Definition 1 of 2
Definition
A reading on a flight instrument that reflects an aircraft condition through a secondary effect rather than measuring the condition itself. The instrument shows the result of something happening, not the thing directly. For example, the altimeter is an indirect indication of pitch attitude, because pitch changes alter altitude, and the altimeter reflects that altitude change.
Plain English
An instrument reading that tells you about a flight condition by showing the effect it produces, not by measuring it directly. You read the clue, not the cause.
Context Anchor
Seen in instrument flying when using the altimeter during straight-and-level flight to recognize whether the airplane is maintaining the assigned altitude.
Derivation
From Latin 'indirectus,' meaning 'not straight' or 'not direct.' In instrument flying, the path from cause to reading goes through a step in between — the instrument shows the consequence, not the original action.
Why Pilots Care
On instruments, no single gauge directly shows pitch attitude except the attitude indicator. Pilots must learn to read pitch from a combination of indirect indicators — altimeter, vertical speed indicator, and airspeed indicator — and trust what the pattern is telling them, especially if the attitude indicator fails.
Grounding Statement
If the altimeter starts moving away from the altitude you meant to hold, it is pointing to a change in level flight even though it is not showing nose position directly.
Intuition Check
Do not read “indirect” as meaning vague or unreliable. Here it means the instrument is showing a related result, not the condition itself.
Example Sentence 1
With the attitude indicator failed, the pilot used the altimeter and vertical speed indicator as indirect indications of pitch.
Example Sentence 2
In straight-and-level flight the pilot uses the indirect indication from the altimeter to hold a constant altitude.