Definition
An automated function of the air traffic control computer system that alerts the controller when a tracked aircraft under their control is below, or is predicted to go below, a predetermined minimum safe altitude along its en route path. The system compares the aircraft's reported altitude against terrain and obstruction clearance data for the route segment and triggers an aural and visual alert at the controller's position when a conflict is detected.
Plain English
A computer safety net at the controller's radar screen. If a plane in cruise flight gets too close to the ground or a tall obstacle, the system flashes a warning and sounds an alert so the controller can tell the pilot to climb.
Context Anchor
You may encounter E-MSAW in FAA glossary material, Air Traffic Control discussions, and situations where a controller issues an urgent low-altitude safety instruction.
Derivation
The 'E' stands for En Route, meaning the cruise phase of flight between airports. MSAW (Minimum Safe Altitude Warning) was originally developed for terminal areas around airports; the 'E-' prefix extends the same protection out into the en route environment.
Why Pilots Care
The system protects against controlled flight into terrain by prompting controllers to issue an immediate altitude advisory to the pilot.
Intuition Check
Do not assume E-MSAW is a warning shown in the cockpit. It is primarily an Air Traffic Control system alert that prompts the controller to warn or instruct the pilot.
Example Sentence 1
When the aircraft descended below the minimum en route altitude, E-MSAW triggered an alert at the controller's scope and she immediately instructed the pilot to climb.
Example Sentence 2
E-MSAW monitoring continues throughout the flight until the aircraft is handed off to the next facility.