Definition 1 of 2
Definition
A range of altitudes, depicted on an instrument approach procedure profile view, between which the aircraft must be flown over a specified segment or fix. The aircraft must remain at or above the lower altitude and at or below the upper altitude while crossing that portion of the procedure.
Plain English
A height window you must stay inside while flying over a particular point on an instrument approach. You can be anywhere between the bottom number and the top number, but not below the bottom and not above the top.
Context Anchor
Seen on the profile view of instrument procedures, where altitude limits are shown for specific points or parts of the route.
Derivation
‘Mandatory’ comes from Latin mandare, meaning to command or require. ‘Block’ here means a range with a defined top and bottom — like a block of time. Together: a required altitude range you must fly inside.
Why Pilots Care
Maintains vertical separation from other traffic and ensures compliance with ATC instructions.
Intuition Check
Do not read “block altitude” as “any altitude I want in the general area.” Here, the block is a required vertical range with a bottom and a top.
Example Sentence 1
Crossing the fix, the pilot maintained 3,500 feet to comply with the mandatory block altitude of 3,000 to 4,000 feet shown on the approach chart.
Example Sentence 2
We remained in the mandatory block altitude while deviating around weather.