Off-course correction calculator
You reach a landmark and find yourself a few miles to one side of where you meant to be. This works out how much to turn, both to stop the drift getting worse and to rejoin the track at the destination.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The track angle error is how far your actual path has diverged from the one you planned. Correcting by that amount alone makes you parallel the intended track without ever rejoining it.
The closing angle is the extra turn needed to converge on the destination. Adding the two gives the total correction, which is the figure to apply.
Once you are back on track, take the closing angle off again and keep the track angle error applied, otherwise you will fly straight through and off the other side.
When this is useful
- Visual navigation, at the moment you identify a landmark and realise you are not over it.
- Any leg flown on a forecast wind that turns out to be wrong.
- Training, because the underlying 1 in 60 rule is worth being able to do in your head.
How it is calculated
Both angles come from basic trigonometry. The track angle error is the angle whose sine is the distance off track divided by the distance flown. The closing angle is the angle whose sine is the distance off track divided by the distance remaining.
The 1 in 60 rule is the mental arithmetic version: 1 degree of error moves you 1 NM off track over 60 NM. Multiply the miles off track by 60 and divide by the miles flown. The calculator uses the exact trigonometry and tells you when the approximation agrees.
total correction = asin(off track / flown) + asin(off track / remaining)
Worked example
- Distance flown
- 60 NM
- Distance off track
- 4 NM to the right
- Distance remaining
- 90 NM
Turn left by about 6 degrees to rejoin the track at the destination.
Check: The 1 in 60 rule gives 60 multiplied by 4 divided by 60, which is 4 degrees, plus 60 multiplied by 4 divided by 90, which is 2.7 degrees. That totals 6.7 degrees against the exact 6.4, close enough for mental arithmetic and confirming the exact result.
Limitations
- It assumes the wind that put you off track stays the same. If the error came from a one off, such as a deviation around cloud, a smaller correction is enough.
- The geometry assumes you have flown a reasonably straight line since the last known fix.
- It gives a correction, not a new heading. Apply it to the heading you were already flying.
Common mistakes
- Turning the wrong way. If you are right of track you turn left, and the calculator states the direction to avoid exactly this.
- Applying only the track angle error, which stops the drift growing but never brings you back.
- Leaving the full correction on after regaining track, which sends you off the other side.
Sources
- Pilot's Handbook of Aeronautical Knowledge, navigation chapter , US Federal Aviation Administration