Figure 4-36. Terminal arrival area (TAA) design “basic T.”  of 250 feet obstacle clearance is also incorporated into the  final approach segment for straight-in approaches, and a  maximum 400-ft/NM descent gradient is permitted.  The approach design criteria are different for approaches  that use vertical guidance provided by a Baro-VNAV  system. Because the Baro-VNAV guidance is advisory and  not primary, Baro-VNAV approaches are not authorized in  areas of hazardous terrain, nor are they authorized when  a remote altimeter setting is required. Due to the inherent  problems associated with barometric readings and cold  temperatures, these procedures are also temperature  limited. Additional approach design criteria for RNAV  Approach Construction Criteria can be found in the  appropriate FAA Order 8260-series orders.  4-61 