installed in an aircraft, and what it is approved to do. It is  also important to understand that the VNAV function of  non-WAAS capable or non-WAAS equipped IFR-approved  GPS receivers does not make the aircraft capable of flying  approaches to LNAV/VNAV minimums.  FMS are the primary tool for most modern aircraft, air  carriers, and any operators requiring performance based  navigation. Most of the modern FMS are fully equipped  with LNAV/VNAV and WAAS. The FMS provides flight control  steering and thrust guidance along the VNAV path. Some  less integrated systems may only advise the flight crew  of the VNAV path but have no auto-throttle capability.  These less integrated systems require an increase in pilot  workload during the arrival/approach phase in order to  maintain the descent path.  The need to plan the IFR descent into the approach gate  and airport environment during the preflight planning  stage of flight is particularly important for turbojets. TOD  from the en route phase of flight for high performance  aircraft is often used in this process and is calculated  manually or automatically through a FMS based upon the  altitude of the approach gate. A general rule of thumb for  initial IFR descent planning in jets is the 3 to 1 formula.  This means that it takes 3 NM to descend 1,000 feet. If  an airplane is at FL 310 and the approach gate or initial  approach fix is at 6,000 feet, the initial descent requirement  equals 25,000 feet (31,000–6,000). Multiplying 25 times  3 equals 75; therefore, begin descent 75 NM from the  approach gate, based on a normal jet airplane, idle thrust,  speed Mach 0.74 to 0.78, and vertical speed of 1,800–2,200  fpm. For a tailwind adjustment, add 2 NM for each 10 knots  of tailwind. For a headwind adjustment, subtract 2 NM for  each 10 knots of headwind. During the descent planning  stage, try to determine which runway is in use at the  destination airport, either by reading the latest aviation  routine weather report (METAR) or checking the automatic  terminal information service (ATIS) information. There can  be big differences in distances depending on the active  runway and STAR. The objective is to determine the most  economical point for descent.  An example of a typical jet descent-planning chart is  depicted in Figure 3-7. Item 1 is the pressure altitude from  which the descent begins; item 2 is the time required  for the descent in minutes; item 3 is the amount of fuel  consumed in pounds during descent to sea level; and item  4 is the distance covered in NM. Item 5 shows that the chart  is based on a Mach .80 airspeed until 280 knots indicated  airspeed (KIAS) is obtained. The 250 knot airspeed limitation  To make use of WAAS; however, the aircraft must be  equipped with an IFR-approved GPS receiver with WAAS  signal reception that integrates WAAS error correction  signals into its position determining processing. The WAAS  enabled GPS receiver [Figure 3-5] allows the pilot to load an  RNAV approach and receive guidance along the lateral and  vertical profile shown on the approach chart. [Figure 3-6]  It is very important to know what kind of equipment is  3-6  Note: WAAS makes use of a collection of ground stations  that are used to detect and correct inaccuracies in the  position information derived from the GPS. Using WAAS,  the accuracy of vertical position information is increased  to within three meters.  Lateral navigation/vertical navigation (LNAV/VNAV)  equipment is similar to an instrument landing system (ILS)  in that it provides both lateral and vertical approach course  guidance. Since precise vertical position information is  beyond the current capabilities of the GPS, approaches with  LNAV/VNAV minimums make use of certified barometric  VNAV (baro-VNAV) systems for vertical guidance and/or  the wide area augmentation system (WAAS) to improve  GPS accuracy for this purpose.  The two types of VNAV paths that the FMS use is either a  performance path or a geometric path. The performance  path is computed using at idle or near idle power from  the TOD to the first constrained waypoint. [Figure 3-3]  The geometric path is computed from point to point  between two constrained waypoints or when on an  assigned vertical angle. The geometric path is shallower  than the performance path and is typically a non-idle  path. [Figure 3-4]  Descent Planning for High Performance Aircraft  LNAV/VNAV Equipment  Figure 3-5. WAAS data provide lateral and vertical guidance.  GPS  ENR  FLAGGED  119.000  121.600  114.10  116.00  1200  A  C  T  A  C  T  S  B  Y  S  B  Y  S  B  Y  --.- vor  MSG PROC  FPL  NAV  ETE GS DIST BRG 120 kt  04:52  9.74 n  m  332° TRK 332°  Lnav/Vnav  C345  SUNOL  RW34Lm  DTK 346°  This GPS/RNAV provides lateral and vertical  guidance during an RNAV approach. 