TVMDC Compass Heading Chain Calculator
True → Variation → Magnetic → Deviation → Compass, every arrow shown — the full chain from chart course to the number you actually steer.
“True Virgins Make Dull Company — Add Whiskey” encodes the order and the rule: traveling T→C, add westerly errors. The deviation value comes from the compass card for the heading nearest your magnetic heading.
Formula
⚠️ For flight planning and education only — verify with official sources and certified equipment. Not for primary navigation.
True → Variation → Magnetic → Deviation → Compass, every arrow shown — the full chain from chart course to the number you actually steer.
About TVMDC Compass Heading Chain Calculator
Between the course you drew and the number you steer stand two corrections in strict order: the planet's (variation) and your airplane's (deviation). The TVMDC chain — True, Variation, Magnetic, Deviation, Compass — is how navigators have organized them for a century, and this calculator walks it explicitly, printing every intermediate value, with the mnemonic's add-whiskey rule (add westerly) applied mechanically in the T-to-C direction.
How to use TVMDC Compass Heading Chain Calculator
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula T ± V = M ± D = C (add westerly, subtract easterly errors going T→C) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use TVMDC Compass Heading Chain Calculator?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula T ± V = M ± D = C (add westerly, subtract easterly errors going T→C) with sources cited on the page
- ✓“True Virgins Make Dull Company — Add Whiskey” encodes the order and the rule: traveling T→C, add westerly errors. The deviation value comes from the compass card for the heading nearest your magnetic heading.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
What is deviation physically?+
Your aircraft's own magnetism bending the compass: engine block, avionics currents, speaker magnets, structural steel. It differs per heading (the interference rotates with the airframe), which is why the correction card lists values every 30°. A compass 'swing' on an airport compass rose measures and minimizes it; the card records what remains.
Which card row do I use for the deviation value?+
The one nearest your magnetic heading — deviation is tabulated against what the compass should read. Interpolate between rows for in-between headings (it varies smoothly). Cards reading 'FOR 120 STEER 122' have pre-applied the arithmetic: that's the compass heading directly, no further math.
Why add westerly and subtract easterly going T to C?+
A westerly error means north-as-measured sits west of the reference north, so any direction measured against it reads numerically higher — to command the same physical direction you steer a higher number. Easterly, the reverse. Both variation and deviation obey the same logic, which is why one rule ('add whiskey') runs the whole chain.
Does the chain still matter in a glass cockpit?+
The AHRS magnetometer applies these corrections continuously — until it doesn't: the wet compass remains the regulatory backup (91.205), and partial-panel work, ferry flights in older airframes and every knowledge test still run TVMDC by hand. Five minutes with this trainer before a checkride is cheap insurance against the examiner's favorite gotcha (the sign flip).
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