI), and was formerly known as a rate-of-climb
indicator. It is a rate-of-pressure change instrument that gives
an indication of any deviation from a constant pressure level.
Inside the instrument case is an aneroid very much like the one
in an ASI. Both the inside of this aneroid and the inside of the
instrument case are vented to the static system, but the case
is vented through a calibrated orifice that causes the pressure
inside the case to change more slowly than the pressure inside
the aneroid. As the aircraft ascends, the static pressure becomes
lower. The pressure inside the case compresses the aneroid,
moving the pointer upward, showing a climb and indicating
the rate of ascent in number of feet per minute (fpm).
When the aircraft levels off, the pressure no longer changes.
The pressure inside the case becomes equal to that inside
the aneroid, and the pointer returns to its horizontal, or
zero, position. When the aircraft descends, the static
pressure increases. The aneroid expands, moving the pointer
downward, indicating a descent.
The pointer indication in a VSI lags a few seconds behind the
actual change in pressure. However, it is more sensitive than
an altimeter and is useful in alerting the pilot of an upward or
downward trend, thereby helping maintain a constant altitude.
Some of the more complex VSIs, called instantaneous vertical
speed indicators (IVSI), have two accelerometer-actuated air
pumps that sense an upward or downward pitch of the aircraft
and instantaneously create a pressure differential. By the time
the pressure caused by the pitch acceleration dissipates, the
altitude pressure change is effective.
Dynamic Pressure Type Instruments
Airspeed Indicator (ASI)
An ASI is a differential pressure gauge that measures the
dynamic pressure of the air through which the aircraft is
flying. Dynamic pressure is the difference in the ambient
static air pressure and the total, or ram, pressure caused by
the motion of the aircraft through the air. These two pressures
are taken from the pitot-static system.
The mechanism of the ASI in Figure 5-11 consists of a thin,
corrugated phosphor bronze aneroid, or diaphragm, that
receives its pressure from the pitot tube. The instrument
case is sealed and connected to the static ports. As the
pitot pressure increases or the static pressure decreases, the
diaphragm expands. This dimensional change is measured by
a rocking shaft and a set of gears that drives a pointer across
the instrument dial. Most ASIs are calibrated in knots, or
nautical miles per hour; some instruments show statute miles
per hour, and some instruments show both.