Definition
An ATC clearance authorizing a pilot to operate within a specified range of altitudes, rather than at a single assigned altitude. The pilot may climb, descend, and maneuver freely between the lower and upper limits of the assigned block without further ATC approval.
Plain English
ATC gives you a band of altitudes to fly within — for example, between 10,000 and 12,000 feet — and you can move up or down inside that band as you need to, without asking again.
Context Anchor
Seen in instrument flying when a pilot needs some altitude flexibility, such as while dealing with turbulence or searching for smoother air.
Derivation
‘Block’ here means a contiguous range or chunk — the same sense as a ‘block of time.’ The clearance gives you a block of airspace stacked vertically, rather than a single flight level.
Why Pilots Care
It lets the pilot climb or descend within the block to find smoother air without requesting a new clearance each time.
Intuition Check
Do not read “block altitude clearance” as clearance to one altitude. It means clearance to use any altitude inside the assigned altitude range.
Example Sentence 1
Encountering moderate turbulence at 11,000 feet, the pilot requested a block altitude clearance between 10,000 and 12,000 feet to ride out the rough air.
Example Sentence 2
With the block altitude clearance in hand, the aircraft descended 1,500 feet to smoother air without further calls to ATC.