Figure 2-11.  Center radar displays. Figure 2-12.  A center controller’s scope. Figure 2-13.  A/FD center frequencies listing. Another common Center clearance is “Leaving (altitude)  fly (heading) or proceed direct when able.” This keeps the  terrain/obstruction clearance responsibility in the flight deck  until above the minimum IFR altitude. A controller cannot  issue an IFR clearance until an aircraft is above the minimum  IFR altitude unless it is able to climb in VFR conditions. On a Center controller’s scope, 1 NM is about  1 ⁄28 of an inch.  When a Center controller is providing Approach/Departure  control services at an airport many miles from the radar  antenna, estimating headings and distances is very difficult.  Controllers providing vectors to final must set the range on  their scopes to not more than 125 NM to provide the greatest  possible accuracy for intercept headings. Accordingly, at  locations more distant from a Center radar antenna, pilots  should expect a minimum of vectoring. A common clearance in these situations is “When able,  proceed direct to the Astoria VOR…” The words “when able”  mean to proceed to the waypoint, intersection, or NAVAID  when the pilot is able to navigate directly to that point using  onboard available systems providing proper guidance, usable  signal, etc. If provided such guidance while flying VFR, the  pilot remains responsible for terrain and obstacle clearance.  Using the standard climb gradient, an aircraft is 2 miles  from the departure end of the runway before it is safe to  turn (400 feet above ground level (AGL)). When a Center  controller issues a heading, a direct route, or says “direct  when able,” the controller becomes responsible for terrain  and obstruction clearance. 2-10