AIM
4/3/14
5−4−20
Arrival Procedures
acquire and avoid 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.
2.
Additional protection for the visual segment
below the MDA is provided if a VDP is published and
descent below the MDA is started at or after the VDP.
Protection is also provided if a Visual Glide Slope
Indicator (VGSI); e.g., VASI or PAPI, is installed and
the aircraft remains on the VGSI glide path angle
from the MDA. In either case, a chart note will
indicate if the VDP or VGSI are not coincident with
the VDA. On RNAV approach charts, a small shaded
arrowhead shaped symbol (see the legend of the U.S.
Terminal Procedures books, page H1) from the end of
the VDA to the runway indicates that the 34:1 visual
surface is clear.
3.
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.
4.
Since one of the reasons for publishing a circ-
ling only instrument landing procedure is that the
descent rate required exceeds the maximum allowed
for a straight in approach, circling only procedures
may have VDAs which are considerably steeper than
the standard 3 degree angle on final. In this case, the
VDA provides the crew with information about the
descent rate required to land straight in from the FAF
or step down fix to the threshold. This is not intended
to imply that landing straight ahead is recommended,
or even possible, since the descent rate may exceed
the capabilities of many aircraft. The pilot must
determine how to best maneuver the aircraft within
the circling obstacle clearance area in order to land.
5.
In rare cases the LNAV minima may have a
lower HAT than minima with a glide path due to the
location of the obstacles. This should be a clear indic-
ation to the pilot that obstacles exist below the MDA
which the pilot must see 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 the rules for a nonprecision
approach are applied at the MDA. However, the pilot
must keep in mind the information in this paragraph
j. Pilot Operational Considerations When
Flying Nonprecision Approaches.
The missed
approach point (MAP) on a nonprecision approach
is not designed with any consideration to where
the aircraft must begin descent to execute a safe
landing. It is developed based on terrain, obstruc-
tions, NAVAID location and possibly air traffic
considerations. Because the MAP may be located
anywhere from well prior to the runway threshold to
past the opposite end of the runway, the descent from
the Minimum Descent Altitude (MDA) to the runway
threshold cannot be determined based on the MAP
location. Descent from MDA at the MAP when the
MAP is located close to the threshold would require
an excessively steep descent gradient to land in the
normal touchdown zone. Any turn from the final
approach course to the runway heading may also be
a factor in when to begin the descent.
1.
Pilots are cautioned that descent to a
straight−in landing from the MDA at the MAP may
be inadvisable or impossible, on a nonprecision
approach, even if current weather conditions meet the
published ceiling and visibility. Aircraft speed, height
above the runway, descent rate, amount of turn and
runway length are some of the factors which must be
considered by the pilot to determine if a landing can
be accomplished.
2.
Visual descent points (VDPs) provide pilots
with a reference for the optimal location to begin
descent from the MDA, based on the designed
vertical descent angle (VDA) for the approach
procedure, assuming required visual references are
available. Approaches without VDPs have not been
assessed for terrain clearance below the MDA, and
may not provide a clear vertical path to the runway at
the normally expected descent angle. Therefore,
pilots must be especially vigilant when descending
below the MDA at locations without VDPs. This does
not necessarily prevent flying the normal angle; it
only means that obstacle clearance in the visual
segment could be less and greater care should be
exercised in looking for obstacles in the visual
segment. Use of visual glide slope indicator (VGSI)
systems can aid the pilot in determining if the aircraft
is in a position to make the descent from the MDA.
However, when the visibility is close to minimums,
the VGSI may not be visible at the start descent point