Christchurch to Timaru: map fix, revised heading and lowering cloud
You are flying a C152 from Christchurch (NZCH) to Timaru (NZTU), 75 NM, planned track 210°M at 3,500 ft. TAS is 95 kt, forecast wind 300/25, planned groundspeed about 92 kt. You have 55 L usable fuel, burn 28 L/h, and require a 30-minute reserve. Twenty-five minutes after departure you identify the Rangitata River mouth: it lies 4 NM left of your intended track and you estimate you are 36 NM along the route. Your DR position had put you 6 NM inland, so the fix is significant. Ahead, the cloud base lowers to 1,500 ft AGL over the Canterbury Plains and there is haze. Timaru is still 39 NM away. You have no GNSS, only a VNC and Topo50. You must revise heading, recalculate ETA and fuel, and decide whether the low cloud changes your plan.
1. From the Rangitata mouth fix, calculate the track error and the revised magnetic heading to Timaru.
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You are 4 NM left of track after 36 NM flown:
Track error = (4/36) × 60 = 6.7°. Because you are left of track, you need a right correction.
Remaining distance to Timaru is 39 NM, and the closing angle is:
Closing angle = (4/39) × 60 = 6.2° right.
Total correction = 6.7 + 6.2 = 12.9°, rounded to 13° right.
Planned track is 210°M, so the revised heading is 210 + 13 = 223°M. This is a map-reading correction, not a new wind triangle; after changing heading, use the next river, road or railway line to confirm that the new track is converging on Timaru.
2. Calculate actual groundspeed, revised ETA, and fuel remaining at Timaru with reserve. Is the fuel plan still acceptable?
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You covered 36 NM in 25 minutes:
Groundspeed = 36 × (60/25) = 86.4 kt.
Remaining 39 NM at 86.4 kt = 39/86.4 × 60 = 27.1 min.
Fuel used so far = 28 × 25/60 = 11.7 L. Remaining fuel = 55 − 11.7 = 43.3 L.
Fuel to Timaru = 28 × 27.1/60 = 12.6 L. Reserve = 30 min at 28 L/h = 14 L. Total required from the fix = 12.6 + 14 = 26.6 L.
Margin = 43.3 − 26.6 = 16.7 L. The fuel plan is still acceptable, but the margin is smaller than planned because the actual groundspeed is below the planned 92 kt.
3. The cloud base ahead is 1,500 ft AGL and you are at 3,500 ft. What map-reading and terrain considerations determine whether you can continue visually to Timaru, and what would you do if the cloud lowers to 1,000 ft AGL?
Read the worked answer
First confirm the terrain and obstacles along the remaining route. The Canterbury Plains are mostly flat, but you still need to identify towns, roads, rivers and any rising ground, and ensure you can remain clear of cloud and in sight of the surface. At 1,500 ft AGL cloud base, a descent to about 2,000 ft AMSL may keep you visual, but you must check the VFR minima and the minimum safe altitude for the area. Haze reduces the horizon and makes feature identification harder, so use the Rangitata and Orari rivers, roads and towns as positive fixes rather than relying on a general sense of direction.
If the cloud lowers to 1,000 ft AGL, the route is still over flat plains, but options reduce sharply. Set a personal limit: if you cannot maintain clear visual contact with the ground and a safe forced-landing area, turn back or land before entering the lowering cloud. Do not descend below a safe altitude and press on hoping for improvement.