Time and Distance
Formulas for calculating travel time, groundspeed, and distance.
Calculating travel times is fundamental in IFR. The pilot must constantly estimate when they will reach the next waypoint.
Basic Formula
Time (min) = Distance (NM) × 60 / Groundspeed (kt)Example: Covering 20 NM at 150 kt groundspeed:
Time = 20 × 60 / 150 = 8 minutesMental Math Tips
Base Factor Method
First calculate the base factor (time for 1 NM):
Base factor = 60 / Gs (kt)| Gs (kt) | Base factor | Time for 10 NM |
|---|---|---|
| 120 | 0.50 min/NM | 5 min |
| 150 | 0.40 min/NM | 4 min |
| 180 | 0.33 min/NM | 3 min 20 s |
| 210 | 0.286 min/NM | 2 min 52 s |
| 240 | 0.25 min/NM | 2 min 30 s |
Tip: Memorize the factors for common speeds. For 150 kt, it's simple: 1 NM every 24 seconds, or 10 NM in 4 minutes.
Fraction Method
To simplify the calculation:
- Gs = 120 kt → Distance / 2 = time in minutes
- Gs = 180 kt → Distance / 3 = time in minutes
- Gs = 240 kt → Distance / 4 = time in minutes
Example: 15 NM at 180 kt groundspeed → 15 / 3 = 5 minutes
Groundspeed Calculation
Using DME
If you have a DME, time the distance change:
Gs (kt) = Δdistance (NM) × 60 / Δtime (min)Example: The DME goes from 25 NM to 22 NM in 1 minute:
Gs = 3 × 60 / 1 = 180 ktBy Timing Between Two Points
If you know the distance between two waypoints:
Gs (kt) = Distance (NM) × 60 / Travel time (min)True Airspeed Calculation
True airspeed (TAS) is derived from indicated airspeed (IAS) by accounting for altitude and temperature:
TAS ≈ IAS + (IAS × altitude_ft / 60000) + temperature_correctionSimplified rule: Add approximately 2% per 1,000 ft of altitude to the IAS.
| Altitude | Approximate Correction |
|---|---|
| 5,000 ft | +10% |
| 10,000 ft | +20% |
| 20,000 ft | +40% |
| 30,000 ft | +60% |
In HOLD: The TAS is displayed on the true airspeed indicator. The groundspeed is automatically calculated by the simulation engine based on the wind.