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DESCENT RATES AND GLIDEPATHS FOR
NONPRECISION APPROACHES
Maximum Acceptable Descent Rates: Operational
experience and research have shown that a descent
rate of greater than approximately 1,000 FPM is unacceptable during the final stages of an approach (below
1,000 feet AGL). This is due to a human perceptual
limitation that is independent of the type of airplane
or helicopter. Therefore, the operational practices and
techniques must ensure that descent rates greater than
1,000 FPM are not permitted in either the instrument
or visual portions of an approach and landing operation.
For short runways, arriving at the MDA at the MAP
when the MAP is located at the threshold may require a
missed approach for some airplanes. For nonprecision
approaches a descent rate should be used that will
ensure that the airplane reaches the MDA at a distance
from the threshold that will allow landing in the touchdown zone. On many IAPs this distance will be annotated by a VDP. To determine the required rate of
descent, subtract the TDZE from the FAF altitude and
divide this by the time inbound. For example if the
FAF altitude is 2,000 feet MSL, the TDZE is 400 feet
MSL and the time inbound is two minutes, an 800
FPM rate of descent should be used.
To verify the airplane is on an approximate 3° glidepath, use a calculation of “300-foot-to 1 NM.” The
glidepath height above TDZE is calculated by multiplying the NM distance from the threshold by 300.
For example, at 10 NM the aircraft should be 3,000
feet above the TDZE, at 5 NM 1,500 feet, at 2 NM
600 feet, at 1.5 NM 450 feet, etc., until a safe landing
can be made. In the above example the aircraft should
arrive at the MDA (800 feet MSL) approximately 1.3
NM from the threshold and in a position to land in the
touchdown zone. Techniques for deriving a “300-to1” glidepath include using distance measuring equipment (DME), distance advisories provided by
radar-equipped control towers, RNAV (exclusive of
Omega navigation systems), GPS, dead reckoning,
and pilotage when familiar features on the approach
course are visible. The runway threshold should be
crossed at a nominal height of 50 feet above the
TDZE.
TRANSITION TO VISUAL
The transition from instrument flight to visual
flight during an instrument approach can be very
challenging, especially during low visibility operations. Additionally, single-pilot operations make
the transition even more challenging. Approaches
with vertical guidance add to the safety of the
transition to visual because the approach is
already stabilized upon visually acquiring the
required references for the runway. One hundred
to 200 feet prior to reaching the DA, DH, or MDA,
most of the PM’s attention should be outside of
the aircraft in order to visually acquire at least one
visual reference for the runway, as required by the
regulations. The PF should stay focused on the
instruments until the PM calls out any visual aids
that can be seen, or states “runway in sight.” The
PF should then begin the transition to visual
flight. It is common practice for the PM to call out
the V/S during the transition to confirm to the PF
that the instruments are being monitored, thus
allowing more of the PF’s attention to be focused
on the visual portion of the approach and landing.
Any deviations from the stabilized approach criteria should also be announced by the PM.
Single-pilot operations can be much more challenging because the pilot must continue to fly by the
instruments while attempting to acquire a visual
reference for the runway. While it is important for
both pilots of a two-pilot aircraft to divide their
attention between the instruments and visual references, it is even more critical for the single-pilot
operation. The flight visibility must also be at least
the visibility minimum stated on the instrument
approach chart, or as required by regulations. CAT
II and III approaches have specific requirements
that may differ from CAT I precision or nonprecision approach requirements regarding transition to
visual and landing. This information can be found
in the operator’s OpsSpecs or Flight Operations
Manual.
The visibility published on an approach chart is
dependent on many variables, including the height
above touchdown for straight-in approaches, or height
above airport elevation for circling approaches. Other
factors include the approach light system coverage, and
type of approach procedure, such as precision, nonprecision, circling or straight-in. Another factor determining the minimum visibility is the penetration of the
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Instrument Procedures Handbook (IPH)仪表程序手册下(119)
