Figure 2-6. Boston Air Route Traffic Control Center.  Figure 2-7. Flight progress strips.  2-4  21 ARTCC’s in the United States. [Figure 2-5] Any aircraft  operating under IFR within the confines of an ARTCC’s  airspace is controlled by air traffic controllers at the Center.  This includes all sorts of different types of aircraft: privately  owned single engine aircraft, commuter airlines, military  jets, and commercial airlines.  The largest component of the NAS is the ARTCC. Each  ARTCC covers thousands of square miles encompassing  all or part of several states. ARTCCs are built to ensure safe  and expeditious air travel. All Centers operate 7-days a  week, 24-hours a day, and employ a combination of several  hundred ATC specialists, electronic technicians, computer  system specialists, environmental support specialists, and  administrative staff. Figure 2-6 is an example of the Boston  ARTCC. The green lines mark the boundaries of the Boston  Center area, and the red lines mark the boundaries of  Military Operations Areas (MOAs), Prohibited, Restricted,  Alert, and Warning Areas.  The primary means of controlling aircraft is accomplished  by using highly sophisticated computerized radar systems.  In addition, the controller maintains two-way radio  communication with aircraft in his or her sector. In this  way, the specialist ensures that the aircraft are separated  by the following criteria:  Safe Separation Standards  The strips, one for each en route point from which the  pilot reports his or her position, are posted on a slotted  board in front of the air traffic controller. [Figure 2-7] At a  glance, he or she is able to see certain vital data: the type  of aircraft and who is flying it (airline, business, private,  or military pilot), aircraft registration number or flight  number, route, speed, altitude, airway designation, and the  estimated time of arrival (ETA) at destination. As the pilot  calls in the aircraft’s position and time at a predetermined  location, the strips are removed from their slots and filed.  Any change from the original flight plan is noted on the  strips as the flight continues. Thus, from a quick study of  the flight progress board, a controller can assess the overall  traffic situation and can avoid possible conflicts.  The controllers can accomplish this separation by issuing  instructions to the pilots of the aircraft involved. Altitude  assignments, speed adjustments, and radar vectors are  examples of instructions that might be issued to aircraft.  En route control is handled by pinpointing aircraft positions  through the use of flight progress strips. These strips are  pieces of printed paper containing pertinent information  extracted from the pilot’s flight plan. These strips are printed  20 minutes prior to an aircraft reaching each Center’s sector.  A flight progress strip tells the controller everything needed  to direct that aircraft. If the flight progress strips of each  aircraft approaching a sector are arranged properly, it is  possible to determine potential conflicts long before the  aircraft are even visible on the Center controller’s display.  In areas where radar coverage is not available, this is the  sole means of separating aircraft.  • Laterally—5 miles  • Vertically—  • 1,000 feet (if the aircraft is below FL 290, or  between FL 290 and FL 410 for RVSM compliant  aircraft) • 2,000 feet (if the aircraft is at FL 290 or above) 