Time, speed & distance / ETA

Your real groundspeed is rarely what you planned. For an honest ETA, fly to a checkpoint, work out the speed you actually made, then re-time the rest of the leg with it.

• Checked groundspeed from the leg you just flew: GS = 60 · d1 / t1 (d1 in nm, t1 in minutes); the 60 turns minutes into hours so the answer is in knots.
• Revised time for the distance still ahead: remaining min = d2 · t1 / d1, i.e. d2 / GS × 60. Add that to the time right now for a fresh ETA.
• Keep distance in nm. Groundspeed in knots is nm per hour, so a distance quoted in statute miles or km must be converted first (1 nm = 1.152 SM = 1.852 km). Using SM or km straight in over-times the leg.

Trap: using the planned groundspeed instead of the checked one, applying it to the leg already flown instead of the distance ahead (d2), or feeding a statute-mile/km distance into a knots groundspeed.

Practice