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– Agonic is the line that joins places of zero magnetic variation.
Definitions
• Easy way to remember when to add or
subtract variation to get magnetic heading:
“Variation West, Magnetic Best
Variation East, Magnetic Least”
Definitions
• Deviation
– Combined metal and electrical components of
an aircraft create a magnetic field which
interferes with the compass
– Deviation cards are provided for correction
Definitions
• Lubber line
– Is a line on the compass which shows the
heading on the compass card
Principles
– A direct reading compass is filled with a liquid
which; supports its weight, reduces friction
and dampens oscillations
Principles
000
180
270
090
N
S
N
• The aircraft turns around a magnet which
is aligned north/south
Considerations
• Magnetic Dip
– At equator, lines of magnetic force are parallel to Earth’s surface
– Approaching magnetic poles, lines of magnetic force dip toward
them and the compass aligns to these lines
• Magnetic dip is zero at equator and maximum at the poles.
N S
S N
Pivot Line
Weight
Considerations
• Minimising Dip
– By placing the pivot point above the CoG, it opposes the dip
force
– The greater the dip force, the greater distance of the CoG
outwards, and the greater the restoring force
– In the southern hemisphere, the compass CoG is north of
the pivot line.
Side View
Considerations
N S
N S
Pivot Line
Weight
N
S
• Acceleration errors
– Due to CoG displacement, acceleration errors
occur. As aircraft accelerates, pivot point
moves with the aircraft, but the magnet is left
behind causing errors, until it is realigned.
Side View Top View
Considerations
• In the southern hemisphere SAND
• South Accelerate, North Decelerate
West Apparent turn to the South Apparent turn to the North
South None None
East Apparent turn to the South Apparent turn to the North
North None None
HDG Acceleration Error Deceleration Error
000
180
270
090
N
S
Considerations
• Turning Errors
– The centripetal force of the turn pulls the pivot point of the
compass into the turn, but the magnet is left behind again
• In the southern hemisphere ONUS
• Overshoot through North, Undershoot through South
HDG 330
HDG 030
000
180
270
090
N
S
030
HDG 330
HDG 030
000
180
270
090
N
S
330
Considerations
• Turning through south
HDG 150
HDG 210
000
180
270
090
N
S
150
HDG 150
HDG 210
000
180
270
090
N
S
210
Considerations
• DG failure
– You can turn onto headings using a watch by
timing the turn
– Rate one turn is 360 degrees in 2 minutes
(120 seconds)
– This is 3 degrees per second
– How many seconds would it then take to turn
90 degrees?
Considerations
• Turbulent weather
– Compass may become unreadable in very
bumpy weather since the compass is not
stabilised in any way.
– Use DG instead of compass
– However aligning DG to compass could be
difficult
Airmanship
– Always avoid placing ferrous/metallic/electronic materials
near the compass
– Apply SAND and ONUS rules where applicable
– Always apply magnetic variation when flight planning
“What you see is true, what you hear is Magnetic”
– Always apply compass deviation
– Check compass for fluid leaks
– Never Align Compass to DG, in a climb, descent, turn,
acceleration or deceleration
– You can always use your watch in a timed rate one turn to
turn onto a heading if you have a DG failure
– Look at the compass from head on, otherwise you will get a
small parallax error
Compasses Mass Brief
Aim
• To gain an understanding of the principles
and limitations of using an aeroplane
compass in flight.
Application
• To align the DG and compass in VFR flight
• When the DG fails
Overview
• Definitions
• Principles
• Considerations
• Airmanship
Definitions
• Simple Bar Magnet
　
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