Palmerston North to Napier: track error, closing angle and fuel after a fix
You are flying a C172 from Palmerston North (NZPM) to Napier (NZNR), 68 NM, planned track 040°M. TAS is 110 kt and the forecast wind is 300/15, giving a planned groundspeed of 112 kt and an ETA of 36 minutes. You depart with 75 L usable fuel and burn 32 L/h; reserve is 45 minutes. Twenty minutes after departure you obtain a visual fix: you are 5 NM right of track and 35 NM from Palmerston North. The remaining distance to Napier is 33 NM. The wind at your level feels stronger than forecast, and the GNSS is showing a groundspeed of 105 kt. You have no autopilot and are map-reading. There is rising terrain ahead but cloud base is 4,000 ft and you are at 5,500 ft. You must decide whether to adjust heading, recompute ETA and fuel, and whether a diversion is warranted.
1. Calculate the track error and closing angle from the fix, and give the revised magnetic heading to Napier.
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Track error is the angle between the planned track and the track actually made good. You have flown 35 NM and are 5 NM right of track:
Track error = (5/35) × 60 = 8.6°. You are right of track, so the correction is to the left.
Closing angle is the additional correction needed to converge on the destination from the fix. Remaining distance is 33 NM:
Closing angle = (5/33) × 60 = 9.1° left.
Total heading change = 8.6 + 9.1 = 17.7°, rounded to 18° left.
Planned track is 040°M, so the revised heading is 040 − 18 = 022°M. This assumes the wind stays similar; after changing heading, take another fix in about 10 minutes to confirm the new track.
2. Using the fix, calculate actual groundspeed, revised ETA, and fuel remaining at Napier including reserve. Is fuel adequate?
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You covered 35 NM in 20 minutes:
Groundspeed = 35 × (60/20) = 105 kt. This agrees with the GNSS groundspeed.
Remaining 33 NM at 105 kt = 33/105 × 60 = 18.9 min.
Fuel used so far = 32 × 20/60 = 10.7 L. Remaining fuel = 75 − 10.7 = 64.3 L.
Fuel to Napier = 32 × 18.9/60 = 10.1 L. Reserve = 45 min at 32 L/h = 24 L. Total required from the fix = 10.1 + 24 = 34.1 L.
Fuel margin at Napier = 64.3 − 34.1 = 30.2 L. Fuel is adequate. Revised ETA is about 20 + 19 = 39 minutes after departure.
3. The stronger-than-forecast wind suggests your drift may increase. What in-flight checks would you make, and at what point would you divert to Dannevirke?
Read the worked answer
Take another fix within 10 minutes to see whether the track error is increasing. Compare the new drift with the 8.6° already experienced; if it is growing, the wind is stronger than forecast and the revised heading will need further adjustment. Monitor groundspeed and fuel flow, not just the GNSS.
Diversion criteria should be set before the flight: divert if the revised ETA means the fixed reserve would be breached, if drift exceeds about 20° and you cannot hold the corrected track, if cloud or terrain prevents maintaining VFR at 5,500 ft, or if fuel remaining at the next fix falls below the fuel needed to reach Napier plus reserve plus a personal margin. Dannevirke is 20 NM south of track and has fuel; it is a sensible alternate if Napier becomes marginal. Do not wait for the reserve to be infringed before deciding.