mean sea level) is only 3,500 feet at Point B. The fact that  the altitude indication is not always true lends itself to the  memory aid, “When flying from hot to cold or from a high  to a low, look out below.”  [Figure 5-8] altitudes. 14 CFR part 91 requires the altitude transmitted by  the transponder to be within 125 feet of the altitude indicated  on the instrument used to maintain flight altitude. Below 31,000 feet, a 1,000 foot separation is the minimum  required between usable flight levels. Flight levels (FLs)  generally start at 18,000 feet where the local pressure is  29.92 "Hg or greater. All aircraft 18,000 feet and above use  a standard altimeter setting of 29.92 "Hg, and the altitudes  are in reference to a standard hence termed FL. Between FL  180 and FL 290, the minimum altitude separation is 1,000  feet between aircraft. However, for flight above FL 290  (primarily due to aircraft equipage and reporting capability;  potential error) ATC applied the requirement of 2,000 feet of  separation. FL 290, an altitude appropriate for an eastbound  aircraft, would be followed by FL 310 for a westbound  aircraft, and so on to FL 410, or seven FLs available for flight.  With 1,000-foot separation, or a reduction of the vertical  separation between FL 290 and FL 410, an additional six  FLs become available. This results in normal flight level and  direction management being maintained from FL 180 through  FL 410. Hence the name is Reduced Vertical Separation  Minimum (RVSM). Because it is applied domestically, it is  called United States Domestic Reduced Vertical Separation  Minimum (DRVSM). However, there is a cost to participate in the DRVSM program  which relates to both aircraft equipage and pilot training. For  example, altimetry error must be reduced significantly and  operators using RVSM must receive authorization from the  appropriate civil aviation authority. RVSM aircraft must meet  required altitude-keeping performance standards. Additionally,  operators must operate in accordance with RVSM policies/ procedures applicable to the airspace where they are flying. The aircraft must be equipped with at least one automatic  altitude control— It is not sufficient in the airspace system for only the pilot  to have an indication of the aircraft’s altitude; the air traffic  controller on the ground must also know the altitude of the  aircraft. To provide this information, the aircraft is typically  equipped with an encoding altimeter. When the ATC transponder is set to Mode C, the encoding  altimeter supplies the transponder with a series of pulses  identifying the flight level (in increments of 100 feet) at  which the aircraft is flying. This series of pulses is transmitted  to the ground radar where they appear on the controller’s  scope as an alphanumeric display around the return for the  aircraft. The transponder allows the ground controller to  identify the aircraft and determine the pressure altitude at  which it is flying. A computer inside the encoding altimeter measures the  pressure referenced from 29.92 "Hg and delivers this data to  the transponder. When the pilot adjusts the barometric scale  to the local altimeter setting, the data sent to the transponder  is not affected. This is to ensure that all Mode C aircraft are  transmitting data referenced to a common pressure level. ATC  equipment adjusts the displayed altitudes to compensate for  local pressure differences allowing display of targets at correct  That aircraft must be equipped with an altitude alert system  that signals an alert when the altitude displayed to the flight  crew deviates from the selected altitude by more than (in most  cases) 200 feet. For each condition in the full RVSM flight  Altimeter Enhancements (Encoding)  •  Within a tolerance band of ±65 feet about an acquired  altitude when the aircraft is operated in straight-and- level flight. •  Within a tolerance band of ±130 feet under no  turbulent, conditions for aircraft for which application  for type certification occurred on or before April 9,  1997 that are equipped with an automatic altitude  control system with flight management/performance  system inputs. 5-7 Reduced Vertical Separation Minimum (RVSM) Figure 3-9. Effects of nonstandard pressure on an altimeter. Figure 5-8. Effects of nonstandard pressure on an altimeter of an  aircraft flown into air of lower than standard pressure (air is less  dense). 28.86 "Hg 1,000 feet 27.82 "Hg 2,000 feet 26.82 "Hg 3,000 feet 25.84 "Hg 4,000 feet 24.90 "Hg 5,000 feet 28.4  28.5  28.3  28.2  30.0  29.9  29.8  29.92 "Hg