when it is not located at the runway threshold.  RNAV encompasses a variety of underlying navigation  systems and, therefore, approach criteria. This results in  different sets of criteria for the final approach segment  of various RNAV approaches. RNAV instrument approach  criteria address the following procedures:  The original GPS approach procedures provided  authorization to fly non-precision approaches based  on conventional, ground-based NAVAIDs. Many of  these approaches have been converted to stand-alone  approaches, and the few that remain are identified by  the name of the procedure and “or GPS.” These GPS non- precision approaches are predicated upon the design  criteria of the ground-based NAVAID used as the basis of  the approach. As such, they do not adhere to the RNAV  design criteria for stand-alone GPS approaches, and are not  considered part of the RNAV (GPS) approach classification  for determining design criteria. [Figure 4-38]  The number of GPS stand-alone approaches continues to  decrease as they are replaced by RNAV approaches. RNAV  (GPS) approaches are named so that airborne navigation  databases can use either GPS or RNAV as the title of the  approach. This is required for non-GPS approach systems,  such as VOR/DME based RNAV systems. In the past,  these approaches were often referred to as "stand-alone  GPS" approaches. They are considered non-precision  approaches, offering only LNAV and circling minimums.  Precision minimums are not authorized, although LNAV/  VNAV minimums may be published and used as long as  the on-board system is capable of providing approach  approved VNAV. The RNAV (GPS) Runway 14 approach for  Lincoln, Nebraska, incorporates only LNAV and circling  minimums. [Figure 4-39]  For a non-vertically guided straight-in RNAV (GPS)  approach, the final approach course must be aligned within  15° of the extended runway centerline. The final approach  segment should not exceed 10 NM, and when it exceeds 6  NM, a stepdown fix is typically incorporated. A minimum  er altitude without an approach clearance, request the  lower TAA altitude from ATC. Pilots not sure of the clear­ ance should confirm their clearance with ATC or request a  specific clearance. Pilots entering the TAA with two−way  radio communications failure (14 CFR § 91.185, IFR Op­ erations: Two−way Radio Communications Failure), must  maintain the highest altitude prescribed by 14 CFR §  91.185(c)(2) until arriving at the appropriate IAF.  Once cleared for the approach, pilots may descend in the  TAA sector to the minimum altitude depicted within the  defined area/subdivision, unless instructed otherwise by  air traffic control. Pilots should plan their descent within  the TAA to permit a normal descent from the IF/IAF to the  FAF.  U.S. Government charts depict TAAs using icons located  in the plan view outside the depiction of the actual ap­ proach procedure. Use of icons is necessary to avoid ob­ scuring any portion of the “T” procedure (altitudes, cours­ es, minimum altitudes, etc.). The icon for each TAA area  will be located and oriented on the plan view with respect  to the direction of arrival to the approach procedure, and  will show all TAA minimum altitudes and sector/radius  subdivisions. The IAF for each area of the TAA is included  on the icon where it appears on the approach to help the  pilot orient the icon to the approach procedure. The IAF  name and the distance of the TAA area boundary from the  IAF are included on the outside arc of the TAA area icon.  TAAs may be modified from the standard size and shape  to accommodate operational or ATC requirements. Some  areas may be eliminated, while the other areas are ex­ panded. The “T” design may be modified by the proce­ dure designers where required by terrain or ATC consider­ ations. For instance, the “T” design may appear more like  a regularly or irregularly shaped “Y,” an upside down “L,”  or an “I.”  When an airway does not cross the lateral TAA boundar­ ies, a feeder route will be established from an airway fix  or NAVAID to the TAA boundary to provide a transition  from the en route structure to the appropriate IAF. Each  feeder route will terminate at the TAA boundary and will  be aligned along a path pointing to the associated IAF.  Pilots should descend to the TAA altitude after crossing  the TAA boundary and cleared for the approach by ATC.  Each waypoint on the “T” is assigned a pronounceable 5−  letter name, except the missed approach waypoint. These  names are used for ATC communications, RNAV data­ bases, and aeronautical navigation products. The missed  approach waypoint is assigned a pronounceable name  4-60  RNAV Approach Types  GPS Stand-Alone/RNAV (GPS) Approach  GPS Overlay of Nonprecision Approach  •  GPS overlay of pre-existing nonprecision approaches.  •  VOR/DME based RNAV approaches.  •  Stand-alone RNAV (GPS) approaches.  •  RNAV (GPS) approaches with vertical guidance (APV).  •  RNAV (GPS) precision approaches (WAAS and LAAS). 