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 instruments, are prone to various errors stemming from their mechanical nature (e.g., wear, hysteresis) and installation.
• Atmospheric conditions significantly affect altimeter accuracy; changes in barometric pressure and temperature (density) can lead to considerable discrepancies between indicated and actual altitude.
• Maintaining altimeter accuracy is critical for flight safety, requiring pilots to understand potential errors, perform pre-flight checks (e.g., ensuring <75 ft deviation from field elevation), and consistently input correct local altimeter settings.
• Human error in setting the altimeter and neglecting to update settings with changing weather conditions are significant contributors to altimetry inaccuracies.

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