Drift Correction and Revised ETA on a 168 NM Leg
You are flying a single-engine piston aeroplane from Wattle Ridge (elevation 1,180 ft) to Kooringa (elevation 640 ft). The planned route is 168 NM with a true track of 075°T. You plan to cruise at 6,500 ft with a TAS of 115 kt. The forecast wind is 345°T at 25 kt. Magnetic variation is 9°E, and the compass deviation on the planned heading is 2°E. Fuel burn is 34 L/h, usable fuel is 140 L, and the fixed reserve is 45 min. You depart at 0210 UTC. After 40 min, a visual fix over a racecourse and highway intersection places you 8 NM right of track, with 95 NM remaining to Kooringa. The GNSS is serviceable but you are using it only as a backup. ATC has no surveillance coverage below 8,000 ft, so you are relying on dead reckoning and visual fixes.
1. Calculate the true heading, compass heading, ground speed and ETA for Kooringa. Show the wind-triangle reasoning and the fuel check.
Read the worked answer
Wind is from 345°T, track 075°T: the wind is 90° off the track from the left, so crosswind = 25 kt and along-track component = 0. Drift = arcsin(25/115) = 12.5°; heading must be offset left into wind. TH = 075 - 12.5 = 062.5°T. GS = sqrt(115² - 25²) = 112.3 kt. MH = TH - variation (east) = 062.5 - 9 = 053.5°M. CH = MH - deviation (east) = 053.5 - 2 = 051.5°C, set 052°C. Time = 168/112.3 = 1.496 h = 1 h 30 min. ETA = 0210 + 0130 = 0340 UTC. Fuel = 1.496 × 34 = 50.9 L; reserve 45 min = 25.5 L; total 76.4 L, within 140 L usable. The calculation assumes the forecast wind holds at 6,500 ft; the 052°C heading is a planning heading, not a guarantee.
2. At the 40 min fix, calculate the heading change to arrive at Kooringa and the revised ETA. If the revised ETA is after last light, what action do you take and why?
Read the worked answer
At 40 min, distance flown = 112.3 × 40/60 = 74.9 NM. Track error = 8/74.9 × 60 = 6.4°. Closing angle to destination = 8/95 × 60 = 5.1°. Total correction = 11.5°, turn left (toward track). New compass heading = 052 - 11.5 ≈ 040°C. Remaining distance = sqrt(95² + 8²) = 95.3 NM. At 112 kt, time = 51 min. ETA = 0250 + 0051 = 0341 UTC, 1 min later than planned. If last light is, say, 0345 UTC, you would have only a 4 min buffer, which is not enough. You should not press on: either divert to an aerodrome with lighting, turn back while daylight remains, or land before last light. The 1-in-60 correction is approximate; use the next visual fix to refine it. Also note the correction changes the heading and therefore the compass deviation slightly; use the deviation for the new heading if available.