GNSS Failure and a VOR Fix Near Controlled Airspace
You are on a VFR flight from inland to the coast at 4,500 ft. The GNSS fails, and the tablet overheats and shuts down. You have a current chart, a VOR receiver, and an ADF. The visibility is 10 km in haze, the horizon is indistinct, and the terrain is flat. You identify a town with a railway line and a small lake. The VOR is 28 DME on the 315° radial (magnetic). The destination aerodrome is 55 NM from the VOR on the 090° radial. Class C airspace begins 20 NM seaward of the coast and has a lower limit of 3,500 ft; you are at 4,500 ft. The coast is visible 15 NM ahead. Your last positive fix was 20 min ago; groundspeed is 110 kt. You need to decide whether to climb, descend, or turn, and you have no clearance into Class C.
1. Calculate your position relative to the VOR and the magnetic track and distance to the destination.
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Take the VOR as the origin. On the 315° radial at 28 DME, you are 28 sin 315° = 19.8 NM west and 28 cos 315° = 19.8 NM north of the VOR. The destination is on the 090° radial at 55 NM: 55 NM east of the VOR. Vector from you to destination: 74.8 NM east and 19.8 NM south. Distance = sqrt(74.8² + 19.8²) = sqrt(5,595 + 392) = 77.4 NM. Track = 180° - atan(74.8/19.8) = 180° - 75.2° = 104.8°M, round to 105°M. At 110 kt, no-wind time = 77.4/110 = 0.704 h = 42 min. This is a magnetic track; apply wind drift and deviation to get a compass heading. The fix is a VOR/DME fix, so it is independent of GNSS.
2. Will your current route and altitude enter Class C? What action do you take?
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You are at 4,500 ft; Class C lower limit is 3,500 ft. The coast is 15 NM ahead, and Class C begins 20 NM seaward of the coast, so if you continue on the same track you could cross the Class C boundary about 35 NM ahead. Because 4,500 ft is above 3,500 ft, you would enter Class C without a clearance. The destination is 77 NM away on 105°M, so you cannot simply assume it is outside Class C. Action: either obtain a clearance before the boundary (if the aircraft and radio are suitable) or descend to 3,000 ft to remain clear, provided terrain/obstacle clearance and VFR minima allow. Do not climb. If you descend, set a minimum safe altitude based on the chart; flat terrain does not mean no obstacles. If you cannot get a clearance and cannot descend safely, turn away from the boundary and replan.
3. If the VOR fails too, describe the lost procedure you would use and the heading you would fly.
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If the VOR fails, revert to chart, visual features and the ADF. First maintain the last known heading and note the time; do not make random turns. Climb if safe for better visibility, but you are already at 4,500 ft and the terrain is flat. Use the town, railway, lake and the coast 15 NM ahead as a line feature: fly to the coast, identify a distinctive bend or town on the chart, and fix your position. Then fly 105°M plus a wind-drift correction toward the destination; without a wind estimate, use the forecast wind and refine with the next feature. If uncertain, call ATC, squawk emergency/ident, and fly a deliberate lost procedure: maintain VFR, conserve fuel, and follow the coast to a known aerodrome. The heading is not simply 105°M compass; apply variation and deviation. The key is to navigate by a known line feature rather than dead reckoning blindly.