The Federal Aviation Administration needs to urgently address aging air traffic control systems that raise safety risks and in some cases might not be upgraded for a decade or more, the U.S. Government Accountability Office said Monday.

The non-partisan GAO said the FAA had assessed that of its 138 systems, 51, or 37%, were unsustainable and 54, or 39%, were potentially unsustainable.

The FAA has 64 ongoing investments aimed at modernizing 90 of the 105 systems but "has been slow to modernize the most critical and at-risk systems."