Definition
An automated function of the air traffic control computer system used by en route controllers (those handling aircraft between terminal areas) that generates an aural and visual alert when a tracked aircraft is, or is predicted to be, below a predetermined minimum safe altitude for the airspace it is flying through. The alert prompts the controller to advise the pilot and confirm a safe altitude.
Plain English
It is a computer alarm in the en route controller's system that warns the controller when an airplane is flying too low, or is about to be too low, for the area it is in. The controller then calls the pilot to check the altitude.
Context Anchor
Seen in Pilot/Controller Glossary and air traffic control discussions of safety alerts during the en route portion of a flight.
Derivation
The 'E' marks this as the en route version, used by Air Route Traffic Control Centers, as opposed to the terminal version (MSAW) used in the approach and tower environment. 'Minimum Safe Altitude' is the lowest altitude judged safe for the airspace, accounting for terrain and obstacles.
Why Pilots Care
Helps prevent controlled flight into terrain by providing early warning to controllers who can then advise the pilot.
Intuition Check
Do not read E-MSAW as a cockpit warning system or as a guarantee of terrain clearance. It is an air traffic control alert that helps controllers notice when an aircraft may be too low.
Example Sentence 1
Center called us with a low altitude alert, almost certainly triggered by E-MSAW, and we immediately verified our altimeter setting and climbed back to the assigned altitude.
Example Sentence 2
E-MSAW systems monitor IFR flights across en route airspace to maintain safe terrain clearance.