Compass, Wind, and a GPS Failure Over Featureless Terrain
You are flying a 72 nm leg from A to B in an ultralight at 70 kt TAS. The forecast wind is 300/25. Your planned true track is 090°. Magnetic variation is 8°E, and the compass deviation card shows +2° on east headings. Your GPS fails 20 nm after departure. You have a map, compass, watch, and a VOR receiver; B has a VOR 12 nm southwest of the strip. Fuel is 50 L, burn 12 L/h, and the reserve is 30 minutes. Visibility is 20 km, but the ground is featureless and haze obscures detail below 3,000 ft. You departed at 10:15 local and estimate 14 minutes have passed since departure. Decide how to navigate and whether to continue.
1. Calculate the true heading, magnetic heading, compass heading, and ground speed.
Read the worked answer
Wind is from 300° at 25 kt, track 090°: angle between wind direction and track is 150°. Crosswind component = 25 sin 150° = 12.5 kt. WCA = arcsin(12.5/70) ≈ 10.3°, and the wind is from the left, so turn left: true heading ≈ 080°. Tailwind component = 25 cos 150° = −21.7 kt, so ground speed ≈ 70 cos 10.3° + 21.7 ≈ 90.5 kt. Magnetic heading = 080 − 8 = 072°M. Compass heading = 072 − 2 = 070°C. Round to 070°C and 90 kt for planning.
2. What are the ETA and fuel state at B, and what assumptions must you check?
Read the worked answer
Leg time at 90 kt = 72/90 h = 0.80 h = 48 min. Distance remaining = 72 − 20 = 52 nm; time = 52/90 × 60 ≈ 35 min. ETA = 10:15 + 48 min = 11:03 local. Fuel burn = 12 L/h = 0.2 L/min; used ≈ 2.8 L; remaining ≈ 47.2 L; reserve 6 L. Fuel is ample. Recalculate if the actual ground speed differs; a 10 kt error changes leg time by about 5 min. The 20 nm already flown is approximate, so confirm it against time and the VOR.
3. How do you confirm position and decide whether to continue?
Read the worked answer
Use the VOR: B’s VOR is 12 nm southwest of the strip, so identify the radial that places you on a known line and cross-check with time and map. If you cannot get a positive fix within about 10 min, do not continue on dead reckoning into featureless haze. Climb if terrain and cloud base allow, or turn toward the VOR or a known landmark. If uncertain of position near controlled airspace, stay clear and ask ATC for a position fix. A GPS failure is not an emergency, but continued flight without a fix is poor judgment.