Understand how the magnetic compass works, the associated errors, how to correct for these errors, and rate of turns.

Have you ever walked with a bowl of soup? When you turn, how does the soup turn? This is similar to a compass: rigidity in space.

In an Emergency without the AHRS, you will need to use the compass for headings given by ATC.

Definitions:

Variation: caused by the difference in the physical location of the magnetic north pole and the geographic north pole.

Deviation: caused by local magnetic fields within the aircraft. Deviation error is different on each heading

Standard Rate Turns: 3°/second turns in heading.

Errors:

Variation: caused by the difference in the physical location of the magnetic north pole and the geographic north pole.

• Does it affect me as an IFR pilot? No—we don’t use true north

Magnetic or True?

• IFR charts? Magnetic
• VOR Magnetic
• GPS? Magnetic
• Runways? Magnetic

Deviation: caused by local magnetic fields within the aircraft. Deviation error is different on each heading

• Deviation card shows what the mechanics are unable to correct/adjust
• Used mainly for setting directional gyro. If electrics fail/off, the heading is not accurate because it has been corrected for the magnetic interference on the ground

Magnetic Dip: indication errors caused by an unbalanced compass

• Teeter-totter balance analogy– at the magnetic equator north and south forces are balanced. Increase latitude, north pole is pulling down more than south pole
• Above 60° latitude, compass is unusable—too much dip

ANDS: Accelerate North, Decelerate South—this is what the compass does, caused by inertial forces

UNOS: Undershoot North, Overshoot South.

Rule of thumb: latitude minus turn out (half your bank angle in degrees heading as usual). Utah ~40° latitude: 40°-10° (20° bank)=30° for Northerly headings. 20° for 330 and 030, 10° for 300 and 060

Standard Rate Turns: 3° heading/second: 2 minutes for 360° turn

• Bank angle required: Standard rate turns: 15% of TAS or ~20% IAS (easier to calculate)
• TAS: 2% per thousand feet indicated
• Compass limitation: 16°-20° bank becomes unusable—compass measures the horizontal plane

Standard Rate Turns—Set easy rule based on airspeed

1. Within white arc: 10° bank
2. Above the white arc: 15° bank

Timed Turns (less than 30°heading change)—Set easy rule based on airspeed: use half standard rate:

1. Within white arc: 5° bank
2. Above the white arc: 10° bank

Gyroscopic instruments—Attitude Indicator

• Rigidity in space
• Gyroscopic precession