Definition
In the primary and supporting instrument method of attitude instrument flying, the primary instrument is the one that gives the most direct, accurate, and unchanging information about a specific aircraft performance parameter once a desired flight condition has been established. For straight-and-level flight, the altimeter is primary for pitch, the heading indicator is primary for bank, and the airspeed indicator is primary for power.
Plain English
The instrument you watch most closely to hold a specific value steady, like altitude, heading, or airspeed. It is the one that tells you straight away whether you are still on target.
Context Anchor
In straight-and-level instrument flight, the Airspeed Indicator may be treated as the primary instrument for power because it shows whether the selected power setting is producing the desired speed.
Derivation
From Latin primarius, meaning 'of the first rank' or 'principal.' In this method the instrument is 'first' in the sense of being the one you check first to confirm you are still holding the target value.
Why Pilots Care
Correct identification of primary instruments allows efficient scan patterns and accurate aircraft control in instrument meteorological conditions.
Intuition Check
Primary does not mean the only instrument that matters. It means the instrument that gets first priority for a specific control task in that phase of flight.
Example Sentence 1
Once level at 5,000 feet, the altimeter becomes the primary instrument for pitch.
Example Sentence 2
The altimeter becomes the primary instrument for pitch control when maintaining a constant altitude.