CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

1. Flight instrument systems and their operating characteristics
   
   
   d. Magnetic Compass
   
   1) Operating principle
   a) Earth is magnet whose lines of flux leave its surface at magnetic north pole and reenter at magnetic south pole
   b) Most direction indicators make use of one of two important magnetic principles
   i) Free magnets align with lines of flux (magnetic compass)
   ii) Electrical current is induced in conductor that cuts across lines of flux (flux gate compass)
   2) Magnetic compass is required by 14 CFR part 91 for both VFR and IFR flight
   3) Components
   a) Two small magnets attached to a
   b) Metal float that is sealed inside a
   c) Bowl of clear fluid similar to kerosene
   d) Graduated scale, the card, marked with cardinal direction letters, numbers every 30° between letters, and long and short marks representing 10° and 5° respectively
   e) Lubber line reference indicating magnetic direction
   f) Jewel-and-pivot type mounting of float and card assembly allows free rotation and tilting to about 18° bank
   g) Compensator assembly allows aviation maintenance technician (AMT) to correct deviation error
   4) Compass errors
   a) Variation
   i) Difference between true and magnetic directions is called variation
   ii) Isogonic lines connect points with the same variation and are numbered to indicate degrees of variation along each one
   
   iii) Along the agonic line, passing near Chicago, the variation is zero
   iv) East of agonic line is the west variation area (magnetic north pole is west of geographic north pole) where west variation is added to true course (TC) to obtain magnetic course (MC) to be flown:
   
   MC = TC + West Variation
   v) West of agonic line is the east variation area (magnetic north pole is east of geographic north pole) where east variation is subracted from true course to obtain magnetic course:
   
   MC = TC - East Variation
   b) Deviation
   i) Caused by local magnetic fields within the aircraft
   ii) Different on each heading
   iii) AMT minimizes by "swinging the compass" - adjusting compensating magnets as needed every 30° while aircraft is aligned with magnetic directions indicated on a compass rose on the surface
   iv) Deviation errors that cannot be eliminated by AMT are recorded on a compass correction card placed near the magnetic compass for use by pilot in flight
   c) Compass course (CC)
   i) Course to be maintained under the magentic compass lubber line during cross country flight leg in order to fly appropriate true course over the ground
   ii) True course (TC) corrected for variation (V) and deviation (D) errors:
   
   TC +/- V = MC
   MC +/- D = CC
   d) Dip errors
   i) Northerly turning error
   • Caused by pull of vertical component of Earth's magnetic field
   • When turning east (or west) from a north heading, compass initially shows a turn in the opposite direction (compass lags)
   • When turning east (or west) from a south heading, compass initiallly shows an exaggerated turn in the same direction (compass leads)
   • NOSE ("North Opposite, South Exaggerates")
   ii) Acceleration error
   • Acceleration in SLF on a heading of east or west causes magnetic compass to indicate a turn to the north
   • Deceleration in SLF on a heading of east or west causes magnetic compass to indicate a turn to the south
   • ANDS (Accelerate North, Decelerate South)
   e) Oscillation error
   i) Combination of all other errors results in compass swinging back and forth around heading being flown
   ii) Set gyroscopic HI to average compass heading between swings
   5) Compass turns
   a) With bank angle 15-18° (approximately standard rate turn), amount of lead or lag when turning to north or south varies with and is approximately equal to the current local latitude (e.g., 41° at KLPR)
   b) Turn to north: lead for roll out = normal lead + latitude
   Near KLPR lead to N = 5+41 = 46°, i.e.,
   Start rollout at approximately 45° or 315°
   c) Turn to south: lead for roll out = pass south heading by latitude - normal lead
   Near KLPR lead to S = 41-5 = 36° past S, i.e.,
   Start rollout at approximately 145° or 215°
   d) Turn to east or west from north, lead by approximately 10°
   e) Turn to east of west from south, lead by approximately 5°
   f) Interpolate for other directions
   g) Use smooth control technic and accurate pitch-and-bank control

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