Definition
In Air Traffic Control instructions, the lowest altitude assigned to or authorized for a procedure, route segment, or block of airspace. On a Standard Terminal Arrival (STAR) or other published procedure, it is the lowest altitude a pilot is permitted to descend to at a given fix unless ATC issues a different clearance.
Plain English
The lowest altitude you are allowed to fly at a particular point on a route or procedure. You can be at that altitude or higher, but not below it.
Context Anchor
Seen on published arrival procedures and in air traffic control clearances that let an aircraft descend along an arrival route.
Derivation
“Bottom” means the lower end or lowest part of something. “Altitude” comes from a Latin word meaning “height.” Together, the phrase points to the low end of the allowed altitude on an arrival procedure.
Why Pilots Care
Observing the bottom altitude keeps the aircraft above terrain and inside the protected airspace of the published procedure.
Intuition Check
Do not read “bottom” as “as low as the airplane can safely go.” Here, Bottom Altitude means the lowest altitude authorized by the arrival procedure or by air traffic control.
Example Sentence 1
The STAR shows a bottom altitude of 8,000 feet at LINDEN, so the crew levels off there until ATC clears them lower.
Example Sentence 2
On the STAR, we descended to the bottom altitude of 4,000 feet before receiving further clearance.