Definition
An automated function of the en route air traffic control computer system that alerts the controller when a tracked aircraft is below, or is predicted to descend below, a predetermined minimum safe altitude for the area in which it is operating. When triggered, the controller receives a visual and aural warning so that corrective instructions can be issued to the pilot.
Plain English
A safety alarm built into the en route ATC computer. If a plane drops too low, or looks like it's about to, the controller's screen flashes and beeps so they can call the pilot and get them back up to a safe height.
Context Anchor
Pilots may encounter this term in air traffic control and terrain-clearance discussions. The warning appears to controllers, not as a cockpit alert to the pilot.
Derivation
"En route" comes from French, meaning "on the way" — used in aviation for the cruise portion of flight between departure and arrival phases. "Minimum safe altitude" is plain English: the lowest altitude at which the aircraft can fly safely clear of terrain and obstacles in that area. Together the term describes a warning system that watches altitudes during the en route phase.
Why Pilots Care
Gives controllers an immediate cue to issue a safety alert, helping prevent controlled flight into terrain during the en route phase.
Intuition Check
Do not read “minimum safe” as a guarantee that the aircraft is safe. Here it means the system has compared the aircraft’s altitude with a protected altitude threshold and may warn the controller if the aircraft is too low.
Example Sentence 1
The controller issued a low altitude alert after the E MSAW system flagged the aircraft descending below the minimum safe altitude for that sector.
Example Sentence 2
EMSAW monitoring continues throughout the en route segment until the aircraft enters the next terminal area.