Navigation with a Failing GPS and a Magnetic Anomaly
You are on a VFR cross-country flight in a single-engine aircraft. Your route crosses a region known for magnetic anomalies. Your GPS has begun to show a position that is inconsistent with your dead reckoning, and the track error is growing. You have a VOR receiver, a magnetic compass and a chart. The visibility is 20 km, the cloud base is 4,000 ft, and you are at 3,500 ft. Your destination is 45 minutes ahead. There is a controlled airspace boundary 10 nautical miles to your right. The magnetic compass is swinging erratically.
1. How should you diagnose the navigation problem?
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Step 1: Cross-check sources. Compare the GPS position with dead reckoning from the last known fix and with any VOR bearing. If the GPS is inconsistent with both, suspect the GPS. If the VOR also disagrees, suspect the magnetic compass or the chart.
Step 2: Consider the anomaly. A magnetic anomaly can cause the compass to swing and can also affect a magnetic-based heading reference. A GPS is not magnetic, so a GPS error is more likely to be a receiver or satellite problem.
Step 3: Establish a fix. Tune a VOR and take a bearing. Use two bearings or a bearing and a known distance to fix your position. If no VOR is available, use visual landmarks and the chart.
Step 4: Decide. Once you have a reliable fix, update your dead reckoning and continue, or divert if the uncertainty is too great.
2. What are the risks of continuing with an unreliable position?
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Airspace risk. The controlled airspace boundary is 10 nm to your right. An undetected position error could put you inside controlled airspace without a clearance.
Terrain and obstacle risk. Without a reliable position, you cannot be sure of your clearance from terrain and obstacles, especially at 3,500 ft with a 4,000 ft cloud base.
Fuel and time risk. A growing track error means your destination may be further than planned, reducing your fuel margin.
Compass risk. An erratic compass makes heading-based navigation unreliable, so you cannot fall back on it without a fix.
Decision. If you cannot establish a reliable fix within a few minutes, divert to a known point or request assistance from ATC.
3. What should you tell ATC and how should you plan the remainder of the flight?
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ATC communication. Advise ATC of your situation: unreliable GPS, erratic compass, and your last confirmed fix. Request a radar identification or a position report. If you are unsure of your position relative to controlled airspace, say so and request clearance or avoidance instructions.
Planning. Once you have a fix, replan using VOR radials and visual landmarks rather than the GPS. Note the magnetic anomaly on your chart and avoid relying on the compass in that region. Recompute fuel for the remaining leg and identify an alternate. If the destination is not achievable with reserves, divert early.