Your Altimeter Lies

The instruments in our so-called "steam-gauge" panels are marvels of ingenuity. A collection of springs, tubing, gears, bellows, shafts and dials, their basic design predates most of the pilots staring at them. While they have, for the most part, been rendered obsolete by the latest microelectronics and air-data computers, they still work as advertised. Well, pretty much. The fact is our faithful mechanical instruments are regularly susceptible to certain errors. Too, they can fall victim to not-so-regular problems, mostly brought about through neglect or damage. The good news is many of these errors are predictable, if we take the time to understand how the instruments work and how the errors may manifest themselves.

Key Takeaways:

• Mechanical altimeters, despite their ingenious design, are susceptible to various instrument-specific errors (e.g., mechanical, scale, installation, hysteresis) that can affect accuracy, though many are predictable and can be mitigated through maintenance and pre-flight checks.
• Altimeter readings are significantly influenced by atmospheric conditions, with changes in pressure and temperature leading to discrepancies between indicated and actual altitude; pilots must frequently update Kollsman window settings to account for these environmental factors.
• Accurate altimetry is crucial for flight safety, especially in IFR operations, necessitating diligent pre-flight checks (recalibrating if off by more than 75 feet) and the correct, consistent input of altimeter settings to prevent errors, many of which are human-induced.

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