Definition
Airport signs with a red background and white inscription that identify the entrance to a runway, a critical area, or an area prohibited to aircraft. A pilot must not cross a mandatory instruction sign without specific clearance from air traffic control at a towered airport, or without first confirming the area is clear at a non-towered airport.
Plain English
Red signs at an airport that tell you to stop and not go past unless you have permission. They mark the edge of runways and other places you are not allowed to enter on your own.
Context Anchor
Seen while taxiing on the airport surface, especially near runway entrances and other protected areas.
Derivation
Mandatory comes from the Latin mandatum, meaning a command or order. The sign is a command, not a suggestion — you must obey it.
Why Pilots Care
Ignoring these signs can cause runway incursions, creating immediate collision risk with landing or departing aircraft.
Grounding Statement
If the sign is red with white writing, treat it as a required stop-or-do-not-enter instruction until you are cleared or otherwise allowed to proceed.
Intuition Check
Do not read “mandatory instruction signs” as general airport advice signs. In this FAA context, they are required instructions that control whether an aircraft may enter or pass a specific point.
Example Sentence 1
The pilot held short at the mandatory instruction sign for Runway 27 and waited for takeoff clearance.
Example Sentence 2
During night operations the pilot used the red mandatory instruction signs to confirm the correct hold-short point before crossing.