Definition
An altitude depicted on an instrument procedure chart at which an aircraft is required to cross the associated fix or segment. The aircraft must be at that exact altitude — neither above nor below — when crossing the specified point.
Plain English
An altitude you must hit precisely at a given point on the procedure. Not higher, not lower — exactly that altitude.
Context Anchor
Seen on instrument departure procedures, especially SIDs, where climb instructions include required altitudes at specific points along the departure path.
Derivation
From Latin mandatum, meaning 'a command or order.' On a chart, the altitude is not a suggestion or a limit — it is a commanded crossing altitude.
Why Pilots Care
Maintains the designed vertical path for terrain clearance, traffic separation, and ATC compliance during departures.
Intuition Check
Mandatory does not mean “recommended” or “normally a good idea.” In this context, it means the altitude is required unless air traffic control gives a different instruction.
Example Sentence 1
The SID showed a mandatory altitude of 5,000 feet at HOBTT, so the crew leveled off precisely at 5,000 when crossing the fix.
Example Sentence 2
ATC expects the aircraft to cross the waypoint at the mandatory altitude to stay within the protected airspace.