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:

• Aircraft altimeters, while ingenious mechanical devices, are susceptible to various inherent errors, including mechanical malfunctions, installation issues, and lag (hysteresis).
• Environmental factors like changes in atmospheric pressure and temperature significantly impact altimeter accuracy, leading to potential discrepancies between indicated and actual altitude, notably in "high to low" pressure scenarios.
• Accurate altimeter readings rely on diligent pre-flight checks (e.g., <75 ft error tolerance), proper maintenance, and crucially, the correct input of local barometric pressure settings into the Kollsman window.
• Many critical altimeter errors stem from human factors, such as failing to properly update altimeter settings, underscoring the need for meticulous procedural adherence for flight safety.

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