Definition
The underlying physical mechanism by which an instrument senses a quantity and converts it into an indication on its display. For the sensitive altimeter, the principle of operation is that a stack of sealed aneroid wafers expands as outside (static) air pressure decreases with altitude and contracts as pressure increases, and this mechanical movement is geared to pointers on the altimeter face to show height above a selected pressure reference.
Plain English
How the instrument actually works inside — what it senses and how it turns that into the number you see on the dial.
Context Anchor
Seen in instrument handbook sections that explain how pitot/static instruments, including the sensitive altimeter, create their cockpit indications.
Derivation
From Latin principium, meaning 'beginning' or 'foundation,' and operari, 'to work.' Together: the foundational way something works. In aviation texts, it signals 'here is the basic mechanism behind this instrument.'
Why Pilots Care
Knowing this principle explains why non-standard pressure and temperature cause altimeter errors that must be corrected for safe flight.
Intuition Check
Do not read principle of operation as a checklist or operating procedure. Here it means the basic cause-and-effect that makes the instrument give a reading.
Example Sentence 1
Before learning altimeter errors, the student first studied the principle of operation so she could see why a stuck static port would freeze the indication.
Example Sentence 2
A pilot reviews the principle of operation before flight to anticipate how a low-pressure area will affect the altimeter reading.