Magnetic Compass

Private Pilot License (PPL) Notes

This section covers the construction, operation, and errors associated with the magnetic compass in aviation, ensuring student pilots understand its importance despite the availability of more modern instruments.

Introduction to the Magnetic Compass

1. The magnetic compass is a required instrument in training airplanes.
   1. It is usually the only north-seeking instrument in such airplanes.
2. While the heading indicator is primarily used for directional navigation:
   2. It is subject to precession errors.
   3. Knowing how the magnetic compass functions is essential.

Construction and Operation of the Magnetic Compass

1. The compass consists of:
   1. Two magnetized needles mounted under a float.
   2. The compass card attached around the float.
2. The assembly is mounted on a pivot, allowing the card to tilt up to 18 degrees.
3. The compass is sealed in a chamber filled with high-grade petroleum distillate:
   1. The fluid dampens oscillations and lubricates the pivot.
   2. It reduces some weight off the pivot.
4. The magnets and compass card remain stationary while the airplane turns around them.
5. The compass card numbers and letters appear backward because:
   1. We are viewing the back of the compass card.

Earth's Magnetic Field and Its Effects

1. Earth has a magnetic field similar to a huge bar magnet along its axis.
2. "Opposites attract" applies in magnetism:
   1. The south pole end of the compass needles is attracted to Earth's magnetic north pole.
3. Lines of magnetic force:
   1. Are parallel to the surface at the magnetic equator.
   2. Dip downward increasingly towards the magnetic poles.
4. The magnetic dip (vertical component) causes major compass errors.

Compass Errors Due to Banking and Turning

1. In straight and level flight, the compass design keeps the card level.
2. During banking and turning:
   1. The card tilts, compromising between polar and dip attractions.
   2. The compass does not show the actual heading.
3. The error is greatest on north and south headings:
   1. On northerly headings, the compass lags behind the airplane.
   2. On southerly headings, the compass leads the airplane.
4. Lead and lag errors:
   1. Are minimal at the magnetic equator.
   2. Greatest at the magnetic poles.
   3. In the lower 48 states, errors are generally 30 to 35 degrees.
5. Compass turns should be made with shallow banks to prevent card locking.

Understanding Lead and Lag Errors

1. On turns from northerly headings:
   1. The compass initially indicates a turn in the opposite direction.
   2. Example: Turning east from north, the compass shows northwest by north (330 degrees).
2. On turns from southerly headings:
   1. The compass shows the correct direction but exaggerates the rate of turn.
3. There is no lead or lag error on east and west headings.
4. Mnemonic "NOSE" helps remember:
   1. N - North
   2. O - Opposite (compass shows opposite turn)
   3. S - South
   4. E - Exaggerate (compass exaggerates rate)

Compass Errors Due to Acceleration and Deceleration

1. Acceleration and deceleration affect compass readings on east and west headings:
   1. During acceleration, the compass indicates a turn to the north.
   2. During deceleration, the compass indicates a turn to the south.
2. This error diminishes towards north and south headings.
3. Mnemonic "ANDS" helps remember:
   1. A - Acceleration
   2. N - North
   3. D - Deceleration
   4. S - South
4. There is no rule of thumb for predicting the magnitude of this error.

Using the Heading Indicator and Magnetic Compass

1. The heading indicator is the primary directional instrument.
2. It should be reset every 15 minutes to match the magnetic compass:
   1. Adjusted for deviation as per the deviation card.
3. Ensure the airplane is in straight and level flight when reading the compass.

Importance of Understanding the Magnetic Compass

1. Despite seeming complicated, the compass is valuable:
   1. Self-powered, requiring no electricity or vacuum.
   2. Essential if the heading indicator fails.
2. Understanding compass quirks ensures accurate navigation.