0˚
15˚N
30˚N
45˚N
60˚N
70˚N70˚N
70˚N70˚N
15˚S
30˚S
45˚S
60˚N
0˚
15˚N
30˚N
45˚N
60˚N
60˚N
15˚S
30˚S
45˚S
0˚15˚W30˚W45˚W60˚W90˚W 75˚W105˚W120˚W135˚W150˚W165˚W180˚W 180˚W
180˚W 180˚W
15˚E 30˚E 45˚E 60˚E 90˚E75˚E 105˚E 120˚E 135˚E 150˚E 165˚E
0˚15˚W30˚W45˚W60˚W90˚W 75˚W105˚W120˚W135˚W150˚W165˚W 15˚E 30˚E 45˚E 60˚E 90˚E75˚E 105˚E 120˚E 135˚E 150˚E 165˚E
60
50
40
30
20
10
10
20
0
0
10
0
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0
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130 110
100 90 80
70 60
50
40
30
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70
20
10
-40
-90
-100
-110 -120 -130
-50
-40-30
-20
-10
-30
-20
-10
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-30
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-10
-80
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-10
Main field declination (D)
Contour interval:
2 degrees
red contours positive (east)
blue negative (west)
pink (agonic) zero line.
Mercator Projection.
Position of dip poles
Figure 8-33. Isogonic lines are lines of equal variation.
card from its backside. When the pilot is flying north, as the
compass indicates, 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 rotate around it. Because of this setup, the magnetic
compass can be confusing to read.
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 8-33 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.
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.
Some causes for magnetic fields in aircraft include flowing
electrical current, magnetized parts, and conflict with the
Earth’s magnetic field. These aircraft magnetic fields create
a compass error called deviation.
Deviation, unlike variation, depends on the aircraft heading.
Also unlike variation, the aircraft’s geographic location
does not affect deviation. While no one can reduce or
change variation error, an aviation maintenance technician
(AMT) can provide the means to minimize deviation error
by performing the maintenance task known as “swinging
the compass.”
Figure 8-35. A compass correction card shows the deviation
correction for any heading.
030
060
E
120
150S
210
240
W
300
330 N
True north
Figure 8-34. Utilization of a compass rose aids compensation for
deviation errors.
To swing the compass, an AMT positions the aircraft
on a series of known headings, usually at a compass
rose. [Figure 8-34] A compass rose consists of a series
of lines marked every 30° on an airport ramp, oriented to
magnetic north. There is minimal magnetic interference at
the compass rose. The pilot or the AMT, if authorized, can
taxi the aircraft to the compass rose and maneuver the aircraft
to the headings prescribed by the AMT.
As the aircraft is “swung” or aligned to each compass rose
heading, the AMT adjusts the compensator assembly located
on the top or bottom of the compass. 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.
The adjustments position the compensating magnets to
minimize the difference between the compass indication and
the actual aircraft magnetic heading. The AMT records any
remaining error on a compass correction card like the one
in Figure 8-35 and places it in a holder near the compass.
Only AMTs can adjust the compass or complete the compass
correction card. Pilots determine and fly compass headings
using the deviation errors noted on the card. Pilots must also
note the use of any equipment causing operational magnetic
interference such as radios, deicing equipment, pitot heat,
radar, or magnetic cargo.
The corrections for variation and deviation must be applied
in the correct sequence as 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 Earth's magnetic field runs parallel to its surface only at
the Magnetic Equator, which is the point halfway between
the Magnetic North and South Poles. As you move away
from the Magnetic Equator towards the magnetic poles, the
angle created by the vertical pull of the Earth's magnetic field
in relation to the Earth’s surface increases gradually. This
angle is known as the dip angle. The dip angle increases in
a downward direction as you move towards the Magnetic
North Pole and increases in an upward direction as you move
towards the Magnetic South Pole.
If the compass needle were mounted so that it could pivot
freely in three dimensions, it would align itself with the
magnetic field, pointing up or down at the dip angle in the
direction of local Magnetic North. Because the dip angle is
of no navigational interest, the compass is made so that it can
21 S 15 12
21
S 15 12
21 S 15
12
Dip effect
CARD CARD
Left turn No error Right turn
Dip effect
DIP DIP DIP
B
3 N 33 30
3 N 33
30
3
N 33 30CARD
Dip effect
CARD
Dip effect
Dip effect
Left turn No error Right turnA
DIP DIP
DIP
Dip effect
Dip effect
Dip effect
Figure 8-36. Northerly and southerly turning errors.
rotate only in the horizontal plane. This is done by lowering
the center of gravity below the pivot point and making the
assembly heavy enough that the vertical component of the
magnetic force is too weak to tilt it significantly out of the
horizontal plane. The compass can then work effectively at
all latitudes without specific compensation for dip. However,
close to the magnetic poles, the horizontal component of
the Earth’s field is too small to align the compass which
makes the compass unusable for navigation. Because of this
constraint, the compass only indicates correctly if the card
is horizontal. Once tilted out of the horizontal plane, it will
be affected by the vertical component of the Earth’s field
which leads to the following discussions on northerly and
southerly turning errors.
Northerly Turning Errors
The center of gravity of the float assembly is located lower
than the pivotal point. As the aircraft turns, the force that
results from the magnetic dip causes the float assembly to
swing in the same direction that the float turns. The result is
a false northerly turn indication. Because of this lead of the
compass card, or float assembly, a northerly turn should be
stopped prior to arrival at the desired heading. This compass
error is amplified with the proximity to either magnetic pole.
One rule of thumb to correct for this leading error is to stop
the turn 15 degrees plus half of the latitude (i.e., if the aircraft
is being operated in a position near 40 degrees latitude, the
turn should be stopped 15+20=35 degrees prior to the desired
heading). [Figure 8-36A]
Southerly Turning Errors
When turning in a southerly direction, the forces are such that
the compass float assembly lags rather than leads. The result
is a false southerly turn indication. The compass card, or float
assembly, should be allowed to pass the desired heading prior
to stopping the turn. As with the northerly error, this error
is amplified with the proximity to either magnetic pole. To
correct this lagging error, the aircraft should be allowed to
pass the desired heading prior to stopping the turn. The same
rule of 15 degrees plus half of the latitude applies here (i.e.,
if the aircraft is being operated in a position near 30 degrees
latitude, the turn should be stopped 15+15=30 degrees after
passing the desired heading). [Figure 8-36B]
Acceleration Error
The magnetic dip and the forces of inertia cause magnetic
compass errors wh en acceleratin g and decelerating on
easterly and westerly headings. Because of the pendulous-
type mounting, the aft end of the compass card is tilted
upward when accelerating and downward when decelerating
during changes of airspeed. When acceler ating on either
an easterly or westerly headin g, the error appears as a
turn indication toward north. When decelerating on either
of these head ings, the comp ass indicates a turn toward
south. A mnemonic, or memory jogg er, for the effect of
acceleration error is the word “ANDS” (Acceleration-
North/Deceleration-South) may help you to remember the
acceleration error. [Figure 8-37] Acceleration causes an
N
W
S
E
33
30
24
21 15
12
6
3
Figure 8-38. Vertical card magnetic compass.
NORTH
South
Figure 3-21. The effects of acceleration error.
OBS
N
E
S
W
3 33
24
21 15
12
30
6
NAV
GS
View is from the pilot’s
perspective, and the
movable card is reset
after each turn.
Figure 8-37. The effects of acceleration error.
indication toward north; deceleration causes an indication
toward south.
Oscillation Error
Oscillation is a combination of all of the errors previously
mentioned and results in fluctuation of the compass card in
relation to the actual heading direction of the aircraft. 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 vertical card magnetic compass eliminates some of the
errors and confusion encountered with the magnetic compass.
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. [Figure 8-38]
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
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.
20
0
-20
-40
-60
40
60
100
140
120
80
F
20
0
-20
-40
40
60
C
Figure 8-39. Outside air temperature (OAT) gauge.
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 8-39]
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 pitot-
static system and associated instruments, the vacuum system
and associated instruments, the gyroscopic instruments, and
the magnetic compass.