One-in-Sixty Rule
Rule-of-thumb linking a lateral track error to an angular deviation: 1 nm off after 60 nm is about 1° of error.
Formula
Variables
Example
2 nm off after 30 nm: θ = 60×2÷30 = 4°
How it works
The One-in-Sixty Rule gives a quick estimate of how far a aircraft has turned away from its planned track by turning the sideways displacement into an angle. It comes from the small-angle approximation in navigation: for small angles, 3 theta in radians is roughly cross-track error divided by distance flown, and converting radians to degrees gives the factor of about 60. That is why 1 nautical mile off track after about 60 nautical miles corresponds to about 1 0B0.
When to use it
Use this in basic dead reckoning or en-route navigation when you want a fast mental estimate of heading or track error without doing trigonometry. It is most useful for small deviations over moderate distances, such as checking whether a drift or heading error is becoming significant.
Worked example, step by step
An aircraft is 3 nm off its planned track after flying 90 nm. Step 1: Write the formula: b8 = (60 d7 d_off) / d_flown Step 2: Substitute the values: b8 = (60 d7 3) / 90 Step 3: Calculate the numerator: 60 d7 3 = 180 Step 4: Divide: 180 / 90 = 2 Final answer: the track error is about 2 0B0.
Common mistakes to avoid
- Using statute miles instead of nautical miles
- Applying the rule to large angles or very short distances where the approximation is poor
- Confusing distance off track with total distance flown
Did You Know?
Course-deviation indicators for VOR and GPS are built around this rule — each dot of needle deflection represents a fixed angular error from the desired track.
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