Definition
The instrument that gives the most direct, accurate, and stable indication of a specific flight parameter (pitch, bank, power, or yaw) for the maneuver being flown. In the primary/supporting concept of instrument flying, the primary instrument is the one the pilot references to hold a desired value steady, while supporting instruments confirm the trend.
Plain English
For each thing you are trying to hold constant in flight — like altitude, airspeed, or heading — there is one instrument that shows it most directly. That instrument is the one you watch closely to keep the value pinned. Other instruments back it up, but the primary one is the main reference.
Context Anchor
Seen in instrument flying when learning which instrument to rely on most during a specific maneuver, such as a climb, descent, turn, or level-off.
Derivation
Primary' comes from the Latin primus, meaning 'first' or 'chief.' Here it means the first instrument you look to for a given value — the chief reference, not the only one.
Why Pilots Care
Correct identification of the primary instrument for each parameter lets the pilot maintain precise control with less effort and fewer errors when flying solely by reference to instruments.
Intuition Check
Do not assume "primary flight instrument" means one permanently most important instrument. Here it means the instrument that is first for a specific job at a specific moment.
Example Sentence 1
During a constant airspeed climb, the airspeed indicator is the primary flight instrument for pitch, because pitch is adjusted to hold the target airspeed.
Example Sentence 2
The attitude indicator remains the primary flight instrument for both pitch and bank during straight-and-level flight on instruments.