approved aircraft may transit through RVSM airspace  provided they are given continuous climb throughout the  designated airspace, and 2,000 feet vertical separation is  provided at all times between the non-RVSM flight and all  others for the duration of the climb/descent.  Critics of the change were concerned that by reducing  the space between aircraft, RVSM may increase the  number of mid-air collisions and conflicts. In the ten  years since RVSM was first implemented, not one  collision has been attributed to RVSM. In the United  States, this program was known as the Domestic  Reduced Vertical Separation Minimum (DRVSM).  The term “cruise” may be used instead of “maintain” to  assign a block of airspace to an aircraft. The block extends  from the minimum IFR altitude up to and including the  altitude that is specified in the cruise clearance. On a cruise  clearance, you may level off at any intermediate altitude  within this block of airspace. You are allowed to climb or  descend within the block at your own discretion. However,  once you start descent and verbally report leaving an  altitude in the block to ATC, you may not return to that  altitude without an additional ATC clearance. A cruise  clearance also authorizes you to execute an approach at  the destination airport.  When the barometric pressure is 31.00 " Hg or less and  pilots are flying below 18,000 feet MSL, use the current  reported altimeter setting. When an aircraft is en route  on an instrument flight plan, air traffic controllers furnish  this information at least once while the aircraft is in the  controller’s area of jurisdiction. When the barometric  pressure exceeds 31.00 " Hg, the following procedures are  placed in effect by NOTAM defining the geographic area  affected: Set 31.00 " Hg for en route operations below  18,000 feet MSL and maintain this setting until beyond  the affected area. ATC issues actual altimeter settings and  advises pilots to set 31.00 " Hg in their altimeter, for en route  operations below 18,000 feet MSL in affected areas. If an  aircraft has the capability of setting the current altimeter  setting and operating into airports with the capability of  measuring the current altimeter setting, no additional  restrictions apply. At or above 18,000 feet MSL, altimeters  should be set to 29.92 " Hg (standard setting). Additional  procedures exist beyond the en route phase of flight.  The lowest usable flight level is determined by the  atmospheric pressure in the area of operation. As local  altimeter settings fall below 29.92 " Hg, pilots operating  in Class A airspace must cruise at progressively higher  indicated altitudes to ensure separation from aircraft  are available from weather reporting facilities operated or  approved by the U.S. National Weather Service, or a source  approved by the FAA. Some commercial operators have  the authority to act as a government-approved source of  weather information, including altimeter settings, through  certification under the FAA’s Enhanced Weather Information  System.  Flight level operations at or above 18,000 feet MSL require  the altimeter to be set to 29.92 inches of mercury (" Hg). A  flight level (FL) is defined as a level of constant atmospheric  pressure related to a reference datum of 29.92 " Hg. Each  flight level is stated in three digits that represent hundreds of  feet. For example, FL 250 represents an altimeter indication  of 25,000 feet. Conflicts with traffic operating below 18,000  feet MSL may arise when actual altimeter settings along the  route of flight are lower than 29.92 " Hg. Therefore, 14 CFR  Part 91, § 91.121 specifies the lowest usable flight levels for  a given altimeter setting range.  Reduced vertical separation minimums (RVSM) is a term  used to describe the reduction of the standard vertical  separation required between aircraft flying at levels  between FL 290 (29,000 feet) and FL 410 (41,000 feet)  from 2,000 feet to 1,000 feet. The purpose; therefore,  increases the number of aircraft that can safely fly in a  particular volume of airspace. Historically, standard vertical  separation was 1,000 feet from the surface to FL 290, 2,000  feet from FL 290 to FL 410 and 4,000 feet above this. This  was because the accuracy of the pressure altimeter (used  to determine altitude) decreases with height. Over time,  air data computers (ADCs) combined with altimeters have  become more accurate and autopilots more adept at  maintaining a set level; therefore, it became apparent that  for many modern aircraft, the 2,000-foot separation was not  required . It was, therefore, proposed by ICAO that this be  reduced to 1,000 feet.  Between 1997 and 2005, RVSM was implemented in all of  Europe, North Africa, Southeast Asia, North America, South  America, and over the North Atlantic, South Atlantic, and  Pacific Oceans. The North Atlantic implemented initially in  March 1997, at FL 330 through FL 370. The entire western  hemisphere implemented RVSM FL 290–FL 410 on January  20, 2005.  Only aircraft with specially certified altimeters and  autopilots may fly in RVSM airspace, otherwise the aircraft  must fly lower or higher than the airspace, or seek special  exemption from the requirements. Additionally, aircraft  operators (airlines or corporate operators) must receive  specific approval from the aircraft’s state of registry in  order to conduct operations in RVSM airspace. Non-RVSM  2-43  Lowest Usable Flight Level  Reduced Vertical Separation Minimums (RSVM)  Cruise Clearance 