Definition
A warning indicator displayed on certain cockpit instruments — typically the airspeed indicator or Mach/airspeed indicator — consisting of a pattern of crossed diagonal lines that appears in the instrument's display window when the instrument's information is unreliable, unpowered, or has failed.
Plain English
A striped or crossed-line warning pattern that pops up on an instrument's face to tell the pilot the instrument isn't working properly and its reading should not be trusted.
Context Anchor
Seen on some altimeter faces and in pitot/static instrument diagrams, especially when learning how to read altitude indications.
Derivation
The word 'crosshatch' comes from the drawing technique of shading an area with two sets of parallel lines crossing each other. The flag uses the same visual pattern, which is why it carries the name.
Why Pilots Care
Tells the pilot the reading cannot be trusted and backup instruments or procedures must be used instead.
Grounding Statement
When the crosshatch flag is visible, treat it as a reading aid that says, “This indication is in the lower altitude range.”
Intuition Check
Do not assume “flag” means the instrument has failed. In this term, the crosshatch flag is a normal visual cue on the altimeter, not a failure warning.
Example Sentence 1
During the climb, the pilot noticed a crosshatch flag covering the Mach window and cross-checked airspeed against the standby instrument.
Example Sentence 2
With the crosshatch flag visible on the altimeter the pilot turned to the backup instruments for altitude.