Definition
An ATC altitude restriction phrase used in clearances and instructions, requiring the aircraft to be at the specified altitude precisely when crossing the named fix — neither earlier nor later, and not above or below that altitude at the moment of crossing.
Plain English
When you reach this point on the chart, you must be exactly at this altitude. Not above it, not below it, not approaching it — at it.
Context Anchor
Heard in ATC clearances during arrivals, departures, and route changes, especially when a controller needs an aircraft at a certain height at a certain point.
Derivation
Altitude comes from the Latin altus, meaning high. Fix comes from the idea of a fixed position; in aviation, it means a named point in space that pilots and controllers can both identify.
Why Pilots Care
Maintains safe separation from other traffic and ensures compliance with charted altitudes and terrain clearance.
Intuition Check
Do not read “at” as “start the climb or descent there.” It means you must already be at the assigned altitude when you reach the fix.
Example Sentence 1
ATC instructed us to cross LIBBY at 6,000, so I started the descent early to make sure we were level at 6,000 right at the fix.
Example Sentence 2
The pilot planned the descent to cross the fix at the published crossing altitude.