Definition
Inaccuracies in altimeter indication caused by the physical and mechanical limitations of the instrument itself, rather than by atmospheric conditions or pilot setting errors. These include scale error (the altimeter reading differs from true pressure altitude even when correctly set), hysteresis (a lag in the response of the aneroid wafers after a rapid altitude change), reversal error (a momentary indication in the wrong direction during abrupt pitch changes), and friction error (the pointer sticking briefly due to internal mechanical resistance).
Plain English
Small reading errors that come from the way the altimeter is built, not from the weather or how it was set. The internal parts can lag, stick, or briefly read the wrong way, especially during rapid climbs, descents, or sharp pitch changes.
Context Anchor
Seen when studying the altimeter and when checking whether the altitude shown in the cockpit can be trusted in real flight conditions.
Derivation
Mechanical' comes from the Greek 'mekhane' meaning machine or device. It points to the fact that these errors come from the physical workings of the instrument — the wafers, gears, and linkages inside — rather than from outside air conditions.
Why Pilots Care
Uncorrected errors can place the aircraft hundreds of feet higher or lower than the pilot believes, directly affecting terrain clearance and traffic separation.
Grounding Statement
The key idea is that the altimeter is not directly measuring height with a tape measure; it is estimating altitude from air pressure, so anything that affects that pressure reading can affect the altitude shown.
Intuition Check
Do not assume mechanical altimeter errors only mean the instrument is broken. In this context, the altimeter can be mechanically sound and still indicate an incorrect altitude because of setting, pressure, temperature, or air-supply problems.
Example Sentence 1
During the preflight check, the pilot allowed for small mechanical altimeter errors when comparing the indicated altitude to the known field elevation.
Example Sentence 2
In cold, low-pressure air the crew adjusted for mechanical altimeter errors before descending toward the minimum safe altitude.