In the real world, mag compasses are so inaccurate, and the compass correction cards so out of date, that deviation is almost irrelevant. For tests, though -2 means subtract 2. Magnetic variation published on sectional charts is usually decades old, and winds aloft used for planning are forecasts, not observations. Add to that the fact that most pilots cannot hand-fly to an accuracy of two degrees and it all boils down to "garbage in, garbage out."
Bob Gardner
Charts should be no more than five years out of date for variation. That's the WMM update cycle.In the real world, mag compasses are so inaccurate, and the compass correction cards so out of date, that deviation is almost irrelevant. For tests, though -2 means subtract 2. Magnetic variation published on sectional charts is usually decades old, and winds aloft used for planning are forecasts, not observations. Add to that the fact that most pilots cannot hand-fly to an accuracy of two degrees and it all boils down to "garbage in, garbage out."
Bob Gardner
He's very good at that! Read "Say Again Please".Very succinctly, and accurately, stated.
Well done.
A reg for what? The ten degree thing? Sure: 23.1327
(a) Except as provided in paragraph (b) of this section -
(1) Each magnetic direction indicator must be installed so that its accuracy is not excessively affected by the airplane's vibration or magnetic fields; and
(2) The compensated installation may not have a deviation in level flight, greater than ten degrees on any heading.
(b) A magnetic nonstabilized direction indicator may deviate more than ten degrees due to the operation of electrically powered systems such as electrically heated windshields if either a magnetic stabilized direction indicator, which does not have a deviation in level flight greater than ten degrees on any heading, or a gyroscopic direction indicator, is installed. Deviations of a magnetic nonstabilized direction indicator of more than 10 degrees must be placarded in accordance with § 23.1547(e).
If deviation of magnetic compass says -2degrees on the paper, do we have to add -2 to our magnetic heading/course ?
A correction card isn't even REQUIRED until you get to more than 10 degree error in some direction.
All compasses require a correction card.
FTFY.Steps:
1. Fly to magenta line.
Every DR leg I've flown using pre planned headings has been very accurate.Your absolute True or Magentic heading doesn't end up meaning much given the errors in the system and winds aloft.
Charts should be no more than five years out of date for variation. That's the WMM update cycle.
It's the VORs that get decades out of date, and as long as one doesn't misuse VOR roses as actual courses, it works fine.
Remember, many of the airplanes we fly were NOT certified under Part 23, so it's generally inapplicable. CAR 3 says the following about the mag compass:
"§ 3.666 Magnetic direction indicator. The magnetic direction indicator shall be so installed that its accuracy shall not be excessively affected by the airplane’s vibration or magnetic fields. After the direction indicator has been compensated, the installation shall be such that the deviation in level flight does not exceed 10 degrees on any heading. A suitable calibration placard shall be provided as specified in § 3.758."
-and-
"§ 3.758 Magnetic direction indicator. A placard shall be installed on or in close proximity to the magnetic direction indicator which contains the calibration of the instrument in a level flight attitude with engine(s) operating and radio receiver(s) on or off (which shall be stated). The calibration readings shall be those to known magnetic headings in not greater than 30-degree increments."