About the Magnetic Compass
One of the oldest and simplest instruments for indicating direction is the magnetic compass. It is also one of the basic instruments required by Title 14 of the Code of Federal Regulations (14 CFR) part 91 for both VFR and IFR flight.
A magnet is a piece of material, usually a metal containing iron, that attracts and holds lines of magnetic flux. Regardless of size, every magnet has two poles: north and south. When one magnet is placed in the field of another, the unlike poles attract each other, and like poles repel.
An aircraft magnetic compass has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid similar to kerosene. A graduated scale, called a card, is wrapped around the float and viewed through a glass window with a lubber line across it. The card is marked with letters representing the cardinal directions, north, east, south, and west, and a number for each 30º between these letters. The final “0” is omitted from these directions. For example, 3 = 30º, 6 = 60º, and 33 = 330º. There are long and short graduation marks between the letters and numbers, each long mark representing 10º and each short mark representing 5º. – Source: Pilot's Handbook of Aeronautical Knowledge.
Magnetic Compass Deviation
The magnets in a compass align with any magnetic field. Some causes for magnetic fields in aircraft include flowing electrical current, magnetized parts, and conflict with the Earth’s magnetic field. These aircraft magnetic fields create a compass error called deviation.
Deviation, unlike variation, depends on the aircraft heading. Also unlike variation, the aircraft’s geographic location does not affect deviation. While no one can reduce or change variation error, an aviation maintenance technician (AMT) can provide the means to minimize deviation error by performing the maintenance task known as “swinging the compass.” Source: Pilot's Handbook of Aeronautical Knowledge.
Magnetic Compass Variation
The Earth rotates about its geographic axis; maps and charts are drawn using meridians of longitude that pass through the geographic poles. Directions measured from the geographic poles are called true directions. The magnetic North Pole to which the magnetic compass points is not collocated with the geographic North Pole, but is some 1,300 miles away; directions measured from the magnetic poles are called magnetic directions. In aerial navigation, the difference between true and magnetic directions is called variation. This same angular difference in surveying and land navigation is called declination. Source: Pilot's Handbook of Aeronautical Knowledge.
Northerly Turning Errors
The center of gravity of the float assembly is located lower than the pivotal point. As the aircraft turns, the force that results from the magnetic dip causes the float assembly to swing in the same direction that the float turns. The result is a false northerly turn indication. Because of this lead of the compass card, or float assembly, a northerly turn should be stopped prior to arrival at the desired heading. This compass error is amplified with the proximity to either magnetic pole. One rule of thumb to correct for this leading error is to stop the turn 15 degrees plus half of the latitude. Source: Pilot's Handbook of Aeronautical Knowledge.
Southerly Turning Errors
When turning in a southerly direction, the forces are such that the compass float assembly lags rather than leads. The result is a false southerly turn indication. The compass card, or float assembly, should be allowed to pass the desired heading prior to stopping the turn. As with the northerly error, this error is amplified with the proximity to either magnetic pole. To correct this lagging error, the aircraft should be allowed to pass the desired heading prior to stopping the turn. The same rule of 15º plus half of the latitude applies here. Source: Pilot's Handbook of Aeronautical Knowledge.
Airpath C2300 Compensation Instructions
Before attempting to compensate compass, every effort should be made to place the aircraft in simulated flight conditions. Check to see that canopy is closed, flaps are in neutral position and the sailplane is in a level attitude. All electrical switches, flight computers, transponder, radio, electric vario, etc., should be in the position they will normally be for navigation flight.
- Set adjustment screws of compensator on zero. Zero position of adjustment screw is obtained by lining up the dot on the screw with the dot on the compensator frame.
- Head aircraft on magnetic North heading. Adjust N–S adjustment screw until compass reads exactly North.
- Head aircraft on magnetic East heading. Adjust E–W adjustment screw until compass reads exactly East.
- Head aircraft on magnetic South heading. Note the resulting South error. Adjust the N–S adjusting screw until one–half of this error is removed.
- Head aircraft on magnetic West heading. Note the resulting West error. Adjust the E–W adjusting screw until one–half of this error is removed.
- Head aircraft in successive magnetic 30–º headings and record all errors on the deviation card furnished with the compass.
For best results, all extraneous magnetism causing over 30–35º compass errors should be removed from the sailplane, or the compass should be relocated to a position where uncompensated error does not exceed 30–35º. Use a brass or other non–ferrous material screwdriver when making compensator adjustments.
In flight compensation for sailplanes with a directional gyro:
- Set directional gyro from a sectional line or runway. (Allow for magnetic variation to ensure gyro corresponds to magnetic heading)
- Follow procedures 1 through 6 above.
- Re–check directional gyro occasionally for possible precision, and allow for such precision error in recording results on magnetic compass deviation card.
NOTE: If your sailplane is equipped, GPS can be used (allow for deviation) to establish reference headings for compass compensation. This technique will eliminate possible errors caused by gyro precision.
Common Airpath C2300 Compensation Problems
Any time there is a maintenance or repair to your sailplane, it is recommended that the compass be compensated. This is particularly true if there is work associated with the removal of old and/or installation of new equipment in the instrument panel. New radios and relocation of speakers or intercoms could affect the compensation required. New hardware (i.e. screws, nuts, etc.) installed during maintenance can sometimes be the cause of excessive errors if the hardware is steel or magnetic.
Loose electrical grounds, lighting, or extended periods of parking in North–South alignment on the ramp can lead to the magnetization of some airframes. This is often evidenced by excessive uncompensated compass error (more than 30–35º). Engine mounts on single engine aircraft and center windshield posts becoming magnetized can lead to compensation problems. Demagnetizing (degaussing) the airframe component or relocating the compass will solve this problem.
Remember that every sailplane is different. Following the set–up procedures outlined above prior to compensation is important. As stated, in–flight compensation will achieve the best results. Landing gear position can sometimes affect deviation. Other factors to consider in airplanes are: yoke position, cruise configuration, pilot heat, and de–icing equipment (particularly windshield anti–ice).
You should consider removing any jewelry while compensating compasses. Such things as watches, rings, and eyeglasses can affect the amount of compensation required. If above method does not give satisfactory results, determine the amount of uncompensated error by aligning the reference dots on the compensator adjustment screws and frame or by removing the compensator assembly from the compass. If the uncompensated error is in excess of 30–35º, troubleshoot for magnetization of aircraft components or excessive electrical interference.
