Definition 1 of 2
Definition
An apparent shift in the indicated value of an instrument reading caused by viewing the instrument from an angle rather than straight on. In an airplane, parallax error most commonly affects how the pilot reads the attitude indicator, altimeter, and other panel instruments when looking at them from the left seat instead of from directly in front.
Plain English
When you look at a gauge or indicator from the side instead of straight on, the needle or marking can appear to be in a slightly different place than it really is. That apparent difference is parallax error.
Context Anchor
In establishing a turn, a pilot may notice parallax error when using the airplane’s nose, wing, windshield, or another cockpit reference to judge bank and direction against the outside view.
Derivation
From the Greek 'parallaxis,' meaning 'change' or 'alternation.' It describes how an object's apparent position changes when the viewer's position changes. Knowing this helps the pilot remember that the error is about viewing angle, not a fault in the instrument.
Why Pilots Care
An uncorrected parallax error can cause the pilot to overbank or underbank, resulting in uncoordinated flight, altitude loss, or heading deviations.
Analogy
A car’s speedometer can look slightly different from the driver’s seat than from the passenger seat if you view the needle from an angle. The needle did not move; your viewing angle changed the apparent reading.
Intuition Check
Parallax error is not a problem with the airplane or the instrument. It is a viewing-position problem: the same reference can appear different when your eye position changes.
Example Sentence 1
Sitting in the left seat, the pilot allowed for parallax error when reading the centerline-mounted attitude indicator during a steep turn.
Example Sentence 2
Checking the attitude indicator from the side introduced parallax error and produced an incorrect bank indication.