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 ingenuity, are susceptible to inherent mechanical errors (e.g., wear, scale issues, hysteresis) and external factors like static system variations.
• Environmental conditions, primarily pressure and temperature changes, significantly affect altimeter accuracy, leading to common errors such as being lower than indicated when flying from high to low pressure or into cold air.
• Accurate altimeter readings depend on correctly setting the local barometric pressure in the Kollsman window and understanding how air density variations impact indicated altitude.
• Regular maintenance, required 24-month checks, and vigilant pilot practices—including preflight checks (tolerance of 75 feet) and constant verification of settings—are crucial for minimizing errors and ensuring flight safety.

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