Definition
A range of altitudes assigned by ATC within which a pilot may operate, rather than being held to a single specific altitude. The clearance specifies a lower and upper limit (for example, 'maintain block altitude 8,000 through 10,000'), and the aircraft may climb, descend, or cruise anywhere within that band without further coordination.
Plain English
Instead of holding one exact altitude, ATC gives you a vertical window of airspace and lets you fly anywhere inside it.
Context Anchor
Seen on Standard Instrument Departure charts and sometimes heard in air traffic control clearances when a range of allowed altitudes is used instead of one assigned altitude.
Derivation
Block' here means a reserved chunk or section of something — the same sense as a 'block of time.' ATC is reserving a vertical block of airspace for the aircraft.
Why Pilots Care
Provides flexibility to select the best altitude inside the block for weather avoidance, turbulence, or aircraft performance without new clearances.
Analogy
Think of it like a speed zone that says “between 40 and 50.” The instruction is not one exact number; it is a safe allowed range.
Intuition Check
Do not read “block” as meaning blocked off or unavailable. Here it means a permitted altitude range with a bottom and a top.
Example Sentence 1
Center cleared the flight to maintain a block altitude of 10,000 through 12,000 to allow the crew to find smoother air above the broken cloud layer.
Example Sentence 2
We used the block altitudes to stay above the buildups without requesting a new altitude.