An Alert Area depicted on an aeronautical chart indicates:
AAirspace permanently closed to all civil aircraft.
BAirspace with high volumes of pilot training or unusual aerial activities where all aircraft share equal collision avoidance responsibility.✓
CAirspace requiring ATC authorization for sUAS at all altitudes.
Why →Alert Areas are established to inform non-participating pilots of areas containing high volumes of student pilot training or other unusual activities. No authorization is required to enter, unlike Restricted or Prohibited Areas. Both civil and military aircraft share equal collision avoidance responsibility.
The trap →No clearance is required for Alert Areas; the designation is a warning, not a restriction. That does not make them safe. Student pilots flying unpredictably are a real hazard.
Field note →Alert Areas are common around flight training airports. For drone pilots, the heightened risk is from student pilots flying low, slow, and unpredictably, making visual scanning especially important near these areas.
SOURCE → FAA AIM Chapter 3CHECKED JUL 16ACS II.A.K3EASY