As a 1700-hour pilot with what I consider a set of prudent and safe practices, I knew my initial 10-hour engine break-in flights would be VFR-only. Certainly, there was no reason to take chances.
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Three Out Of Four
As a 1700-hour pilot with what I consider a set of prudent and safe practices, I knew my initial 10-hour engine break-in flights would be VFR-only. Certainly, there was no reason to take chances. After liftoff on my second hour of break-in flights, I reported in to our local tower to make them aware I was out there flying around their Class C airspace, near Lexington, Ky. As I reported my altitude, 3000 feet, and well below the cloud deck, the friendly controller politely responded, “We show your Mode C as 2400.”
Key Takeaways:
- An experienced pilot discovered a 600-foot altitude discrepancy between his primary altimeter and ATC's Mode C reading, which was corroborated by his other digital avionics.
- Trusting the ATC and digital systems, the pilot adjusted his altitude, unknowingly flying for several hours with the primary altimeter providing an inaccurate reading due to a perceived fault.
- The root cause was a malfunctioning Mode C encoder, which provided erroneous data not only to ATC but also corrupted the altitude readings across multiple other integrated avionics, leaving the "steam gauge" altimeter as the only accurate source.
- This incident demonstrated how complex avionics integration means a single, hidden component failure can spread incorrect data across an entire system, potentially misleading even highly experienced pilots.
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