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• Descent Groundspeed: 155 knots
Subtract 2,100 feet from 17,000 feet, which equals
14,900 feet. Divide this number by 1,500 feet per
minute, which equals 9.9 minutes, round this off to 10
minutes. Using your flight computer, find the distance
required for the descent by using the time of 10 minutes
and the groundspeed of 155 knots. This gives you a distance of 25.8 NM. You need to begin your descent
approximately 26 NM prior to arriving at your destination airport approach gate.
CRUISE CLEARANCE
The term "cruise" may be used instead of "maintain" to
assign a block of airspace to an aircraft. The block
extends from the minimum IFR altitude up to and
including the altitude that is specified in the cruise
clearance. On a cruise clearance, you may level off at
any intermediate altitude within this block of airspace.
You are allowed to climb or descend within the block at
your own discretion. However, once you start descent
and verbally report leaving an altitude in the block to
ATC, you may not return to that altitude without an
additional ATC clearance. A cruise clearance also
authorizes you to execute an approach at the destination
airport. When operating in uncontrolled airspace on a
cruise clearance, you are responsible for determining
the minimum IFR altitude. In addition, your descent
and landing at an airport in uncontrolled airspace are
governed by the applicable visual flight rules (VFR)
and/or Operations Specifications (OpsSpecs), i.e., CFR,
91.126, 91.155, 91.175, 91.179, etc.
HOLDING PATTERNS
If you reach a clearance limit before receiving a further
clearance from ATC, a holding pattern is required at
your last assigned altitude. Controllers assign holds for
a variety of reasons, including deteriorating weather or
high traffic volume. Holding might also be required following a missed approach. Since flying outside the area
set aside for a holding pattern could lead to an encounter
with terrain or other aircraft, you need to understand the
size of the protected airspace that a holding pattern provides.
Each holding pattern has a fix, a direction to hold from
the fix, and an airway, bearing, course, radial, or route on
which the aircraft is to hold. These elements, along with
the direction of the turns, define the holding pattern.
Since the speed of the aircraft affects the size of a holding pattern, maximum holding airspeeds have been
Figure 4-4. Descent Preflight Planning
4-5
designated to limit the amount of airspace that must be
protected. The three airspeed limits are shown in
Figure 3-31 in Chapter 3 of this book. Some holding
patterns have additional airspeed restrictions to keep
faster airplanes from flying out of the protected area.
These are depicted on charts by using an icon and the
limiting airspeed.
Distance-measuring equipment (DME) and IFR-certified global positioning system (GPS) equipment offer
some additional options for holding. Rather than being
based on time, the leg lengths for DME/GPS holding
patterns are based on distances in nautical miles. These
patterns use the same entry and holding procedures as
conventional holding patterns. The controller or the
instrument approach procedure chart will specify the
length of the outbound leg. The end of the outbound
leg is determined by the DME or the along track distance (ATD) readout. The holding fix on conventional
procedures, or controller-defined holding based on a
conventional navigation aid with DME, is a specified
course or radial and distances are from the DME station for both the inbound and outbound ends of the
holding pattern. When flying published GPS overlay or
standalone procedures with distance specified, the
holding fix is a waypoint in the database and the end of
the outbound leg is determined by the ATD. Instead of
using the end of the outbound leg, some FMSs are programmed to cue the inbound turn so that the inbound
leg length will match the charted outbound leg length.
Normally, the difference is negligible, but in high
winds, this can enlarge the size of the holding pattern.
Be sure you understand your aircraft’s FMS holding
program to ensure that the holding entry procedures
and leg lengths match the holding pattern. Some situations may require pilot intervention in order to stay
within protected airspace. [Figure 4-5]
DESCENDING FROM THE EN ROUTE
ALTITUDE
As you near your destination, ATC issues a descent
clearance so that you arrive in approach control’s airspace at an appropriate altitude. In general, ATC issues
either of two basic kinds of descent clearances.
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Instrument Procedures Handbook (IPH)仪表程序手册下(85)
