TVMDC and turning the true course you drew on the chart into the compass heading you actually fly

The line you draw on a sectional chart is measured against true north, but your airplane’s compass points to magnetic north, and the metal in your panel pulls the needle further still. To turn that line into a heading you can actually fly, you apply a short chain of corrections—variation (the difference between the two norths), deviation (your airplane’s own magnetic error), and wind correction angle—summarized by the acronym TVMDC: True, Variation, Magnetic, Deviation, Compass. Get the direction of each correction right and your nose points where you intended; flip one and you’ll be miles off over a 90-mile leg.

Why doesn’t my compass heading match the course I drew on the chart?

Because there are two different norths, and you measured against the wrong one for your compass.

True north is the geographic North Pole—the actual top of the planet, where all the lines of longitude converge. When you lay a protractor on a line drawn against those longitude lines, you’re reading your true course.

Magnetic north is a different place entirely. It’s the spot your compass actually points to, currently wandering around the Canadian arctic, and it shifts a little every year. Your compass doesn’t know or care about the geographic pole—it only responds to the magnetic one.

The angle between those two norths, measured from where you happen to be standing, is called variation. That’s the first correction you have to make, and it’s why the raw protractor number is, in a sense, a lie.

What is magnetic variation and how do I find it?

Variation is the difference, in degrees, between true north and magnetic north for your specific location on the continent.

The amount changes depending on where you are. Flying over the Carolinas in the east, magnetic north sits to the west of true north from your viewpoint. Out in California, it sits the other way. Your sectional chart tells you exactly how much.

Look for the dashed magenta lines snaking across the chart—these are isogonic lines, lines of equal variation. One might read 6 degrees east, another further over 7 degrees east. Out west you’ll find west variation instead. Those lines hand you the precise correction for your part of the world.

One practical note: variation drifts over time. The isogonic lines on a current sectional are accurate; on an expired chart from three years ago, the variation has wandered. It’s part of why we fly current charts.

Do I add or subtract variation? (East is least, west is best)

Use the classic rule: East is least, west is best.

If your variation is easterly, subtract it from your true course. If it’s westerly, add it. East is least—take it away. West is best—add it on.

But don’t just memorize the rhyme and move on. Understand what you’re doing: you’re translating a number measured against the real geographic pole into a number that means something to your compass. That’s all variation is—a translation between two norths.

Worked example: Say you draw your line and measure a true course of 090 (due east). You’re in an area with 10 degrees east variation. East is least, so subtract: 090 − 10 = 080 magnetic course.

What is compass deviation and why does it matter?

Deviation is the error in your compass caused by the airplane itself—and it’s the correction everybody forgets.

Your compass sits in a metal box surrounded by radios, speakers, wiring, and electrical current. All of it creates small magnetic fields that tug the needle a degree or two off. Variation is the earth’s error; deviation is the airplane’s error.

To find it, look just below the compass in any certified airplane for a small compass correction card. It reads something like: for 100, steer 102; for 200, steer 198. Someone “swung” that compass—physically aligned the airplane to known headings and recorded the error—so the card is specific to that exact aircraft.

Apply the deviation from the card to your magnetic course (usually just a degree or two), and you finally arrive at your compass heading—the number you’d chase if you were flying raw compass with no heading indicator. Actually glance at that card before you fly. Most pilots never do, and it takes five seconds.

How does wind fit into TVMDC?

Here’s the part that confuses people: wind is not in the TVMDC acronym at all.

TVMDC is the instrument chain—it converts a course into what the compass should read, ignoring wind entirely. Variation is the earth, deviation is the airplane. Wind is the air, and it gets layered on separately when you build your navigation log.

Your magnetic course is the path over the ground you want to make, but the air is moving. With a crosswind, you can’t just point the nose along the line—the wind blows you sideways. You crab: angle the nose into the wind just enough that your actual track stays on course. That angle is your wind correction angle (WCA).

The practical rule: wind from your right, turn the nose right and add degrees; wind from your left, turn left and subtract. The stronger the wind and the more it hits you from the side, the bigger the correction.

On your actual nav log, the order is: start with true course, apply wind correction to get true heading, apply variation to get magnetic heading, then apply deviation to get compass heading. The pieces—variation, wind, deviation—are identical to TVMDC; they’re just stacked in the order that makes the nav log flow. What matters to an examiner is that you know variation is the earth, deviation is the airplane, and wind is the air—three different problems, three different fixes.

What’s the most common navigation mistake on a first cross-country?

Applying a correction in the wrong direction.

Picture your first solo cross-country. You built a beautiful nav log the night before—every leg has a true course, magnetic heading, compass heading, groundspeed, time, and fuel. You take off, roll out on your first heading, and ten minutes in, the town that should be off your left wing is off your right wing instead.

Almost always, you added when you should have subtracted—you flipped “east is least, west is best” during the late-night workload—or you crabbed the wrong way into the wind. The corrections are small, just a few degrees. But over 90 miles, a few degrees becomes a few miles, and a few miles is the difference between flying right over your checkpoint and never seeing it.

The fix isn’t panic. Note which way you’re off, make a correction, and press on. Being a little off and catching it isn’t failure—that is cross-country navigation.

How do I verify my heading is right while flying?

Trust the number, then verify it against the world outside the window.

This is why we teach dead reckoning and pilotage together. Dead reckoning is the math—heading, time, and speed. Pilotage is looking outside and matching the ground to the chart.

Fly the heading your nav log gave you, but also pick checkpoints along the route: a bend in a river, a town with a water tower, a highway crossing a railroad. When your watch says you should be there, look. Is the checkpoint where it should be relative to your nose?

If yes, your chain of corrections worked—fly on with confidence. If no, you have information. Adjust and continue. The cycle is simple: plan, fly, check, adjust.

Worth knowing: the Airman Certification Standards (ACS) expect this. Under pilotage and dead reckoning, you’re required to navigate by pre-computed headings, ground speed, and elapsed time, and to correct for and record the differences between your plan and reality. The ACS literally assumes there will be differences—the skill being tested is whether you notice and respond.

Two habits that prevent direction errors

First, do the variation correction out loud when you build your nav log. Say it: “True course 090, variation 10 east, east is least, subtract, magnetic 080.” Talking through it catches direction errors before they lock into your plan.

Second, check your compass correction card before every flight. Open the door, glance at it, and know whether your airplane’s deviation is essentially nothing or something you need to account for.

Key Takeaways

• There are two norths: true north (geographic pole) and magnetic north (where your compass points). The angle between them, for your location, is variation.
• TVMDC = True → Variation → Magnetic → Deviation → Compass. It’s the instrument chain and deliberately excludes wind.
• East is least, west is best: subtract easterly variation, add westerly variation. Apply the same logic care to wind correction—right wind, turn right and add.
• Variation is the earth’s error; deviation is the airplane’s error (read off the compass correction card); wind is the air’s error (applied on your nav log).
• A few degrees of error becomes a few miles over a long leg. Always verify your heading against ground checkpoints—plan, fly, check, adjust.