When taking Mag Compass into account

[John777]

If deviation of magnetic compass says -2degrees on the paper, do we have to add -2 to our magnetic heading/course ?


John

 

[MAKG1]

Here's a trick for getting this type of thing right.

Bring an E-6B.

Seriously.

 

[bobmrg]

In the real world, mag compasses are so inaccurate, and the compass correction cards so out of date, that deviation is almost irrelevant.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

 

[Let'sgoflying!]

I can honestly say I have not used a compass correction card in over 15 years.
I know they must still teach it but when it comes to practicality, I don't think it is worth the effort.

 

[FastEddieB]

Typically, cards don't say +/-.

Rather For/Steer:

 

[Shepherd]

In the real world, mag compasses are so inaccurate, and the compass correction cards so out of date, that deviation is almost irrelevant.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

Very succinctly, and accurately, stated.
Well done.

 

[MAKG1]

In the real world, mag compasses are so inaccurate, and the compass correction cards so out of date, that deviation is almost irrelevant.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Charts should be no more than five years out of date for variation.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.

It's the VORs that get decades out of date, and as long as one doesn't misuse VOR roses as actual courses, it works fine.

 

[jsstevens]

Very succinctly, and accurately, stated.
Well done.

He's very good at that! Read "Say Again Please".

 

[Checkout_my_Six]

People always complement me on the accuracy of my compass correction card.

 

[flyingron]

A correction card isn't even REQUIRED until you get to more than 10 degree error in some direction.

The answer for the tests are:

True Heading
+ West Variation or - East Variation
+ / - the deviation
equals your magnetic course.

Think of it the way you fly. You have a heading on the chart you want to fly to get to your destination and you apply the Variation to get to the magnetic course not speciific to your airplane, and then correct the magnetic course for your airplane's compass.

As mentioned nobody actually specifies deviation as +/- but rather a correction card gives you the "to get this heading fly this value" reading for various headings... just pick the one closest.

 

[Checkout_my_Six]

Do you have the reg for that?

 

[flyingron]

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).

 

[Dan Thomas]

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).

And if we read a little further on, we find this:

23.1547 Magnetic direction indicator.
(a) A placard meeting the requirements of this section must be installed on or near the magnetic direction indicator.

(b) The placard must show the calibration of the instrument in level flight with the engines operating.

(c) The placard must state whether the calibration was made with radio receivers on or off.

(d) Each calibration reading must be in terms of magnetic headings in not more than 30 degree increments.

(e) If a magnetic nonstabilized direction indicator can have a deviation of more than 10 degrees caused by the operation of electrical equipment, the placard must state which electrical loads, or combination of loads, would cause a deviation of more than 10 degrees when turned on.

The max deviation in 23.1327 is addressing the amount the compass can be off as installed, not the amount it can be off without a compass correction card. All compasses require a correction card.

In Canada the law requires compass calibration annually, or whenever something has been changed that might alter its indication.

 

[dmspilot]

If deviation of magnetic compass says -2degrees on the paper, do we have to add -2 to our magnetic heading/course ?

True Course +/- WCA = True Heading
True Heading +/- Variation = Magnetic Heading
Magnetic Heading +/- Deviation = Compass Heading

If you take the compass deviation card into account when you set your heading indicator, then you don't have to worry about it, since most people would fly headings with the heading indicator.

Like FastEddieB said, I've never seen a compass deviation card like that--they usually say FOR and STEER. FOR is the Magnetic Heading, STEER is the Compass Heading.

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.

 

[Sac Arrow]

Steps:

1. Note heading on compass.

2. Note position with respect to magenta line.

3. Note deviation from magenta line.

4. Fly to magenta line.

5. Once flown to magenta line, stop flying to magenta line.

Repeat steps 2 through 5 as necessary.

 

[jordane93]

Steps:
1. Fly to magenta line.

FTFY.

 

[Dan Thomas]

Flying to a magenta line is all well and good until the alternator quits and the standby battery runs dry. Then we're down to a mag compass. Even with a gyroscopic heading indicator, you'll need that mag compass in IMC to keep the gyro pointing right. They all precess some, and the earth's rotation also messes with it.

As fas a compass cards go, they are part of the required placards in any airplane. Unless the airplane is quite new, I find placards missing almost every time I inspect an airplane new to us. The TCDS lists required placards, or they'll refer you to the serialized AFM for them.

23.1541 General.
(a) The airplane must contain—

(1) The markings and placards specified in §§23.1545 through 23.1567; and

(2) Any additional information, instrument markings, and placards required for the safe operation if it has unusual design, operating, or handling characteristics.

(b) Each marking and placard prescribed in paragraph (a) of this section—

(1) Must be displayed in a conspicuous place; and

(2) May not be easily erased, disfigured, or obscured.

(c) For airplanes which are to be certificated in more than one category—

(1) The applicant must select one category upon which the placards and markings are to be based; and

(2) The placards and marking information for all categories in which the airplane is to be certificated must be furnished in the Airplane Flight Manual.

91.9 Civil aircraft flight manual, marking, and placard requirements.
(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.

 

[flyingron]

It doesn't even need to be the magenta line. If you're flying any navigational course be it a GPS line, a VOR, an LOC, etc... you pick a heading, fly it a bit, and see if it's getting you where you want to go and if not pick another.

Your absolute True or Magentic heading doesn't end up meaning much given the errors in the system and winds aloft.

 

[MauleSkinner]

Your absolute True or Magentic heading doesn't end up meaning much given the errors in the system and winds aloft.

Every DR leg I've flown using pre planned headings has been very accurate.

YMMV.

 

[tsts4]

What's a compass? I can't find one in my plane.

 

[bradg33]

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."

 

[bobmrg]

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.

You are correct. I have an email from a Jim Grant at the National Geodetic Survey saying that. Maybe next time I will dig into my Evernote file before making such assertions.

Bob

 

[Dan Thomas]

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."

Which is essentially the same thing FAR 23 says.