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Descending early results in more flight at low altitudes
with increased fuel consumption, and starting down late
results in problems controlling both airspeed and
descent rates on the approach. Top of descent (TOD)
from the en route phase of flight for high performance
airplanes is often used in this process and is calculated
manually or automatically through a flight management
system (FMS) [Figure 4-1], based upon the altitude of
Figure 4-1. Top of Descent and FMS Display.
PROGRESS 2 / 3
SPD / ALT CMD VS@TOD
240 / 3000 2400
TOC FUEL QTY
151 . 5NM / 00 + 23 20000
TOD GROSS WT
1022NM / 02 + 17 62850
AIR DATA FLT SUM
Top of
Descent
4-2
the approach gate. The approach gate is an imaginary
point used by ATC to vector aircraft to the final approach
course. The approach gate is established along the final
approach course 1 nautical mile (NM) from the final
approach fix (FAF) on the side away from the airport
and is located no closer than 5 NM from the landing
threshold. The altitude of the approach gate or initial
approach fix is subtracted from the cruise altitude, and
then the target rate of descent and groundspeed is
applied, resulting in a time and distance for TOD, as
depicted in Figure 4-1 on page 4-1.
Achieving an optimum stabilized, constant rate descent
during the arrival phase requires different procedures
for turbine-powered and reciprocating-engine airplanes. Controlling the airspeed and rate of descent is
important for a stabilized arrival and approach, and it
also results in minimum time and fuel consumption.
Reciprocating-engine airplanes require engine performance and temperature management for maximum
engine longevity, especially for turbocharged engines.
Pilots of turbine-powered airplanes must not exceed the
airplane’s maximum operating limit speed above
10,000 feet, or exceed the 250-knot limit below 10,000
feet. Also, consideration must be given to turbulence
that may be encountered at lower altitudes that may
necessitate slowing to the turbulence penetration speed.
If necessary, speed brakes should be used.
DESCENT PLANNING
Prior to flight, calculate the fuel, time, and distance
required to descend from your cruising altitude to the
approach gate altitude for the specific instrument
approach of your destination airport. In order to plan
your descent, you need to know your cruise altitude,
approach gate altitude or initial approach fix altitude,
descent groundspeed, and descent rate. Update this
information while in flight for changes in altitude,
weather, and wind. Your flight manual or operating
handbook may also contain a fuel, time, and distance to
descend chart that contains the same information. The
calculations should be made before the flight and “rules
of thumb” updates should be applied in flight. For example, from the charted STAR you might plan a descent
based on an expected clearance to “cross 40 DME West
of Brown VOR at 6,000” and then apply rules of thumb
for slowing down from 250 knots. These might include
planning your airspeed at 25 NM from the runway
threshold to be 250 knots, 200 knots at 20 NM, and 150
knots at 15 NM until gear and flap speeds are reached,
never to fall below approach speed.
The need to plan the IFR descent into the approach gate
and airport environment during the preflight planning
stage of flight is particularly important for turbojet powered airplanes. A general rule of thumb for initial IFR
descent planning in jets is the 3 to 1 formula. This means
that it takes 3 NM to descend 1,000 feet. If an airplane is
at flight level (FL) 310 and the approach gate or initial
approach fix is at 6,000 feet, the initial descent requirement equals 25,000 feet (31,000 - 6,000). Multiplying
25 times 3 equals 75; therefore begin descent 75 NM
from the approach gate, based on a normal jet airplane,
idle thrust, speed Mach 0.74 to 0.78, and vertical speed
of 1,800 - 2,200 feet per minute. For a tailwind adjustment, add 2 NM for each 10 knots of tailwind. For a
headwind adjustment, subtract 2 NM for each 10 knots
of headwind. During the descent planning stage, try to
determine which runway is in use at the destination airport, either by reading the latest aviation routine weather
report (METAR) or checking the automatic terminal
information service (ATIS) information. There can be
big differences in distances depending on the active runway and STAR. The objective is to determine the most
economical point for descent.
An example of a typical jet descent-planning chart is
depicted in Figure 4-2. Item 1 is the pressure altitude
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Instrument Procedures Handbook (IPH)仪表程序手册下(83)
