envelope, the largest combined absolute value for residual  static source error plus the avionics error may not exceed 200  feet. Aircraft with TCAS must have compatibility with RVSM  Operations.  Figure 5-9 illustrates the increase in aircraft  permitted between FL 180 and FL 410. Most noteworthy,  however, is the economization that aircraft can take advantage  of by the higher FLs being available to more aircraft.  The VSI in  Figure 5-10 is also called a vertical velocity  indicator (VVI), and was formerly known as a rate-of-climb  indicator. It is a rate-of-pressure change instrument that gives  an indication of any deviation from a constant pressure level. Inside the instrument case is an aneroid very much like the one  in an ASI. Both the inside of this aneroid and the inside of the  instrument case are vented to the static system, but the case  is vented through a calibrated orifice that causes the pressure  inside the case to change more slowly than the pressure inside  the aneroid. As the aircraft ascends, the static pressure becomes  lower. The pressure inside the case compresses the aneroid,  moving the pointer upward, showing a climb and indicating  the rate of ascent in number of feet per minute (fpm). When the aircraft levels off, the pressure no longer changes.  The pressure inside the case becomes equal to that inside  the aneroid, and the pointer returns to its horizontal, or  zero, position. When the aircraft descends, the static  pressure increases. The aneroid expands, moving the pointer  downward, indicating a descent.  The pointer indication in a VSI lags a few seconds behind the  actual change in pressure. However, it is more sensitive than  an altimeter and is useful in alerting the pilot of an upward or  downward trend, thereby helping maintain a constant altitude.  Some of the more complex VSIs, called instantaneous vertical  speed indicators (IVSI), have two accelerometer-actuated air  pumps that sense an upward or downward pitch of the aircraft  and instantaneously create a pressure differential. By the time  the pressure caused by the pitch acceleration dissipates, the  altitude pressure change is effective.  An ASI is a differential pressure gauge that measures the  dynamic pressure of the air through which the aircraft is  flying. Dynamic pressure is the difference in the ambient  static air pressure and the total, or ram, pressure caused by  the motion of the aircraft through the air. These two pressures  are taken from the pitot-static system. The mechanism of the ASI in  Figure 5-11  consists of a thin,  corrugated phosphor bronze aneroid, or diaphragm, that  receives its pressure from the pitot tube. The instrument  case is sealed and connected to the static ports. As the  pitot pressure increases or the static pressure decreases, the  diaphragm expands. This dimensional change is measured by  a rocking shaft and a set of gears that drives a pointer across  the instrument dial. Most ASIs are calibrated in knots, or  nautical miles per hour; some instruments show statute miles  per hour, and some instruments show both. Dynamic Pressure Type Instruments Airspeed Indicator (ASI) Vertical Speed Indicator (VSI) Figure 5-10. Rate of climb or descent in thousands of feet per minute. Figure 5-9. Increase in aircraft permitted between FL 180 and FL 410. FL Without RVSM With RVSM 7 Usable Flight Levels 13 Usable Flight Levels 410 400 390 380 370 360 350 340 330 320 310 300 290 5-8