How can elapsed time be converted into distance travelled and distance remaining?

Distance from speed and time

The distance mode of Crabwise's time–speed–distance screen computes D = GS × t ÷ 60, with ground speed in knots and time in minutes, and returns nautical miles. Subtracting that from the leg length gives the distance still to go, provided both numbers describe the same leg from the same starting point.

When ground speed changes during the leg, one multiplication with the latest speed over the whole elapsed time misstates progress. Splitting the elapsed time into intervals, each with its own speed, keeps the estimate tied to what was actually flown.

Worked example

At a constant 120 kt, 20 elapsed minutes corresponds to 40 NM. On an unchanged 100 NM educational leg that leaves 60 NM, subject to the assumptions behind that progress estimate.

Distance in the stated interval (NM)

40

Track progress interval by interval

1. Write down the leg length and where it starts.
2. For each interval with a steady ground speed, run the distance mode with that speed and its minutes.
3. Add the interval distances to get total progress.
4. Subtract the total from the leg length and keep both numbers in the record.

Two intervals on a 100 NM leg

The first 15 minutes at 120 kt give 120 × 15 ÷ 60 = 30 NM. The next 10 minutes at 105 kt give 105 × 10 ÷ 60 = 17.5 NM. Progress is 47.5 NM, and 52.5 NM of the 100 NM leg remain.

First interval (NM)

30

Second interval (NM)

17.5

Remaining on the leg (NM)

52.5

Applying the later 105 kt to all 25 minutes would give 43.75 NM and overstate the remaining distance by 3.75 NM.

At the current 105 kt, the time mode turns the remaining 52.5 NM into 52.5 ÷ 105 × 60 = 30 minutes. That figure assumes the 105 kt holds for the rest of the leg.

Common mistakes

• Counting distance flown in a turn or a diversion as progress along the planned leg.
• Entering true airspeed in the speed field instead of ground speed.
• Measuring elapsed time from engine start rather than from the leg's starting point.
• Subtracting progress in nautical miles from a leg length measured in statute miles.

Limits of a dead-reckoning estimate

The subtraction is an arithmetic estimate. It does not locate the aircraft or update for wind. The FAA handbook's navigation chapter below covers the same dead-reckoning relation; position comes from navigation sources.

Primary sources and related work

• FAA Pilot’s Handbook: navigation and basic flight calculations
• NIST SI conversion factors