In-flight Navigation Technique

A prepared chart lets the pilot navigate with quick comparisons instead of continuous measuring. Mental dead reckoning keeps the expected position moving between fixes.

Time/Distance Markers

Mark the track in either elapsed time or distance to go. Time marks may show elapsed time from departure or time remaining to the destination. Distance marks may show distance flown or distance to go. Distance-to-go marks are especially useful for closing-angle calculations.

At 120 kt, the 6-minute distance is 12 NM and the 1-minute distance is 2 NM. At 150 kt, the values are 15 NM and 2.5 NM.

Track Error Lines

Draw guide lines at 5 or 10 degrees on either side of track through the departure point and destination. They allow rapid assessment of track error and closing angle. Avoid so many lines that the route becomes unreadable.

Folding Charts

Fold each chart to provide complete track coverage with as few page turns as possible. Number the sheets in use order and keep the next chart accessible. Avoid refolding at a high-workload point.

Mental dead reckoning

A DR position is the estimated position obtained by advancing the last known position using track, groundspeed and elapsed time. Between reliable fixes, the pilot should always know the approximate DR position, the expected time at the next feature and the direction in which the next feature should appear.

If the last positive fix was at 1015 and groundspeed is 150 kt, then at 1027 the aircraft has advanced 150 x 12 / 60 = 30 NM along the estimated track.

In-flight navigation is a repeated cycle of anticipation, observation, correction and recording. The cycle should support lookout and aircraft control, not compete with them.

Anticipate, check, correct, record

1. Anticipate. A few minutes before the checkpoint, calculate the DR position and decide where the feature should appear.
2. Check. Compare the chart with the view, identify the feature by shape, orientation and surrounding features, then note the time.
3. Revise heading. If the confirmed fix is off track, decide whether to regain track or fly directly to the next point.
4. Revise ETA. Use the measured groundspeed and remaining distance to update the next estimate.
5. Record. Enter actual time, new heading, GS, ETA and any fuel consequence in the log.

Navigation Techniques and workload priorities

Keep flying the aircraft, maintain lookout, monitor weather, fuel and systems, and communicate when required. Navigation must be organised into short tasks. If a calculation cannot be completed safely, stabilise the aircraft and use the simplest reliable method.

A useful visual fix depends on feature selection, preparation and cross-checking. The pilot should identify a pattern, not seize on the first object that looks plausible.

Map Reading

Map-to-ground reading starts with the chart and predicts what should be seen. Use it when position and timing are reasonably certain. Ground-to-map reading starts with a feature seen outside and searches the chart for a matching pattern. Use it when position is uncertain or the expected feature is missing.

Visual Checkpoints and Selecting Visual Checkpoints

Choose checkpoints about 5 to 10 minutes apart. A strong checkpoint is visible early, distinctive, reasonably precise and supported by nearby features. A good feature close to track is better than a poor feature exactly on track.

Prepare and Align a map/chart for use in Visual Navigation

1. A few minutes before the point, establish the approximate position and calculate a DR position.
2. Determine where the checkpoint lies relative to the DR position and cockpit view.
3. Relate the chart pattern to the expected view. If the feature is not visible at the expected place, widen the scan.
4. When sighted, estimate range and bearing, note the time, and combine the observation with surrounding features.
5. After confirming the fix, correct track and revise timing.

Angle of Observation

At low level, masts and other vertical features are prominent, while distant features may merge into terrain. At higher level, plan shape becomes easier to recognise but a mast must be taller to remain conspicuous. Near objects are often identified by shape in plan; distant objects are more readily recognised by elevation and skyline.

A positive fix

A fix is the best estimate of the exact aircraft position at a stated time. It may come from visual, radio or surveillance observations. Independent position lines should intersect at a strong angle, preferably near 60 to 90 degrees. A twin-DME fix is often strong because DME distance is precise and the arcs can intersect at a favourable angle.

A groundspeed check needs two known positions, a measured distance and an accurate elapsed time. The result is then used only where the wind and flight condition are reasonably representative.

Groundspeed over a known distance

If two checkpoints are 20 NM apart and elapsed time is 8 minutes, GS = 20 x 60 / 8 = 150 kt.

Revising the ETA

At 1020 a confirmed fix leaves 45 NM to destination. If measured groundspeed is 150 kt, time to go = 45 x 60 / 150 = 18 minutes. Revised ETA is 1038.

Worked checkpoint revisions

When not to extrapolate

Do not apply one groundspeed blindly across a climb, descent, large wind change or major turn. Revise only the legs for which the observation is representative. If the next leg has a different track, use the forecast or a fresh check because the wind component changes.

A fix answers two questions: where is the aircraft now, and what does that imply for the remaining route? The correction should update both track and time.

Navigation in Cruising Flight, Use of Fixes to Revise Navigation Data

Airway centreline tracks may be magnetic while forecast winds are true. Convert all directions to one reference before comparing them. When a fix is obtained, draw or imagine the line from the previous fix to the new fix, calculate TMG and GS, then revise the next heading and ETA.

The 1 in 60 decision

1. Confirm the fix and measure distance off track.
2. Use distance gone to calculate track error.
3. If the fix is halfway to a suitable intercept point, the double-track-error method is quick.
4. If more than halfway to destination, or if direct routing is preferable, add a closing angle and fly directly to the next point.
5. If returning to the old track, remove the closing angle at the intercept.

Navigation in climb and descent

Climb and descent change TAS and wind exposure. Use a mean TAS and a representative mean wind for the vertical segment, or break the segment into stages when conditions differ greatly. Do not use a cruise groundspeed for the whole climb or descent without checking.

Uncertainty is a condition to manage immediately. Continuing and hoping usually enlarges the error. Stabilise, build a DR estimate, use every available aid and avoid new hazards.

DR Positions and Actual Positions

When visibility is good and features are unique, map-to-ground navigation is efficient. In poor visibility or when the expected feature is absent, change to ground-to-map. Observe a prominent pattern outside, find it on the chart and test the identification against timing, direction and neighbouring features.

The standard lost actions

1. Check heading, compass, airspeed and time. Confirm that the planned values are actually being flown.
2. If weather, terrain and airspace permit, climb to improve visual and radio horizon.
3. Use every available aid, including VOR/DME, VDF, surveillance assistance or a visual fix.
4. Calculate a DR position from the last accurate fix using track, GS and elapsed time.
5. Draw or imagine a circle of uncertainty around the DR position.
6. Use ground-to-map reading inside the circle and look for large, unique patterns.
7. If still uncertain, fly safely towards a suitable line feature outside the circle, then follow it to a definite checkpoint.
8. Maintain lookout, monitor fuel and systems, remain clear of terrain and controlled airspace, and request assistance when needed.

The circle of uncertainty

If 70 NM has been flown since the last positive fix, use a radius of about 7 NM. The centre continues to move with the DR position and the circle grows as distance from the last fix increases.

Line features and assistance

Useful line features include a coastline, motorway, major railway or unmistakable river. Select one that can be reached without entering controlled airspace or approaching high terrain. On reaching it, turn and follow it to a definite checkpoint. If uncertainty remains, make an urgency call and use 121.5 MHz as appropriate.

A complete correction links position, track, groundspeed and time. Treat each confirmed fix as a new start rather than carrying an old assumption through the rest of the flight.

Example: checkpoint, track and ETA

A leg is 90 NM. Planned GS is 120 kt, so planned time is 45 minutes. A positive fix after 30 minutes is 6 NM right of track and 54 NM along the planned line. The remaining distance along track is 36 NM.

1. Measured GS = 54 x 60 / 30 = 108 kt.
2. Track error = 60 x 6 / 54 = 6.7 degrees right.
3. Closing angle to destination = 60 x 6 / 36 = 10 degrees.
4. Turn about 17 degrees left to fly directly to destination.
5. Time to go at 108 kt = 36 x 60 / 108 = 20 minutes, slightly more for the diagonal correction.
6. If the fix time is 1030, the first revised ETA is about 1050, then refine it with the next fix.

Standard checks at every fix

Compact formula table