Definition
An automated function within the en route air traffic control system that monitors the altitude of aircraft under ATC control and alerts the controller when an aircraft is below, or is predicted to descend below, a predetermined minimum safe altitude for its position along the route or within the airspace sector.
Plain English
A computer tool used by en route air traffic controllers that sounds an alert if an aircraft drops too low — or looks like it's about to drop too low — for the terrain or obstacles in that area.
Context Anchor
You may hear about E MSAW in discussions of center controller radar systems, low-altitude alerts, and terrain safety during the en route portion of a flight.
Derivation
‘En Route’ refers to the cruise portion of flight handled by Air Route Traffic Control Centers (ARTCCs), as opposed to terminal areas. ‘Minimum Safe Altitude’ is the lowest altitude that provides required clearance from terrain and obstacles. The ‘E’ simply distinguishes this en route version from the terminal MSAW used by approach controllers.
Why Pilots Care
Gives controllers an early alert so they can issue corrective instructions and reduce the chance of controlled flight into terrain.
Grounding Statement
Picture an aircraft crossing rising terrain while a controller’s screen warns that the aircraft may not have enough altitude for the ground ahead.
Intuition Check
Do not read “safe” as a guarantee that the aircraft is safe. Here it means the system is comparing the aircraft’s altitude with a stored minimum altitude for that area and warning when the margin may be unsafe.
Example Sentence 1
When the aircraft descended through the assigned altitude, E MSAW triggered an alert at the Center, and the controller immediately issued a low altitude warning to the pilot.
Example Sentence 2
EMSAW operates automatically on all en route radar sectors to monitor aircraft altitudes against terrain and obstruction data.