Navigation Without GPS in Featureless Terrain
Your GPS fails 20 minutes after launch over a large flat agricultural plain with few roads and no prominent hills. You carry a magnetic compass, a 1:250,000 chart, and a watch. The forecast wind at your altitude is 270°/18 kt. Your intended destination is a village 30 km to the north-east. You can see a straight irrigation canal running roughly north-south about 6 km to your east, and a railway line running east-west about 10 km to your north. Your ground speed is estimated from two landmarks timed 8 minutes apart, giving 22 kt. Fuel remaining allows 70 minutes of flight above reserve.
1. Describe how you would establish your position and track using the available features and the compass.
Read the worked answer
Use the canal and railway as a grid. The canal runs north-south and the railway east-west, so their intersection is a known point on the chart. Estimate the bearing to the canal using the compass and note the time; repeat a few minutes later to derive drift. With a ground speed of 22 kt, 8 minutes covers about 2.9 nautical miles, or roughly 5.4 km, so timing between the railway and canal gives a rough position fix. Compare the compass bearing to the chart bearing to the canal and railway to estimate the wind correction angle. Because the wind is 270°/18 kt, expect significant drift to the east; the compass heading must be offset into wind to hold the intended track. Plot the fixes on the chart and update the track to the village.
2. Determine whether the destination is reachable within the fuel available and state the navigation decision.
Read the worked answer
The village is 30 km north-east, about 16 nautical miles. At 22 kt ground speed, that is roughly 44 minutes of flight, leaving about 26 minutes of margin above reserve. However, the wind is from the west, so the north-east track has a headwind component; the effective ground speed may be lower than 22 kt, increasing the time. The pilot should track progress against the canal and railway, and if the estimated arrival time exceeds 50 minutes, divert to a nearer landing area. Because position fixing without GPS is approximate, the pilot should keep a generous margin and be prepared to land before the fuel reserve is reached. The decision is to continue only while the fixes confirm the destination remains reachable with margin.