controlled flight into terrain (CFIT) incidents. However,  pilots should be aware that the published angle is for  information only − it is strictly advisory in nature. There is  no implicit additional obstacle protection below the MDA.  Pilots must still respect any published stepdown fixes and  the published MDA unless the visual cues stated 14 CFR  § 91.175 are present, and they can visually acquire and avoid both lit and unlit obstacles once below the MDA. The  presence of a VDA does not guarantee obstacle protection  in the visual segment and does not change any of the requirements for flying a nonprecision approach. Pilots may use the published angle and estimated/actual  groundspeed to find a target rate of descent from the rate  of descent table published in the back of the U.S. Terminal  Procedures Publication. This rate of descent can be flown  with the Vertical Velocity Indicator (VVI) in order to use  the VDA as an aid to flying a stabilized descent. No special  equipment is required.  In rare cases, the LNAV minima may have a lower HAT  than minima with a glide path, due to the location of the  obstacles and the nonprecision MAP. This should serve as  a clear indication to the pilot that obstacles exist below  the MDA, which must be seen in order to ensure adequate  clearance. In those cases, the glide path may be treated  as a VDA and used to descend to the LNAV MDA, as long  as all of the rules for a nonprecision approach are applied  at the MDA.  When there are obstacles in the visual area that could  cause an aircraft to destabilize the approach between  the MDA and touchdown, the IAP will not show a vertical  descent angle in the profile view. The charts currently  include the following statement: “Descent Angle NA” or  “Descent Angle NA-Obstacles” [Figure 4-11B ].  procedures also may have nonstandard features such as  nonstandard final approach course alignment, nonstandard  descent gradients, or other features that may or may not be  compatible with the conduct of EFVS operations.  Under § 91.176(a), operators who have been issued OpSpec  C073, MSpec MC073, or LOA C073, may conduct EFVS  operations to touchdown and rollout on certain vertical  navigation (VNAV) IAPs that use an MDA as a DA/DH in  accordance with C073. Additionally, §§ 91.176 and 91.189  permit an authorized EFVS operation to be conducted  during an authorized Category II or Category III operation.  Currently, EFVS operations in rotorcraft can be conducted  only on IAPs that are flown to a runway. Instrument approach  criteria, procedures, and appropriate visual references have  not yet been developed for straight-in landing operations  below DA/DH or MDA under IFR to heliports or platforms.  EFVS cannot be used in lieu of natural vision to descend  below published minimums on copter approaches to a  point-in-space (PinS) followed by a “proceed visual flight  rules (VFR)” visual segment, or on approaches designed  to a specific landing site using a “proceed visually” visual  segment.  One of the advantages of some GPS and multi-sensor FMS  RNAV avionics is the advisory VNAV capability. Traditionally,  the only way to get vertical path information during an  approach was to use a ground-based precision NAVAID.  Modern RNAV avionics can display an electronic vertical  path that provides a constant-rate descent to minimums.  Since these systems are advisory and not primary guidance,  the pilot must continuously ensure the aircraft remains at  or above any published altitude constraint, including step ­ down fix altitudes, using the primary barometric altimeter.  The pilots, aircraft, and operator must be approved to use  advisory VNAV inside the FAF on an instrument approach.  VNAV information appears on selected conventional  nonprecision, GPS, and RNAV approaches (see “Types of  Approaches” later in this chapter). It normally consists of  two fixes (the FAF and the landing runway threshold), a  FAF crossing altitude, a vertical descent angle (VDA), and  may provide a visual descent point (VDP) [Figure 4-11A].  The VDA provides the pilot with advisory information  not previously available on nonprecision approaches. It  provides a means for the pilot to establish a stabilized  descent from the FAF or step-down fix to the MDA.  Stabilized descent is a key factor in the reduction of  Vertical Navigation  4-23 Figure 4-11B. Descent Angle N/A.. 