Definition
An indication on a TCAS (Traffic Alert and Collision Avoidance System) display that a Traffic Advisory exists for an aircraft whose position is beyond the selected display range, so its symbol is shown at the edge of the screen rather than at its true relative position. The traffic is being tracked and is close enough in altitude and closure to warrant a TA, but its horizontal range exceeds the currently selected scale.
Plain English
The traffic system has flagged another aircraft as a concern, but that aircraft is farther away than your display is currently set to show. So the symbol sits on the outer edge of the screen instead of at its real distance.
Context Anchor
Seen on cockpit traffic displays that show nearby aircraft during traffic avoidance monitoring.
Derivation
Off scale' comes from instrumentation language, where a needle or value that exceeds the marked range of the display is said to go 'off the scale.' Here it means the traffic's range is greater than the scale currently selected on the display.
Why Pilots Care
Prompts the pilot to increase display range or perform an outside scan to locate and avoid the conflicting traffic.
Intuition Check
Do not read “off scale” as “the traffic alert is invalid” or “the system failed.” It means the alerted traffic is outside the range currently shown on the display.
Example Sentence 1
Seeing a TA OFF SCALE indication, the pilot increased the traffic display range from 6 nm to 20 nm to see the conflicting aircraft's actual position.
Example Sentence 2
Even with TA OFF SCALE displayed, the crew maintained visual lookout until the other aircraft was sighted.