Definition
In instrument flying, a supporting instrument is one that confirms or backs up the information shown on the primary instrument for a given pitch, bank, or power condition. It indicates a trend or rate that supports what the primary instrument is showing, allowing the pilot to verify that the desired flight condition is being held.
Plain English
A supporting instrument is one you cross-check against the main instrument to confirm the aircraft is doing what you want. It backs up the reading you're already using to control the aircraft.
Context Anchor
Seen in instrument flying discussions of straight climbs, descents, and other maneuvers where the pilot uses several instruments together instead of relying on just one.
Derivation
Support comes from a Latin word meaning “to carry from below” or “hold up.” That helps here because a supporting instrument does not lead the maneuver; it helps hold up or confirm the pilot’s understanding of what is happening.
Why Pilots Care
Using supporting instruments correctly prevents over-controlling and keeps altitude, airspeed, and heading within standards during instrument flight.
Intuition Check
A supporting instrument is not necessarily an emergency backup instrument. It is a normal instrument used as a confirming check during a specific maneuver.
Example Sentence 1
While holding altitude in level flight, the altimeter is the primary instrument and the vertical speed indicator acts as a supporting instrument, confirming the aircraft is neither climbing nor descending.
Example Sentence 2
The airspeed indicator serves as a supporting instrument when the pilot cross-checks power during a constant-airspeed descent.