Dead Reckoning Navigation
Estimating position using known heading, airspeed, time, and wind — the fundamental navigation skill underpinning all flight planning, taught at PPL level and required throughout aviation.
What is Dead Reckoning?
Dead reckoning (DR) is the process of estimating a current position based on a known starting point, then projecting forward using a known heading, true airspeed, and elapsed time — corrected for wind effect.
- Requires no radio aids or GPS — relies only on the aircraft's instruments and pre-flight planning data
- Accuracy degrades progressively — small heading or speed errors compound over time
- Position fixes (visual, radio, or GPS) must be used to update and reset the DR position
- Mandatory knowledge for PPL and CPL; the E6B flight computer is the traditional tool
- Modern avionics perform DR automatically but understanding the principles remains essential
Origin of the term: "Dead reckoning" likely derives from "deduced reckoning" (abbreviated "ded. reckoning"), though maritime historians debate this. The principle is the same: deduce where you must be based on where you started and how you have moved.
Key Terms and Definitions
| Term | Abbreviation | Definition |
|---|---|---|
| True Course | TC | Direction from departure to destination measured on a chart relative to True North |
| True Heading | TH | Direction the aircraft nose points, relative to True North; TC ± Wind Correction Angle |
| Magnetic Heading | MH | True Heading corrected for magnetic variation; what the compass/DI reads |
| True Airspeed | TAS | Speed of the aircraft through the airmass (IAS corrected for altitude and temperature) |
| Groundspeed | GS | Actual speed over the ground — TAS modified by the wind component along the track |
| Wind Correction Angle | WCA | Angle between the True Course and the True Heading required to maintain that course |
| Track | TR | Actual path of the aircraft over the ground; should equal TC if WCA is correct |
| Track Angle Error | TAE | Angular difference between planned track and actual track observed at a position fix |
| Estimated Time of Arrival | ETA | Calculated arrival time = departure time + (distance ÷ groundspeed) |
The Wind Triangle
The wind triangle is a vector diagram representing three forces that determine an aircraft's movement over the ground. Understanding it is the conceptual foundation of all DR navigation.
Vector 1 (blue): aircraft heading + TAS through the airmass. Vector 2 (amber dashed): wind moves the airmass over the ground. Vector 3 (green): resultant track and groundspeed over the ground.
Mental Model
Imagine walking across a moving walkway at an angle. You point your body (heading) in one direction, the walkway (wind) pushes you sideways, and your actual path across the floor (track) is the combined result. You must angle into the walkway to walk in a straight line — that angle is your wind correction.
- Headwind component: reduces groundspeed
- Tailwind component: increases groundspeed
- Crosswind component: causes drift — corrected by applying WCA into wind
- A pure crosswind requires maximum WCA with minimal effect on groundspeed
- A pure headwind requires no WCA but maximum groundspeed reduction
Calculating Wind Correction Angle
The WCA can be calculated precisely using vector trigonometry or approximately using the formula below. The E6B flight computer (or equivalent digital tool) solves the triangle graphically.
Wind Angle = angle between wind direction and desired track
Result in degrees; positive = angle from track into wind
Approximate formula limitations: The sine formula gives good results when WCA is small (<20°). For large WCA values or precise flight planning, use the E6B flight computer, CRP-5, or navigation software which solves the vector triangle exactly. Always verify against reasonable expectations.
Worked Examples
Example 1 — Pure Crosswind
90° (wind from north, track east = pure crosswind)20 kt11.5°090 − 11.5 ≈ 078°T≈ 98 ktExample 2 — Mixed Head and Crosswind
60° (wind from SW, track S)26 kt (from right)15 kt12.5° → ~13°R (wind from right → steer right)193°T~105 ktTrack Angle Error (TAE) and Correction
When a position fix reveals that the aircraft is off its planned track, the Track Angle Error (TAE) is the angle between the planned track and the actual track from departure.
Regain Track and Parallel
Steer a correction heading to intercept the planned track, then turn back to the planned heading.
Correction: steer (TAE × 2) toward track
→ reaches track at midpoint of remaining distance
Aim Directly at Destination
Calculate the closing angle from current position directly to the destination and steer that heading.
Closing angle = TAE + additional angle
to destination from current position
Double TAE Rule (Practical Shortcut)
The double TAE rule is a quick mental calculation for regaining track in half the remaining distance:
- Determine TAE from current position fix (angle between planned and actual track from departure)
- Estimate the angle from current position to destination (closing angle)
- Total correction heading change = TAE + closing angle
- Steer this heading until back on track, then resume planned heading
Simplified rule: If halfway to destination and off track by 5°, steer 10° toward track (double the TAE). This will put you back on track at the destination if the track error was consistent. In practice, recalculate WCA and apply a revised heading once back on track.
ETA Calculation and Revision
Estimated Time of Arrival is the fundamental output of DR navigation planning:
Time (minutes) = (Distance ÷ GS) × 60
ETA Calculation Example
150 nm | Planned GS: 120 kt1.25 hours = 1 hr 15 min10:15 UTCEn-Route ETA Revision
50 nm covered (planned: 60 nm)100 kt (planned was 120 kt)100 nm1.0 hr = 60 min10:30 UTC (15 min later than planned)ETA Revision Triggers
- Any position fix that reveals actual groundspeed differs from planned
- ATC-assigned altitude change (affects TAS and thus GS)
- Updated wind forecast significantly different from planned
- Diversion to alternate — recalculate from current position
- Always update destination fuel estimate alongside ETA revision