7-1

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 7-1]

Flight 

Instruments

Chapter 7
7-2

Figure 7-1. Pitot-static system and instruments. 

Pitot-static system and instruments

30.0

29.9

29.8

Altimeter

Heater (35 watts)

Static port

Airspeed indicator (ASI) Vertical speed indicator (VSI)

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 which 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 which 

allows moisture to drain from the system should the aircraft 

enter precipitation. Both openings in the pitot tube need to 

be checked prior to flight to insure 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 which 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 

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 

of the flight deck. Due to the venturi effect of the air flowing 
7-3

Figure 7-2. Altimeter.

Altimeter

Aneroid wafers

Static port

1,000 ft. pointer

A crosshatched area appears on 

some altimeters when displaying 

an altitude below 10,000 feet MSL.

Crosshatch flag

Barometric scale adjustment knob

100 ft. pointer

10,000 ft. pointer

Altimeter setting window

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 effect 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 (29.92 "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 7-2]

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 ASI. 

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. 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.
7-4

Figure 7-3. 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 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 inches 

of mercury ("Hg). When the altimeter cannot be set to the 

higher pressure setting, the aircraft actual altitude will be 

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 7-3] 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 colderthan-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 can be employed: 

“FROM HOT TO COLD, LOOK OUT BELOW.” When the 

air is warmer than standard, the aircraft is higher than the 
7-5

Figure 7-4. Look at the chart using a temperature of –10 °C 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.

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

1190

90

280

490

710

950

1210

1500

ReportedTemp 0 °C

200

300

400

500

600

700

800900

1000

1500

2000

3000

4000

5000

altimeter indicates. Altitude corrections for temperature can 

be computed on the navigation computer.

Extremely cold temperatures will also affect altimeter 

indications. Figure 7-4, 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 x 1,000 feet = 250 feet.)
7-6

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 x 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 x 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 should be able to better understand what 

is happening with the aircraft’s altitude. This method always 

yields the correct result and tells a pilot what the altitude is 

and the direction. (The implications of not understanding 

where the errors lie and in what direction are important to a 

safe flight.) If the altitude was lower than actually indicated, 

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 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 to. 

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, 
7-7

Figure 7-5. Vertical speed indicator (VSI).

3

4

2

2

3

VERTICAL SPEED

THOUSAND FT PER MIN

OWN

and obstacle elevations on aeronautical charts are true 

altitudes.

3. Absolute altitude—the vertical distance of an aircraft 

above the terrain, or above ground level (AGL).

4. Pressure altitude—the altitude indicated when 

the altimeter setting window (barometric scale) is 

adjusted to 29.92 "Hg. This is the altitude above the 

standard datum plane, which is a theoretical plane 

where air pressure (corrected to 15 °C) equals 29.92" 

Hg. Pressure altitude is used to compute density 

altitude, true altitude, true airspeed (TAS), and other 

performance data.

5. Density altitude—pressure altitude corrected 

for variations from standard temperature. When 

conditions are standard, pressure altitude and density 

altitude are the same. If the temperature is above 

standard, the density altitude is higher than pressure 

altitude. If the temperature is below standard, the 

density altitude is lower than pressure altitude. This 

is an important altitude because it is directly related 

to the aircraft’s performance.

A pilot must understand how the performance of the aircraft 

is directly related to the density of the air. The density of 

the air affects how much power a naturally aspirated engine 

produces, as well as how efficient the airfoils are. If there are 

fewer air molecules (lower pressure) to accelerate through 

the propeller, the acceleration to rotation speed is longer 

and thus produces a longer takeoff roll, which translates to 

a decrease in performance.

As an example, consider an airport with a field elevation 

of 5,048 feet MSL where the standard temperature is 5 °C. 

Under these conditions, pressure altitude and density altitude 

are the same—5,048 feet. If the temperature changes to 

30 °C, the density altitude increases to 7,855 feet. This 

means an aircraft would perform on takeoff as though the 

field elevation were 7,855 feet at standard temperature. 

Conversely, a temperature of –25 °C would result in a density 

altitude of 1,232 feet. An aircraft would perform much better 

under these conditions.

Instrument Check

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 (AFSS) or any other reliable 

source, such as ATIS, AWOS, or ASOS. 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.

Vertical Speed Indicator (VSI)

The VSI, which is sometimes called a vertical velocity 

indicator (VVI), 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. [Figure 7-5]

Principle of Operation

Although the VSI operates solely from static pressure, it is a 

differential pressure instrument. It contains a diaphragm with 

connecting linkage and gearing to the indicator pointer inside 

an airtight case. 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 air 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 the 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. 
7-8

Figure 7-6. An IVSI incorporates accelerometers to help the 

instrument immediately indicate changes in vertical speed.

3

I

4

0

2

I 2

3

UP

DOWN

.5

.5

Accelerometer

Inlet from static port

Calibrated leak

Figure 7-7. Airspeed indicator (ASI).

Airspeed indicator

100

200

50

150

2

Diaphragm

Static air line Handstaff pinion

Pitot tube

Ram air

Long lever Sector

Pitot connection When the pressure differential stabilizes at a definite ratio, 

the needle indicates the rate of altitude change.

The VSI displays two different types of information:

• Trend information shows an immediate indication of 

an increase or decrease in the aircraft’s rate of climb 

or descent.

• Rate information shows a stabilized rate of change in 

altitude.

The trend information is the direction of movement of the 

VSI needle. For example, if an aircraft is maintaining level 

flight and the pilot pulls back on the control yoke causing the 

nose of the aircraft to pitch up, the VSI needle moves upward 

to indicate a climb. If the pitch attitude is held constant, 

the needle stabilizes after a short period (6–9 seconds) and 

indicates the rate of climb in hundreds of fpm. The time 

period from the initial change in the rate of climb, until the 

VSI displays an accurate indication of the new rate, is called 

the lag. Rough control technique and turbulence can extend 

the lag period and cause erratic and unstable rate indications. 

Some aircraft are equipped with an instantaneous vertical 

speed indicator (IVSI), which incorporates accelerometers to 

compensate for the lag in the typical VSI. [Figure 7-6]

Instrument Check

As part of a preflight 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.

Airspeed Indicator (ASI)

The ASI is a sensitive, differential pressure gauge which 

measures and promptly indicates the difference between pitot 

(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 pitot 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. [Figure 7-7]

The ASI is the one instrument that utilizes both the pitot, 

as well as the static system. 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. The system drives 

the mechanical linkage and the airspeed needle.

Just as in altitudes, there are multiple types of airspeeds. 

Pilots need to be very familiar with each type.

• Indicated airspeed (IAS)—the direct instrument 

reading obtained from the ASI, uncorrected for 

variations in atmospheric density, installation error, 

or instrument error. Manufacturers use this airspeed 

as the basis for determining aircraft performance. 

Takeoff, landing, and stall speeds listed in the AFM/ 

POH are IAS and do not normally vary with altitude 

or temperature.
7-9

Figure 7-8. Airspeed indicator (ASI).

Single-engine airspeed indicator

100

80

120

140

T.A.S.

KTS

PRESS

ALT 0 5 10 15 20

F° 120 90 60 30 0 -30

100

120

40

60

80 100

120

140

160

MPH

MPH

AIRSPEED

KNOTS

VSO

VNE(red line)

Yellow arc

VNO

Green arc VFE

White arc

VS1

• 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 

airspeeds and with certain flap settings, the installation 

and instrument errors may total several knots. This 

error is generally greatest at low airspeeds. In the 

cruising and higher airspeed ranges, IAS and CAS 

are approximately the same. Refer to the airspeed 

calibration chart to correct for possible airspeed 

errors.

• True airspeed (TAS)—CAS corrected for altitude 

and nonstandard temperature. Because air density 

decreases with an increase in altitude, an aircraft has 

to be flown faster at higher altitudes to cause the same 

pressure difference between pitot impact pressure 

and static pressure. Therefore, for a given CAS, TAS 

increases as altitude increases; or for a given TAS, 

CAS decreases as altitude increases. A 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. 

Extremely accurate electronic flight computers are 

also available. Just enter the CAS, pressure altitude, 

and temperature, and the computer calculates the TAS. 

A second method, which is a rule of thumb, provides 

the approximate TAS. Simply add 2 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.

• Groundspeed (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.

Airspeed Indicator Markings

Aircraft weighing 12,500 pounds or less, manufactured after 

1945, and certificated by the FAA, are required to have ASIs 

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. For example, 

if during the execution of a maneuver, it is noted that the 

airspeed needle is in the yellow arc and rapidly approaching 

the red line, the immediate reaction should be to reduce 

airspeed.

As shown in Figure 7-8, ASIs on single-engine small aircraft 

include the following standard color-coded markings:

• White arc—commonly referred to as the flap operating 

range since its lower limit represents the full flap stall 

speed and its upper limit provides the maximum flap 

speed. Approaches and landings are usually flown at 

speeds within the white arc.

• Lower limit of white arc (VS0)—the stalling speed 

or the minimum steady flight speed in the landing 

configuration. In small aircraft, this is the power-off 

stall speed at the maximum landing weight in the 

landing configuration (gear and flaps down).

• Upper limit of the white arc (VFE)—the maximum 

speed with the flaps extended.

• Green arc—the normal operating range of the aircraft. 

Most flying occurs within this range.

• Lower limit of green arc (VS1)—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).

• Upper limit of green arc (VNO)—the maximum 

structural cruising speed. Do not exceed this speed 

except in smooth air.

• Yellow arc—caution range. Fly within this range only 

in smooth air, and then, only with caution.
7-10

Figure 7-9. A blocked pitot tube, but clear drain hole.

Blocked static system

Static port

Drain hole

Pitot tube

Blockage

• Red line (VNE)—never exceed speed. Operating above 

this speed is prohibited since it may result in damage 

or structural failure.

Other Airspeed Limitations

Some important airspeed limitations are not marked on the 

face of the ASI, but are found on placards and in the AFM/

POH. These airspeeds include:

• Design maneuvering speed (VA)—the maximum 

speed at which the structural design’s limit load can 

be imposed (either by gusts or full deflection of the 

control surfaces) without causing structural damage. 

It is important to consider weight when referencing 

this speed. For example, VA may be 100 knots when 

an airplane is heavily loaded, but only 90 knots when 

the load is light.

• Landing gear operating speed (VLO)—the maximum 

speed for extending or retracting the landing gear if 

flying an aircraft with retractable landing gear.

• Landing gear extended speed (VLE)—the maximum 

speed at which an aircraft can be safely flown with 

the landing gear extended.

• Best angle-of-climb speed (VX)—the airspeed at 

which an aircraft gains the greatest amount of altitude 

in a given distance. It is used during a short-field 

takeoff to clear an obstacle.

• Best rate-of-climb speed (VY)—the airspeed that 

provides the most altitude gain in a given period of 

time.

• Single-engine best rate-of-climb (VYSE)—the best 

rate-of-climb or minimum rate-of-sink in a light 

twin-engine aircraft with one engine inoperative. It is 

marked on the ASI with a blue line. VYSE is commonly 

referred to as “Blue Line.”

• Minimum control speed (VMC)—the minimum flight 

speed at which a light, twin-engine aircraft can be 

satisfactorily controlled when an engine suddenly 

becomes inoperative and the remaining engine is at 

takeoff power.

Instrument Check

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 airspeed is increasing at an appropriate rate.

Blockage of the Pitot-Static System

Errors almost always indicate blockage of the pitot tube, the 

static port(s), or both. Blockage may be caused by moisture 

(including ice), dirt, or even insects. During preflight, make 

sure the pitot tube cover is removed. Then, check the pitot and 

static port openings. 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.

Blocked Pitot System

The pitot system can become blocked completely or only 

partially if the pitot tube drain hole remains open. If the pitot 

tube becomes blocked and its associated drain hole remains 

clear, ram air no longer is able to enter the pitot 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, because the ASI senses no difference between ram and 

static air pressure. The ASI no longer operates since dynamic 

pressure can not enter the pitot tube opening. Static pressure 

is able to equalize on both sides since the pitot drain hole 

is still open. The apparent loss of airspeed is not usually 

instantaneous but happens very quickly. [Figure 7-9]

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. The total 

pressure in the pitot tube does not change due to the blockage; 

however, the static pressure will change.

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 pressurizes 

against this static pressure and as a result changes the airspeed 

indication via levers and indicators. [Figure 7-10]
7-11

Figure 7-10. Blocked pitot system with clear static system.

Blocked pitot system with clear static system

Climb

Descent

Static port

Drain hole

Pitot tube

Blockage

For example, take an aircraft and slow it down to zero knots 

at given altitude. If the static port (providing static pressure) 

and the pitot tube (providing dynamic pressure) are both 

unobstructed, the following claims can be made:

1. The ASI would be zero.

2. There must be a relationship between both dynamic 

and static pressure. At zero speed, dynamic pressure 

and static pressure are the same: static air pressure.

3. Because both dynamic and static air pressure are equal 

at zero speed with increased speed, dynamic pressure 

must include two components: static pressure and 

dynamic pressure.

It can be inferred that airspeed indication must be based upon 

a relationship between these two pressures, and indeed it is. 

An ASI uses the static pressure as a reference pressure and 

as a result, the ASI’s case is kept at this pressure behind the 

diaphragm. On the other hand, the dynamic pressure through 

the pitot tube is connected to a highly sensitive diaphragm 

within the ASI case. Because an aircraft in zero motion 

(regardless of altitude) results in a zero airspeed, the pitot 

tube always provides static pressure in addition to dynamic 

pressure.

Therefore, the airspeed indication is the result of two 

pressures: the pitot tube static and dynamic pressure within 

the diaphragm as measured against the static pressure in case. 

What does this mean if the pitot tube is obstructed?

If the aircraft were to descend, the pressure in the pitot 

system including the diaphragm would remain constant. It is 

clogged and the diaphragm is at a single pressure. But as the 

descent is made, the static pressure would increase against 

the diaphragm causing it to compress thereby resulting in an 

indication of decreased airspeed. Conversely, if the aircraft 

were to climb, the static pressure would decrease allowing 

the diaphragm to expand, thereby showing an indication of 

greater airspeed. [Figure 7-10]

The pitot tube may become blocked during flight due to 

visible moisture. Some aircraft may be equipped with pitot 

heat for flight in visible moisture. Consult the AFM/POH for 

specific procedures regarding the use of pitot heat.

Blocked Static System

If the static system becomes blocked but the pitot tube remains 

clear, the ASI continues to operate; however, it is inaccurate. 

The airspeed indicates lower than the actual airspeed when 

the aircraft is operated above the altitude where the static 

ports became blocked, because the trapped static pressure is 

higher than normal for that altitude. When operating at a lower 

altitude, a faster than actual airspeed is displayed due to the 

relatively low static pressure trapped in the system.

Revisiting the ratios that were used to explain a blocked pitot 

tube, the same principle applies for a blocked static port. 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 pitot side is equivalent to an increase 

in dynamic pressure since the pressure can not 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 pitot side, while the pressure on the static side is held 

constant.

A blockage of the static system also affects 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. [Figure 7-11]
7-12

Figure 7-11. Blocked static system.

Blocked static system

30.0 29.9

29.8

Pitot tube

Blockage

Static port

Frozen altimeter

Constant zero indication on VSI

Inaccurate airspeed indications

Some aircraft are equipped with an alternate static source 

in the flight deck. In the case of a blocked static source, 

opening the alternate static source introduces static pressure 

from the flight deck back into the system. Flight deck static 

pressure is lower than outside static pressure. Check the 

aircraft AOM/POH for airspeed corrections when utilizing 

alternate static pressure.

Electronic Flight Display (EFD)

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. Primary electronic instrumentation packages 

are less prone to failure than their analogue counterparts. No 

longer is it necessary for aircraft designers to create cluttered 

panel layouts in order to accommodate all necessary flight 

instruments. Instead, multi-panel digital flight displays 

combine all flight instruments onto a single screen which is 

called a primary flight display (PFD). The traditional “six 

pack” of instruments is now displayed on one liquid crystal 

display (LCD) screen.

Airspeed Tape

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. The TAS is 

displayed at the bottom of the tape through the input to the air 

data computer (ADC) from the outside air temperature probe. 

Airspeed markings for VX, VY, and rotation speed (VR) are 

displayed for pilot reference. An additional pilot-controlled 

airspeed bug is available to set at any desired reference speed. 

As on traditional analogue ASIs, the electronic airspeed tape 

displays the color-coded ranges for the flap operating range, 

normal range, and caution range. [Figure 7-12] The number 

value changes color to red when the airspeed exceeds VNE to 

warn the pilot of exceeding the maximum speed limitation.

Attitude Indicator

One improvement over analogue instrumentation is the 

larger attitude indicator on EFD. The artificial horizon spans 

the entire width of the PFD. [Figure 7-12] 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 (AHRS).

Altimeter

The altimeter is located on the right side of the PFD. 

[Figure 7-12] 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 center of the display tape. 

The altitude is displayed in increments of 20 feet.

Vertical Speed Indicator (VSI)

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. 

[Figure 7-12] Both are equipped with a vertical speed bug. 

Heading Indicator

The heading indicator is located below the artificial horizon 

and is normally modeled after a Horizontal Situation 

Indicator (HSI). [Figure 7-12] As in the case of the attitude 

indicator, the heading indicator receives its information from 

the magnetometer which feeds information to the AHRS unit 

and then out to the PFD.

Turn Indicator

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. [Figure 7-12] Reference for coordinated flight comes 

from accelerometers contained in the AHRS unit.

Tachometer

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 multi-function display (MFD). In the event of a display 

screen failure, it is displayed on the remaining screen with 

the PFD flight instrumentation. [Figure 7-13]
7-13

Figure 7-12. Primary flight display. Note that the actual location of indications vary depending on manufacturers.

XPDR 5537 IDNT LCL10:12:34

INSET PFD OBS CDI DME XPDR IDENT TMR/REF NRST ALERTS

VOR 1 

270°

2

1

1

2

4300

4200

4100

3900

3900

3800

4300

20

80

4000

4000

130

120

110

90

80

70

1

100

9

TAS 106KT

OAT 6°C

NAV1 108.00 113.00 NAV2 108.00 110.60

134.000 118.000 COM1

123.800 118.000 COM2

WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _°T TRK 360° Attitude Indicator Altimeter

Horizontal Situation Indicator

Vertical Speed Indicator (VSI)

Slip Skid Indicator

Air Speed Indicator

Turn Indicator

270°

270°

Slip/Skid Indicator

Turn Rate Indicator Tick Marks

Turn Rate Trend Vector

Figure 7-13. Multi-function display.

Slip/Skid Indicator

The slip/skid indicator [Figure 7-12] is the horizontal 

line below the roll pointer. Like a ball in a turn-and-slip 

indicator, a bar width off center is equal to one ball width 

displacement.

Turn Rate Indicator

The turn rate indicator, illustrated in Figure 7-12, is typically 

found directly above the rotating compass card. Tick marks to 

the left and right of the luber line denote the turn (standardrate 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.
7-14

Figure 7-15. Teledyne’s 90004 TAS/Plus Air Data Computer (ADC) 

computes air data information from the pitot-static pneumatic 

system, aircraft temperature probe, and barometric correction 

device to help create a clear picture of flight characteristics.

Figure 7-16. Airspeed trend vector.

150

140

130

110

100

90

1

120

9

TAS 120KT

1

0

Airspeed trend vector

130

1

20

Figure 7-14. Chelton’s FlightLogic (top) and Avidyne’s Entegra

(bottom) are examples of panel displays that are configurable.

140

120

100

80

60

40

102 10

10

10

10

58

70

65

70

30.30

6710

200 210 220 230 240 250 260 270

UY

UX

VS

IFR APPR

00:03:29

2320B

ANG 239 A

MA239 5800’

239 2.3NM

W S

E N

Individual panel displays are able to be configured for a variety 

of aircraft simply by installing different software packages. 

[Figure 7-14] Manufacturers are also able to upgrade existing 

instrument displays in a similar manner, eliminating the need 

to replace individual gauges in order to upgrade.

Air Data Computer (ADC)

Electronic flight displays utilize the same type of instrument 

inputs as traditional analogue gauges; however, the processing 

system is different. The pitot static inputs are received by an 

ADC. The ADC computes the difference between the total 

pressure and the static pressure, and generates the information 

necessary to display the airspeed 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. [Figure 7-15]

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. In the event of system malfunction, 

the ADC can quickly be removed and replaced in order to 

decrease down time and maintenance turn-around times.

Altitude information is derived from the static pressure port 

just as an analogue system does; however, the static pressure 

does not enter a diaphragm. The ADC computes the received 

barometric pressure and sends a digital signal to the PFD to 

display the proper altitude readout. Electronic flight displays 

also show trend vectors which show the pilot how the altitude 

and airspeed are progressing.

Trend Vectors

Trend vectors are magenta lines which move up and down 

both the ASI and the altimeter. [Figures 7-16 and 7-17] The 
7-15

Figure 7-17. Altimeter trend vector.

150

140

130

110

100

90

1

120

9

TAS 120KT

1

0

Airspeed trend vector

130

1

20

Figure 7-18. Regardless of the position of its base, a gyro tends to

remain rigid in space, with its axis of rotation pointed in a constant 

direction.

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. Additional 

information can be obtained by referencing the Instrument 

Flying Handbook or specific avionics manufacturer’s training 

material.

Gyroscopic Flight Instruments

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.

Gyroscopic Principles

Any spinning object exhibits gyroscopic properties. A wheel 

or rotor designed and mounted to utilize these properties is 

called a gyroscope. 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 upon 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. The wheel or rotor is free to 

rotate in any plane in relation to the base and is balanced so 

that, with the gyro wheel at rest, it remains in the position 

in which it is placed. Restricted or semi-rigidly 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 in space and precession.

Rigidity in Space

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.

By mounting this wheel, or gyroscope, on a set of gimbal 

rings, the gyro is able to rotate freely in any direction. Thus, 

if the gimbal rings are tilted, twisted, or otherwise moved, 

the gyro remains in the plane in which it was originally 

spinning. [Figure 7-18]

Precession

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° 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. The rate 

at which the gyro precesses is inversely proportional to the 

speed of the rotor and proportional to the deflective force.
7-16

Figure 7-19. Precession of a gyroscope resulting from an applied

deflective force.

Plane of Precession

Plane of Force

Plane of Rotation

FORCE

Using the example of the bicycle, precession acts on the 

wheels in order to allow the bicycle to turn. While riding 

at normal speed, it is not necessary to turn the handle bars 

in the direction of the desired turn. A rider 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° 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. [Figure 7-19] 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.

Sources of Power

In some aircraft, all the gyros are vacuum, pressure, or 

electrically operated. In other 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 vanes to spin the rotor at high speed, much like the 

operation of a waterwheel or turbine. The amount of vacuum 

or pressure required for instrument operation varies, but is 

usually between 4.5 "Hg and 5.5 "Hg.

One source of vacuum for the gyros is a vane-type enginedriven pump that is mounted on the accessory case of 

the engine. Pump capacity varies in different airplanes, 

depending on the number of gyros.

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 in Figure 7-20, 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 

suction, from exceeding prescribed limits. After that, the air 

is expelled overboard or used in other systems, such as for 

inflating pneumatic deicing 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. The 

vacuum, or suction, gauge is generally marked to indicate 

the normal range. Some aircraft are equipped with a warning 

light that illuminates when the vacuum pressure drops below 

the acceptable level.

When the vacuum pressure drops below the normal operating 

range, the gyroscopic instruments may become unstable and 

inaccurate. Cross checking the instruments routinely is a 

good habit to develop.

Turn Indicators

Aircraft use two types of turn indicators: turn-and-slip 

indicator and turn coordinator. Because of the way the gyro 

is mounted, the turn-and-slip indicator shows only the rate of 

turn in degrees per second. The turn coordinator is mounted 

at an angle, or canted, so it can initially show roll rate. When 

the roll stabilizes, it indicates rate of turn. Both instruments 

indicate turn direction and quality (coordination), and also 

serve as a backup source of bank information in the event an 

attitude indicator fails. Coordination is achieved by referring 

to the inclinometer, which consists of a liquid-filled curved 

tube with a ball inside. [Figure 7-21]
7-17

Figure 7-20. Typical vacuum system.

Typical vacuum system

8

4 6

2

0 I0

SUCTION

INCHES MERCURT

CT

33

30

24

2I

I5

I2

6

3

20 20

I0 I0

I0 I0

20 20

I

STBY PWR TEST

Vacuum relief valve

Vacuum air filter

Attitude Indicator

Vacuum pump

Heading Indicator

Overboard vent line

Suction

Gauge

Figure 7-21. Turn indicators rely on controlled precession for their operation.

Turn coordinator Turn-and-slip indicator

Gyro rotation

Standard rate turn index

Horizontal gyro

Inclinometer Inclinometer

Gimbal rotation

Standard rate turn index

Gyro rotation

Gimbal rotation

Canted gyro

Gimbal

Turn-and-Slip Indicator

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 

yawing 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 turn-and-slip indicator is incapable of “tumbling” off 

its rotational axis because of the restraining springs. When 

extreme forces are applied to a gyro, the gyro is displaced 

from its normal plane of rotation, rendering its indications 

invalid. Certain instruments have specific pitch and bank 

limits that induce a tumble of the gyro.

Turn Coordinator

The gimbal in the turn coordinator is canted; therefore, its 

gyro can sense both rate of roll and rate of turn. Since turn 

coordinators are more prevalent in training aircraft, this 

discussion concentrates on that instrument. 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.
7-18

Slipping turn Skidding turn

Coordinated turn

TURN COORDINATOR

2 MIN.

D.C.

ELEC.

L R

NO PITCH

INFORMATION

TURN COORDINATOR

2 MIN.

D.C.

ELEC.

L R

NO PITCH

INFORMATION

TURN COORDINATOR

2 MIN.

D.C.

ELEC.

L R

NO PITCH

INFORMATION

Figure 7-22. If inadequate right rudder is applied in a right turn, a 

slip results. Too much right rudder causes the aircraft to skid through 

the turn. Centering the ball results in a coordinated turn.

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. Figure 7-22 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 side of the instrument serve to indicate 

a standard rate of turn. A standard-rate turn is defined as a 

turn rate of 3° per second. The turn coordinator indicates only 

the rate and direction of turn; it does not display a specific 

angle of bank.

Inclinometer

The inclinometer is used to depict aircraft yaw, which is 

the side-to-side movement of the aircraft’s nose. During 

coordinated, straight-and-level flight, the force 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 in Figure 7-22, 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. To correct for these conditions, 

and improve the quality of the turn, remember to “step on 

the ball.” Varying the angle of bank can also help restore 

coordinated flight from a slip or skid. To correct for a slip, 

decrease bank and/or increase the rate of turn. To correct for 

a skid, increase the bank and/or decrease the rate of turn.

Yaw String

One additional tool which can be added to the aircraft is a 

yaw string. A yaw string is simply a string or piece of yarn 

attached to the center of the wind screen. When in coordinated 

flight, the string trails straight back over the top of the wind 

screen. 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.

Instrument Check

During the preflight, check to see that the inclinometer 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 the turn.

Attitude Indicator

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 in the attitude indicator is mounted in a horizontal 

plane and depends upon rigidity in 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. [Figure 7-23]

The gyro spins in the horizontal plane and resists deflection of 

the rotational path. Since the gyro relies on rigidity in 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 the horizon bar to suit the pilot’s line of vision. 

Normally, the miniature aircraft is adjusted so that the wings 

overlap the horizon bar when the aircraft is in straight-andlevel cruising flight. 
7-19

Attitude indicator

20 

I0 I

I0 I0

20 2

TEST BY PWR

 20

0 I0

0 I0

 20

Bank index

Horizon reference arm Roll gimbal

Gimbal rotation

Gyro Pitch gimbal 

Figure 7-23. Attitude indicator.

20 20

I0 I0

I0 I0

20 20

 I

STBY PWR TEST

Bank scale

Descending left bank

20 20

I0 I0

I0 I0

20 20

0 0

 I0

STBY PWR TEST

Straight descent

20 20

I0 I0

I0 I0

20 20

 

I0 I0

STBY PWR TEST

Descending right bank

20 

I0 

 20

 I0

I0 I0

20 20

0 

 20

STBY PWR TEST

Level left bank

20 20

I0 I0

I0 I0

20 20

I0 I0

2

STBY PWR TEST

Level right bank

20 

I0 

 20

 I0

I0 I0

20 20

I0

STBY PWR TEST

Climbing left bank

20 20

I0 I0

I0 I0

20 20

 20

STBY PWR TEST

Straight climb

20 20

I0 I0

I0 I0

20 20

 

STBY PWR TEST

Climbing right bank

20 20

I0 I0

I0 I0

20 20

20 STBY PWR TEST

Artificial horizon Adjustment knob

Pointer

60°

45°

20° 10°

30°

90°

Figure 7-24. Attitude representation by the attitude indicator corresponds to the relation of the aircraft to the real horizon.

The pitch and bank limits depend upon the make and model 

of the instrument. Limits in the banking plane are usually 

from 100° to 110°, and the pitch limits are usually from 60° 

to 70°. If either limit is exceeded, the instrument will tumble 

or spill and will give incorrect indications until realigned. A 

number of modern attitude indicators do not tumble.

Every pilot should be able to interpret the banking scale 

illustrated in Figure 7-24. Most banking scale indicators on 

the top of the instrument move in the same direction from 

that in which the aircraft is actually banked. Some other 

models move in the opposite direction from that in which the 

aircraft is actually banked. This may confuse the pilot if the 

indicator is used to determine the direction of bank. This scale 

should be used only to control the degree of desired bank. 

The relationship of the miniature aircraft to the horizon bar 

should be used for an indication of the direction of bank.
7-20

Heading indicator

33

2I

I5

I2

6

3

Compass card gear

Gimbal Adjustment gears

Main drive gear

Gyro Adjustment knob

Gimbal rotation

Figure 7-25. A heading indicator displays headings based on a 360° 

azimuth, with the final zero omitted. For example, “6” represents 

060°, while “21” indicates 210°. The adjustment knob is used to 

align the heading indicator with the magnetic compass.

Figure 7-26. Attitude and heading reference system (AHRS).

The attitude indicator is reliable and the most realistic flight 

instrument on the instrument panel. Its indications are very 

close approximations of the actual attitude of the aircraft.

Heading Indicator

The heading indicator is fundamentally a mechanical 

instrument designed to facilitate the use of the magnetic 

compass. Errors in the magnetic compass are numerous, 

making straight flight and precision turns to headings difficult 

to accomplish, particularly in turbulent air. A heading 

indicator, however, is not affected by the forces that make 

the magnetic compass difficult to interpret. [Figure 7-25]

The operation of the heading indicator depends upon the 

principle of rigidity in space. The rotor turns in a vertical 

plane and fixed to the rotor is a compass card. Since the rotor 

remains rigid in space, the points on the card hold the same 

position in space relative to the vertical plane of the gyro. The 

aircraft actually rotates around the rotating gyro, not the other 

way around. As the instrument case and the aircraft revolve 

around the vertical axis of the gyro, the card provides clear 

and accurate heading information.

Because of precession caused by friction, the heading 

indicator creeps or drifts from a heading to which it is set. 

Among other factors, the amount of drift depends largely 

upon the condition of the instrument. If the bearings are worn, 

dirty, or improperly lubricated, the drift may be excessive. 

Another error in the heading indicator is caused by the fact 

that the gyro is oriented in space, and the Earth rotates in 

space at a rate of 15° in 1 hour. Thus, discounting precession 

caused by friction, the heading indicator may indicate as 

much as 15° error per every hour of operation.

Some heading indicators referred to as horizontal situation 

indicators (HSI) receive a magnetic north reference from 

a magnetic slaving transmitter, and generally need no 

adjustment. The magnetic slaving transmitter is called a 

magnetometer.

Attitude and Heading Reference System (AHRS)

Electronic flight displays have replaced free-spinning gyros 

with solid-state laser systems that are capable of flight at 

any attitude without tumbling. This capability is the result 

of the development of the Attitude and Heading Reference 

System (AHRS).

The AHRS sends attitude information to the PFD in order 

to generate the pitch and bank information of the attitude 

indicator. The heading information is derived from a 

magnetometer which senses the earth’s lines of magnetic 

flux. This information is then processed and sent out to the 

PFD to generate the heading display. [Figure 7-26]

The Flux Gate Compass System

As mentioned earlier, the lines of flux in the Earth’s magnetic 

field have two basic characteristics: a magnet aligns with 

them, and an electrical current is induced, or generated, in 

any wire crossed by them.

The flux gate compass that drives slaved gyros uses the 

characteristic of current induction. The flux valve is a small, 

segmented ring, like the one in Figure 7-27, made of soft iron 

that readily accepts lines of magnetic flux. An electrical coil 

is wound around each of the three legs to accept the current 
7-21

Figure 7-27. The soft iron frame of the flux valve accepts the flux 

from the Earth’s magnetic field each time the current in the center 

coil reverses. This flux causes current to flow in the three pickup 

coils.

Figure 7-28. The current in each of the three pickup coils changes 

with the heading of the aircraft..

Figure 7-29. Pictorial navigation indicator (HSI, top), slaving meter 

(lower right), and slaving control compensator unit (lower left).

induced in this ring by the Earth’s magnetic field. A coil 

wound around the iron spacer in the center of the frame has 

400 Hz alternating current (AC) flowing through it. During 

the times when this current reaches its peak, twice during 

each cycle, there is so much magnetism produced by this 

coil that the frame cannot accept the lines of flux from the 

Earth’s field.

As the current reverses between the peaks, it demagnetizes 

the frame so it can accept the flux from the Earth’s field. As 

this flux cuts across the windings in the three coils, it causes 

current to flow in them. These three coils are connected in 

such a way that the current flowing in them changes as the 

heading of the aircraft changes. [Figure 7-28]

The three coils are connected to three similar but smaller coils 

in a synchro inside the instrument case. The synchro rotates 

the dial of a radio magnetic indicator (RMI) or a HSI.

Remote Indicating Compass

Remote indicating compasses were developed to compensate 

for the errors and limitations of the older type of heading 

indicators. The two panel-mounted components of a typical 

system are the pictorial navigation indicator and the slaving 

control and compensator unit. [Figure 7-29] The pictorial 

navigation indicator is commonly referred to as an HSI.

The slaving control and compensator unit has a push button 

that provides a means of selecting either the “slaved gyro” 

or “free gyro” mode. This unit also has a slaving meter 

and two manual heading-drive buttons. The slaving meter 

indicates the difference between the displayed heading and 

the magnetic heading. A right deflection indicates a clockwise 

error of the compass card; a left deflection indicates a 

counterclockwise error. Whenever the aircraft is in a turn 

and the card rotates, the slaving meter shows a full deflection 

to one side or the other. When the system is in “free gyro” 

mode, the compass card may be adjusted by depressing the 

appropriate heading-drive button.

A separate unit, the magnetic slaving transmitter is mounted 

remotely, usually in a wingtip to eliminate the possibility of 
7-22

Figure 7-30. Driven by signals from a flux valve, the compass card 

in this RMI indicates the heading of the aircraft opposite the upper 

center index mark. The green pointer is driven by the ADF.

magnetic interference. It contains the flux valve, which is 

the direction-sensing device of the system. A concentration 

of lines of magnetic force, after being amplified, becomes 

a signal relayed to the heading indicator unit, which is also 

remotely mounted. This signal operates a torque motor in 

the heading indicator unit that processes the gyro unit until 

it is aligned with the transmitter signal. The magnetic slaving 

transmitter is connected electrically to the HSI.

There are a number of designs of the remote indicating 

compass; therefore, only the basic features of the system are 

covered here. Instrument pilots must become familiar with 

the characteristics of the equipment in their aircraft.

As instrument panels become more crowded and the pilot’s 

available scan time is reduced by a heavier flight deck 

workload, instrument manufacturers have worked toward 

combining instruments. One good example of this is the 

RMI in Figure 7-30. The compass card is driven by signals 

from the flux valve, and the two pointers are driven by an 

automatic direction finder (ADF) and a very high frequency 

(VHF) omni-directional radio range (VOR).

Heading indicators that do not have this automatic 

northseeking capability are called “free” gyros, and require 

periodic adjustment. It is important to check the indications 

frequently (approximately every 15 minutes) and reset the 

heading indicator to align it with the magnetic compass 

when required. Adjust the heading indicator to the magnetic 

compass heading when the aircraft is straight and level at a 

constant speed to avoid compass errors.

The bank and pitch limits of the heading indicator vary 

with the particular design and make of instrument. On some 

heading indicators found in light aircraft, the limits are 

approximately 55° of pitch and 55° of bank. When either of 

these attitude limits is exceeded, the instrument “tumbles” 

or “spills” and no longer gives the correct indication until 

reset. After spilling, it may be reset with the caging knob. 

Many of the modern instruments used are designed in such 

a manner that they do not tumble.

An additional precession error may occur due to a gyro not 

spinning fast enough to maintain its alignment. When the 

vacuum system stops producing adequate suction to maintain 

the gyro speed, the heading indicator and the attitude indicator 

gyros begin to slow down. As they slow, they become more 

susceptible to deflection from the plane of rotation. Some 

aircraft have warning lights to indicate that a low vacuum 

situation has occurred. Other aircraft may have only a vacuum 

gauge that indicates the suction.

Instrument Check

As the gyro spools up, make sure there are no abnormal 

sounds. While taxiing, the instrument should indicate turns in 

the correct direction, and precession should not be abnormal. 

At idle power settings, the gyroscopic instruments using the 

vacuum system might not be up to operating speeds and 

precession might occur more rapidly than during flight.

Compass Systems

The Earth is a huge magnet, spinning in space, surrounded 

by a magnetic field made up of invisible lines of flux. These 

lines leave the surface at the magnetic north pole and reenter 

at the magnetic South Pole.

Lines of magnetic flux have two important characteristics: 

any magnet that is free to rotate will align with them, and 

an electrical current is induced into any conductor that cuts 

across them. Most direction indicators installed in aircraft 

make use of one of these two characteristics.

Magnetic Compass

One of the oldest and simplest instruments for indicating 

direction is the magnetic compass. It is also one of the basic 

instruments required by Title 14 of the Code of Federal 

Regulations (14 CFR) part 91 for both VFR and IFR flight.

A magnet is a piece of material, usually a metal containing 

iron, which attracts and holds lines of magnetic flux. 

Regardless of size, every magnet has two poles: north and 

south. When one magnet is placed in the field of another, the 

unlike poles attract each other, and like poles repel.
7-23

Figure 7-31. A magnetic compass. The vertical line is called the 

lubber line.

Figure 7-32. Isogonic lines are lines of equal variation.

An aircraft magnetic compass, such as the one in Figure 7-30,

has two small magnets attached to a metal float sealed inside a 

bowl of clear compass fluid similar to kerosene. A graduated 

scale, called a card, is wrapped around the float and viewed 

through a glass window with a lubber line across it. The card 

is marked with letters representing the cardinal directions, 

north, east, south, and west, and a number for each 30° 

between these letters. The final “0” is omitted from these 

directions. For example, 3 = 30°, 6 = 60°, and 33 = 330°. 

There are long and short graduation marks between the letters 

and numbers, each long mark representing 10° and each short 

mark representing 5°.

The float and card assembly has a hardened steel pivot in its 

center that rides inside a special, spring-loaded, hard glass 

jewel cup. The buoyancy of the float takes most of the weight 

off the pivot, and the fluid damps the oscillation of the float 

and card. This jewel-and-pivot type mounting allows the float 

freedom to rotate and tilt up to approximately 18° angle of 

bank. At steeper bank angles, the compass indications are 

erratic and unpredictable.

The compass housing is entirely full of compass fluid. To 

prevent damage or leakage when the fluid expands and 

contracts with temperature changes, the rear of the compass 

case is sealed with a flexible diaphragm, or with a metal 

bellows in some compasses.

The magnets align with the Earth’s magnetic field and the 

pilot reads the direction on the scale opposite the lubber line. 

Note that in Figure 7-31, the pilot sees the compass card from 

its backside. When the pilot is flying north as the compass 

shows, east is to the pilot’s right. On the card, “33”, which 

represents 330° (west of north), is to the right of north. The 

reason for this apparent backward graduation is that the card 

remains stationary, and the compass housing and the pilot turn 

around it, always viewing the card from its backside.

A compensator assembly mounted on the top or bottom 

of the compass allows an aviation maintenance technician 

(AMT) to create a magnetic field inside the compass housing 

that cancels the influence of local outside magnetic fields. 

This is done to correct for deviation error. The compensator 

assembly has two shafts whose ends have screwdriver slots 

accessible from the front of the compass. Each shaft rotates 

one or two small compensating magnets. The end of one shaft 

is marked E-W, and its magnets affect the compass when the 

aircraft is pointed east or west. The other shaft is marked 

N-S and its magnets affect the compass when the aircraft is 

pointed north or south.

Magnetic Compass Induced Errors

The magnetic compass is the simplest instrument in the 

panel, but it is subject to a number of errors that must be 

considered.

Variation

The Earth rotates about its geographic axis; maps and charts 

are drawn using meridians of longitude that pass through the 

geographic poles. Directions measured from the geographic 

poles are called true directions. The magnetic North Pole to 

which the magnetic compass points is not collocated with 

the geographic North Pole, but is some 1,300 miles away; 

directions measured from the magnetic poles are called 

magnetic directions. In aerial navigation, the difference 

between true and magnetic directions is called variation. This 

same angular difference in surveying and land navigation is 

called declination.

Figure 7-32 shows the isogonic lines that identify the number 

of degrees of variation in their area. The line that passes near 

Chicago is called the agonic line. Anywhere along this line 

the two poles are aligned, and there is no variation. East 

of this line, the magnetic North Pole is to the west of the 

geographic North Pole and a correction must be applied to a 

compass indication to get a true direction.
7-24

030

060

E

120

S

150

210

240

W

300

330

N

True north

Figure 7-33. Utilization of a compass rose aids compensation for

deviation errors.

Figure 7-34. A compass correction card shows the deviation 

correction for any heading.

Flying in the Washington, D.C., area, for example, the variation 

is 10° west. If a pilot wants to fly a true course of south (180°), 

the variation must be added to this, resulting in a magnetic course 

of 190° to fly. Flying in the Los Angeles, California, area, the 

variation is 14° east. To fly a true course of 180° there, the pilot 

would have to subtract the variation and fly a magnetic course 

of 166°. The variation error does not change with the heading of 

the aircraft; it is the same anywhere along the isogonic line.

Deviation

The magnets in a compass align with any magnetic field. 

Local magnetic fields in an aircraft caused by electrical 

current flowing in the structure, in nearby wiring or any 

magnetized part of the structure, conflict with the Earth’s 

magnetic field and cause a compass error called deviation.

Deviation, unlike variation, is different on each heading, 

but it is not affected by the geographic location. Variation 

error cannot be reduced or changed, but deviation error can 

be minimized when an AMT performs the maintenance task 

known as “swinging the compass.”

Most airports have a compass rose, which is a series of lines 

marked out on a ramp or maintenance runup area where there 

is no magnetic interference. Lines, oriented to magnetic north, 

are painted every 30°, as shown in Figure 7-33.

The AMT aligns the aircraft on each magnetic heading and 

adjusts the compensating magnets to minimize the difference 

between the compass indication and the actual magnetic 

heading of the aircraft. Any error that cannot be removed 

is recorded on a compass correction card, like the one in 

Figure 7-34, and placed in a cardholder near the compass. The 

pilot can taxi the aircraft to the compass rose and maneuver 

the aircraft to the headings prescribed by the AMT, and if 

authorized to do so, the AMT can also taxi and maneuver the 

aircraft; however, only the AMT can adjust the compass or 

complete the compass coreection card. If the pilot wants to 

fly a magnetic heading of 120° and the aircraft is operating 

with the radios on, the pilot should fly a compass heading 

of 123°.

The corrections for variation and deviation must be applied 

in the correct sequence and is shown below, starting from 

the true course desired.

Step 1: Determine the Magnetic Course

True Course (180°) ± Variation (+10°) = Magnetic Course (190°)

The magnetic course (190°) is steered if there is no deviation 

error to be applied. The compass card must now be considered 

for the compass course of 190°.

Step 2: Determine the Compass Course

Magnetic Course (190°, from step 1) ± Deviation (–2°, from 

correction card) = Compass Course (188°)

NOTE: Intermediate magnetic courses between those listed 

on the compass card need to be interpreted. Therefore, to 

steer a true course of 180°, the pilot would follow a compass 

course of 188°.

To find the true course that is being flown when the compass 

course is known:

Compass Course ± Deviation = Magnetic Course ± Variation= 

True Course

Dip Errors

The lines of magnetic flux are considered to leave the Earth 

at the magnetic North Pole and enter at the magnetic South

Pole. At both locations the lines are perpendicular to the 

Earth’s surface. At the magnetic equator, which is halfway 

between the poles, the lines are parallel with the surface. The 

magnets in a compass align with this field, and near the poles 

they dip, or tilt, the float and card. The float is balanced with 

a small dip-compensating weight, to dampen the effects of 

dip when operating in the middle latitudes of the northern 

hemisphere. This dip (and weight) causes two very noticeable 

errors: northerly turning error and acceleration error.
7-25

Figure 7-35. Northerly turning error.

Figure 7-36. Effects of acceleration error.

The pull of the vertical component of the Earth’s magnetic field 

causes northerly turning error, which is apparent on a heading 

of north or south. When an aircraft flying on a heading of north 

makes a turn toward east, the aircraft banks to the right, and 

the compass card tilts to the right. The vertical component of 

the Earth’s magnetic field pulls the northseeking end of the 

magnet to the right, and the float rotates, causing the card to 

rotate toward west, the direction opposite the direction the turn 

is being made. [Figure 7-35]

If the turn is made from north to west, the aircraft banks to 

the left and the compass card tilts down on the left side. The 

magnetic field pulls on the end of the magnet that causes the 

card to rotate toward east. This indication is again opposite to 

the direction the turn is being made. The rule for this error is: 

when starting a turn from a northerly heading, the compass 

indication lags behind the turn.

When an aircraft is flying on a heading of south and begins 

a turn toward east, the Earth’s magnetic field pulls on the 

end of the magnet that rotates the card toward east, the same 

direction the turn is being made. If the turn is made from 

south toward west, the magnetic pull starts the card rotating 

toward west—again, in the same direction the turn is being 

made. The rule for this error is: when starting a turn from a 

southerly heading, the compass indication leads the turn.

In acceleration error, the dip-correction weight causes the end 

of the float and card marked N (the south-seeking end) to be 

heavier than the opposite end. When the aircraft is flying at 

a constant speed on a heading of east or west, the float and 

card is level. The effects of magnetic dip and the weight are 

approximately equal. If the aircraft accelerates on a heading 

of east [Figure 7-36], the inertia of the weight holds its end of 

the float back and the card rotates toward north. As soon as the 
7-26

Vertical card compass

N

W

S

E

33

30

24

21

15

12

6

3

Figure 7-37. Vertical card compass.

Outside air temperature gauge

20

0

-20

-40

-60

40

60

100

140

120

80

F

20

0

-20

-40

40

60

C

Figure 7-38. Outside air temperature (OAT) gauge.

speed of the aircraft stabilizes, the card swings back to its east 

indication. If, while flying on this easterly heading, the aircraft 

decelerates, the inertia causes the weight to move ahead and the 

card rotates toward south until the speed again stabilizes.

When flying on a heading of west, the same things happen. 

Inertia from acceleration causes the weight to lag, and the 

card rotates toward north. When the aircraft decelerates on 

a heading of west, inertia causes the weight to move ahead 

and the card rotates toward south.

A mnemonic, or memory jogger, for the effect of acceleration 

error is the word “ANDS” (acceleration—north, deceleration— 

south). Acceleration causes an indication toward north; 

deceleration causes an indication toward south.

Oscillation Error

Oscillation is a combination of all of the other errors, and it 

results in the compass card swinging back and forth around 

the heading being flown. When setting the gyroscopic 

heading indicator to agree with the magnetic compass, use 

the average indication between the swings.

The Vertical Card Magnetic Compass

The floating magnet type of compass not only has all the errors 

just described, but also lends itself to confused reading. It is 

easy to begin a turn in the wrong direction because its card 

appears backward. East is on what the pilot would expect to be 

the west side. The vertical card magnetic compass eliminates 

some of the errors and confusion. The dial of this compass 

is graduated with letters representing the cardinal directions, 

numbers every 30°, and tick marks every 5°. The dial is rotated 

by a set of gears from the shaft-mounted magnet, and the nose 

of the symbolic aircraft on the instrument glass represents the 

lubber line for reading the heading of the aircraft from the dial. 

Eddy currents induced into an aluminum-damping cup damp, 

or decrease, oscillation of the magnet. [Figure 7-37]

Lags or Leads

When starting a turn from a northerly heading, the compass 

lags behind the turn. When starting a turn from a southerly 

heading, the compass leads the turn.

Eddy Current Damping

The decreased amplitude of oscillations by the interaction 

of magnetic fields. In the case of a vertical card magnetic 

compass, flux from the oscillating permanent magnet 

produces eddy currents in a damping disk or cup. The 

magnetic flux produced by the eddy currents opposes the flux 

from the permanent magnet and decreases the oscillations.

Outside Air Temperature (OAT) Gauge

The outside air temperature (OAT) gauge is a simple and 

effective device mounted so that the sensing element is 

exposed to the outside air. The sensing element consists 

of a bimetallic-type thermometer in which two dissimilar 

materials are welded together in a single strip and twisted 

into a helix. One end is anchored into protective tube and the 

other end is affixed to the pointer, which reads against the 

calibration on a circular face. OAT gauges are calibrated in 

degrees °C, °F, or both. An accurate air temperature provides 

the pilot with useful information about temperature lapse rate 

with altitude change. [Figure 7-38]

Chapter Summary

Flight instruments enable an aircraft to be operated with 

maximum performance and enhanced safety, especially when 

flying long distances. Manufacturers provide the necessary 

flight instruments, but to use them effectively, pilots need 

to understand how they operate. As a pilot, it is important to 

become very familiar with the operational aspects of the pitotstatic system and associated instruments, the vacuum system 

and associated instruments, the gyroscopic instruments, and 

the magnetic compass.