曝光台 注意防骗
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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.
• ATC may ask you to descend to and maintain a
specific altitude. Generally, this clearance is for en
route traffic separation purposes, and you need to
respond to it promptly. Descend at the optimum
rate for your aircraft until 1,000 feet above the
assigned altitude, then descend at a rate between
500 and 1,500 feet per minute (FPM) to the
assigned altitude. If at any time, other than when
slowing to 250 KIAS at 10,000 feet MSL, you cannot descend at a rate of at least 500 FPM, advise
ATC.
• The second type of clearance allows you to
descend “… at pilot’s discretion.” When ATC
Figure 4-5. Instead of flying for a specific time after passing the holding fix, these holding patterns use distances to mark where the turns are made. The distances come from DME or IFR-certified GPS equipment.
". . . Bonanza 8394K, hold
northeast of the 16 DME
fix on the 030° radial of
the StedmanVORTAC,
five mile legs . . ."
". . . Viking 5786P, hold east of the 20
DME fix on the 265° radial of the Stedman
VORTAC, 5 mile legs . . ."
4-6
issues a clearance to descend at pilot’s discretion,
you may begin the descent whenever you choose
and at any rate you choose. You also are authorized to level off, temporarily, at any intermediate
altitude during the descent. However, once you
leave an altitude, you may not return to it.
A descent clearance may also include a segment where
the descent is at your discretion—such as “cross the
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Instrument Procedures Handbook (IPH)仪表程序手册上(90)
