Turn distance into an arrival time
Use groundspeed to time a 40 NM leg, then work backwards to find the true airspeed for a 30-minute arrival.
What you’ll learn
- Choose the right speed for timing a leg.
- Convert minutes to hours and calculate elapsed time.
- Find the TAS needed against a headwind and calculate an ETA in UTC.
The ground decides how long it takes
A leg drawn on a chart is a distance over the ground. To time it, use groundspeed: the rate at which the aircraft covers that ground distance. True airspeed (TAS) describes motion through the air, so it cannot replace groundspeed when wind is present.
In this exercise, the course is 090° true and the wind comes from 090° true at 20 kt. It is a direct headwind. There is no sideways wind component to correct: subtract 20 kt from TAS to obtain groundspeed. All directions here use true north.
Time a ground distance with groundspeed, not TAS.
Make the units agree
A knot means one nautical mile per hour. If distance is in NM and speed is in kt, dividing distance by speed gives hours. Thirty minutes is 0.5 hours, not 0.30 hours.
You can also calculate directly in minutes: multiply the distance by 60, then divide by groundspeed. A useful sense check is that 60 kt covers 1 NM each minute; 120 kt covers 2 NM each minute.
Keep hours and minutes distinct throughout the calculation.
Start with the required arrival time
Sometimes the time is fixed and the speed is the unknown. To cover 40 NM in 30 minutes, first find the required groundspeed: 40 divided by 0.5 is 80 kt. Then allow for the 20 kt headwind. TAS must be 100 kt.
The lab starts at TAS 80 kt. That produces only 60 kt over the ground, so the same leg takes 40 minutes. Increasing TAS changes both elapsed time and ETA.
Find the required groundspeed first, then account for wind.
Put the elapsed time on the clock
ETA is an estimated time of arrival, not a duration. With departure at 10:15 UTC and a 30-minute leg, arrival is 10:45 UTC. Use the same time reference at both ends; no local-time conversion is needed in this example.
If actual groundspeed differs from the planned value, recalculate the remaining time using the remaining distance. This steady-condition exercise models one leg, not a full flight plan.
Add elapsed time to departure time to obtain ETA.
Choose the speed for a 30-minute leg
Adjust TAS and watch groundspeed, leg time and ETA change. The 40 NM leg, course 090° true, wind from 090° true at 20 kt and departure at 10:15 UTC stay fixed. Then check your setup.
40 NM · Desired track 090° · Wind 090° / 20 kt · Departure · UTC 10:15
Type a value or use the slider. Arrow keys make small changes.
Find the TAS that completes the leg in 30 minutes.
Show a hint
Work backwards from the required groundspeed. A headwind means TAS must be higher than groundspeed.
Explore the controls, then check your setup against the task.
40 NM against a 20 kt headwind
- Convert the target duration: 30 min / 60 = 0.5 h.
- Required groundspeed: 40 NM / 0.5 h = 80 kt.
- Add the direct headwind: 80 + 20 = 100 kt TAS.
- Check the clock: 10:15 UTC + 30 min = 10:45 UTC.
TAS 100 kt gives GS 80 kt, a 30-minute leg and an ETA of 10:45 UTC.
Check your understanding
- Use groundspeed to relate chart distance to elapsed time.
- With a direct headwind, GS is less than TAS; work backwards when the target time is fixed.
- ETA is a clock time. Keep UTC consistent and update estimates when conditions change.
Complete lesson
Solve the interactive exercise and all four checks to complete this lesson.
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Progress saved on this browserSources & syllabus
AMC1 FCL.210; FCL.215 — Navigation 9.1: dead-reckoning navigation and in-flight navigation.
- EASA — Easy Access Rules for Aircrew, October 2026 ↗AMC1 FCL.210; FCL.215 — PPL(A) Navigation syllabus, General Navigation.
- FAA — Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25C ↗Chapter 16, Navigation: dead reckoning, wind effects and time–speed–distance calculations. Used for universal navigation principles.
These references support the syllabus scope and the navigation principles taught here. The exercises and explanations are original Trimbrief material.
Simplified, steady-wind learning model; no live weather or operational navigation data.
Content edition: 2 November 2026