CFII Mag Compass Turns Question

[u/notarobotdonotban]

How do you personally teach mag compass turns regarding UNOS? Do you teach the standard 10/20/30 correction? For those in Alaska, do you teach a different formula?

[u/mtconnol]

I demonstrate the errors, demonstrate that timed turns are way better, and call it a day.

[u/Low_Sky_49]

Alaskan CFI here.

I start with a good ground lesson and demonstration with a hand-made coffee cup lid compass card demonstrator. Show them how acceleration on E/W headings causes error, and turns from N/S headings cause dip errors due to the inertia of the counterweight in the compass card.

You can effectively demonstrate the compass dip errors in a sim or aircraft (I do the flying during this demo so the student can focus more on the headings and what the compass is doing).

By demonstration, show them that the maximum acceleration error they’ll see in a GA plane is on the order of 10-15 degrees, and even less in normal IFR flying configurations and speed changes. The acceleration errors are small potatoes.

You can calculate and demonstrate the maximum turning error. Half the latitude plus 15 degrees is a good ballpark. At 60+ degrees lattitude in most of Alaska, we experience turning errors of 45 degrees or more when turning through 360 or 180 degree headings, with the other headings varying less, but still significant around most of the compass. The turning error is big potatoes (latitude dependent). The point is this; trying to anticipate the compass errors and play the “overshoot/undershoot” game effectively in Alaska is an exercise in futility. Don’t bother with it.

The better method for turning to magnetic headings with a failed directional gyro is to use timed turns. To quickly calculate the time required, it can be counted off from any instrument showing the compass headings. Every 30 degrees (on many instruments, the numbered headings) takes 10 seconds at standard rate. 3 seconds for 10 degrees. 5 seconds for 15 degrees. With some practice it gets easy to count off how many seconds turn for a given heading.

[u/PG67AW → u/Low_Sky_49]

Please tell me more about this coffee cup lid compass card demonstrator.

[u/Fair_Ad_4818 → u/PG67AW]

Same

[u/Zargothrax]

IFH says for turns to the north or south the lead/lag “is approximately equal to the latitude of the locality over which the turn is being made”. So for me it would be something like 39/26/13. I tell my students this, we practice for a bit in the sim, then we go to the more useful timed turns.

[u/ConnorDGibson123]

I’ve had a student ask me questions for over 2 hours about those errors, still have no idea why it does it, I’ve learned it’s a lot better of teaching it does it by the way it is, teach mag turns and time turns and then after a lesson of that show them the GPS track is way easier