The Transponder That Wouldn't Stop Squawking
You are cruising VFR at FL75 in Class C airspace, squawking 7000 with Mode C. ATC clears you to 6,500 ft. As you descend, your transponder begins intermittently showing a Mode C altitude 300 ft off your indicated altitude. You suspect a static source issue. ATC calls: "Confirm level." You are 8 NM from the boundary of a Class D control zone you intend to enter, and your radio is working normally. Your altimeter is set to the local QNH, 1013 hPa is not in use.
1. How should you respond to ATC, and what are your obligations regarding the transponder and airspace?
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Respond to ATC: Acknowledge the call and state your indicated altitude and that you suspect a static source error. Do not simply say "level" if you are unsure - report the discrepancy. Example: "Confirm level, we are indicating 6,500 ft, suspect static source error, Mode C may be unreliable."
Transponder: A transponder showing erroneous Mode C is a safety issue. In Class C airspace, ATC relies on Mode C for separation. If the error is confirmed, you should consider switching off Mode C (ALT) and informing ATC, or following their instructions. Do not continue to present false altitude data without advising ATC.
Airspace: You are still in Class C and approaching a Class D zone. You must not enter the Class D zone without clearance. Given the altitude uncertainty, request clarification of your level and confirm the clearance limit before crossing the boundary. If the static error affects your altimeter, you may not be able to assure terrain or airspace compliance - consider requesting a lower-risk routing or diverting.
2. What pre-flight or in-flight checks could have caught this, and what is the correct altimeter setting action?
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Pre-flight: Check the static ports for blockage, ensure the pitot cover is removed, and verify the altimeter reads the aerodrome elevation on the correct QNH before departure. A leak in the static system can cause altitude errors that only appear in flight.
In-flight: Cross-check the altimeter against GPS altitude and ATC-reported levels. A 300 ft discrepancy is significant. If the static source is suspect, use the alternate static source if fitted.
Altimeter setting: The altimeter should be set to the local QNH (1013 hPa is only for standard pressure at/above transition altitude). Confirm the QNH from ATIS or ATC. Do not change to 1013 hPa below the transition level. If the QNH is correct, the error is in the static system, not the setting.