A 50–70 foot altimeter discrepancy between dual Garmin G5 units and a legacy analog altimeter in a general aviation aircraft under pre-purchase evaluation represents a condition that falls into a well-understood but frequently mischaracterized gray zone in avionics troubleshooting. The G5 employs solid-state MEMS-based pressure transducers and an internal air data computer to derive altitude, while the original steam gauge relies on mechanical aneroid capsule technology that degrades with age, humidity exposure, and vibration cycles. Both instruments draw from the aircraft's static system, meaning any discrepancy must be traced to one of three root causes: a calibration error in the analog unit, a configuration or compensation issue in the G5 installation, or a static system anomaly such as a partial blockage, leak, or mismatched port architecture. The magnitude of this difference — while seemingly minor — is operationally relevant and, more importantly, serves as a diagnostic indicator of the aircraft's overall instrument maintenance posture.
Regarding primary instrument authority, the answer in a dual-G5 installation depends heavily on how the avionics were installed and whether the STC documentation designates the G5s as primary replacements under 14 CFR Part 23 airworthiness standards. Garmin's G5 received FAA TSO approval and subsequent STC authorization to serve as a primary attitude and air data instrument in certified aircraft, which means the G5 altimeter readout carries regulatory credibility — provided the installation was accomplished in accordance with Garmin's Installation Manual and the STC holder's data. In practice, the G5's solid-state sensor is inherently more stable and less susceptible to mechanical wear than a decades-old aneroid altimeter, making it the more reliable source in most GA contexts. However, neither instrument should be presumed accurate without a current static system and altimeter test under 14 CFR 91.411, which mandates that inspection every 24 calendar months for IFR flight — and that test must encompass every altimeter and static port connection in the system, including the G5's air data inputs.
For a prospective buyer, the resolution pathway is both clear and relatively modest in cost. A qualified avionics shop or IA holding appropriate FAA authorization should perform a full static system leak check and altimeter calibration test on all instruments simultaneously. This procedure will isolate whether the analog unit is out of tolerance, whether the G5 was configured with an incorrect offset, or whether a static system leak is causing differential pressure readings between instruments plumbed to different taps. Static system inspections typically run $150–$400 depending on aircraft complexity, and analog altimeter recalibration or overhaul ranges from $200–$600 at an instrument shop. If the analog altimeter is simply aging out of IFR certification tolerance — common in aircraft with original 1970s or 1980s gauges — replacing it with a new certified unit costs $300–$700 and eliminates the discrepancy at its source. In a pre-purchase context, this work should be negotiated as a seller-funded correction or priced into the acquisition.
The broader significance of this scenario reflects a transition dynamic increasingly common across the Part 91 GA fleet: aging certified aircraft being selectively upgraded with modern EFIS technology — G5s, G3Xs, Dynon SkyViews — while retaining original analog instruments as backups or because panel space was insufficient to replace them. This hybrid cockpit architecture creates a dual-source reality where pilots must understand which instrument governs which regulatory requirement and how each system derives its data. The FAA has accommodated this trend through STCs and advisory circular guidance, but it places greater interpretive responsibility on pilots and maintenance personnel to understand instrument hierarchy, static system architecture, and the interaction between legacy and modern avionics. A discrepancy of 50–70 feet that is tolerated without investigation represents a maintenance culture concern at least as much as an instrument accuracy concern — and in a pre-purchase evaluation, it is precisely the kind of deferred item that signals broader attention to aircraft records and mechanical stewardship.