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Pilot's Handbook of Aeronautical Knowledge

Latitude and Longitude (Meridians and

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




If the variation is shown as "9° E," this means that magnetic
north is 9° east of true north. If a true course of 360° is to be
flown, 9° must be subtracted from 360°, which results in a
magnetic heading of 351°. To .y east, a magnetic course of
081° (090° – 9°) would be flown. To fly south, the magnetic
course would be 171° (180° – 9°). To fly west, it would be
261° (270° – 9°). To fly a true heading of 060°, a magnetic
course of 051° (060° – 9°) would be flown.

Remember, if variation is west, add; if east, subtract. One
method for remembering whether to add or subtract variation
is the phrase "east is least (subtract) and west is best (add)."

Determining the magnetic heading is an intermediate step
necessary to obtain the correct compass heading for the flight.
To determine compass heading, a correction for deviation
must be made. Because of magnetic influences within
an aircraft such as electrical circuits, radio, lights, tools,
engine, and magnetized metal parts, the compass needle is
frequently deflected from its normal reading. This deflection
is deviation. The deviation is different for each aircraft, and it
also may vary for different headings in the same aircraft. For
instance, if magnetism in the engine attracts the north end of
the compass, there would be no effect when the plane is on a
heading of magnetic north. On easterly or westerly headings,
however, the compass indications would be in error, as shown
in Figure 15-11. Magnetic attraction can come from many
other parts of the aircraft; the assumption of attraction in the
engine is merely used for the purpose of illustration.

Some adjustment of the compass, referred to as compensation,
can be made to reduce this error, but the remaining correction
must be applied by the pilot.

Proper compensation of the compass is best performed by
a competent technician. Since the magnetic forces within
the aircraft change, because of landing shocks, vibration,
mechanical work, or changes in equipment, the pilot should
occasionally have the deviation of the compass checked. The
procedure used to check the deviation (called "swinging the
compass") is briefly outlined.

Magnetized portions
Figure 15-11. Magnetized portions of the airplane cause the compass to deviate from its normal indications.

The aircraft is placed on a magnetic compass rose, the engine
started, and electrical devices normally used (such as radio)
are turned on. Tailwheel-type aircraft should be jacked up
into flying position. The aircraft is aligned with magnetic
north indicated on the compass rose and the reading shown
on the compass is recorded on a deviation card. The aircraft
is then aligned at 30° intervals and each reading is recorded.
If the aircraft is to be flown at night, the lights are turned on
and any significant changes in the readings are noted. If so,
additional entries are made for use at night.

The accuracy of the compass can also be checked by
comparing the compass reading with the known runway

A deviation card, similar to Figure 15-12, is mounted near
the compass, showing the addition or subtraction required to
correct for deviation on various headings, usually at intervals
of 30°. For intermediate readings, the pilot should be able to
interpolate mentally with sufficient accuracy. For example,
if the pilot needed the correction for 195° and noted the
correction for 180° to be 0° and for 210° to be +2°, it could
be assumed that the correction for 195° would be +1°. The
magnetic heading, when corrected for deviation, is known
as compass heading.

Compass deviation card.
Figure 15-12. Compass deviation card.