Time, speed & distance / ETA
Formulas:
| Calc | Formula | Terms |
|---|---|---|
| Checked GS | 60 · d1 / t1 |
d1 = nm flown, t1 = min |
| Remaining min | d2 / GS × 60 |
d2 = nm still to fly |
| Fresh ETA | time now + remaining min |
Ground speed check:
- When: first positive pinpoint, 10-25 nm from departure. Hold cruise altitude, heading, airspeed for whole check.
- Redo check, revise ETA when track, wind or airspeed change (TP 1102).
- Use checked GS, not planned. Apply to distance ahead.
- Distance in nm. Knots = nm per hour.
- Climb: add 2 min per 1,000 ft (TC sample exam Q88 note).
Mental shortcuts (TP 1102):
| GS | Rate | Time |
|---|---|---|
| 60 KT | 1 nm per min | = distance in min |
| 90 KT | 1.5 nm per min | = ⅔ distance |
| 120 KT | 2 nm per min | = ½ distance |
- Round to nearest speed, adjust a few minutes.
Equal segments (TP 1102):
- Split leg into equal parts.
- Remaining time = first-part time × parts remaining.
- Example: 4 parts, first flown in 6 min:
6 × 3 = 18min.
Trap: using planned GS instead of checked, applying it to leg flown instead of distance ahead, or feeding statute miles into a knots GS.
References
- TP 1102, Pilot Navigation (Ground Speed Check)