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often is flown in level flight at cruising altitude, from top
of climb to top of descent (TOD). Generally, TOD is
used in airplanes with a flight management system
(FMS), and represents the point at which descent is first
initiated from cruise altitude. FMSs also assist in level
flight by cruising at the most fuel saving speed, providing continuing guidance along the flight plan route,
including great circle direct routes, and continuous evaluation and prediction of fuel consumption along with
changing clearance data. Descent planning is discussed
in more detail in the next chapter, “Arrivals.”
AIRCRAFT SPEED AND ALTITUDE
During the en route descent phase of flight, an additional benefit of flight management systems is that the
FMS provides fuel saving idle thrust descent to your
destination airport. This allows an uninterrupted profile
descent from level cruising altitude to an appropriate minimum IFR altitude (MIA), except where
level flight is required for speed adjustment.
Controllers anticipate and plan that you may level off at
10,000 feet MSL on descent to comply with the Part 91
indicated airspeed limit of 250 knots. Leveling off at
any other time on descent may seriously affect air traffic handling by ATC. It is imperative that you make
every effort to fulfill ATC expected actions on descent
to aid in safely handling and expediting air traffic.
ATC issues speed adjustments if you are being radar
controlled to achieve or maintain required or desired
spacing. They express speed adjustments in terms of
knots based on indicated airspeed in 10 knot increments except that at or above FL 240 speeds may be
expressed in terms of Mach numbers in 0.01 increments. The use of Mach numbers by ATC is restricted
to turbojets. If complying with speed adjustments,
pilots are expected to maintain that speed within plus
or minus 10 knots or 0.02 Mach.
Speed and altitude restrictions in clearances are subject
to misinterpretation, as evidenced in this case where a
corporate flight crew treated instructions in a published
procedure as a clearance. “…We were at FL 310 and
had already programmed the ‘expect-crossing 慬瑩瑵摥
3-23
of 17,000 feet at the VOR. When the altitude alerter
sounded, I advised Center that we were leaving FL 310.
ATC acknowledged with a ‘Roger.’ At FL 270, Center
quizzed us about our descent. I told the controller we
were descending so as to cross the VOR at 17,000 feet.
ATC advised us that we did not have clearance to
descend. What we thought was a clearance was in fact
an ‘expect’ clearance. We are both experienced
pilots…which just means that experience is no substitute for a direct question to Center when you are in
doubt about a clearance. Also, the term ‘Roger’ only
means that ATC received the transmission, not that they
understood the transmission. The AIM indicates that
‘expect’ altitudes are published for planning purposes.
‘Expect’ altitudes are not considered crossing restrictions until verbally issued by ATC.”
HOLDING PROCEDURES
The criteria for holding pattern airspace is developed
both to provide separation of aircraft, as well as obstacle
clearance The alignment of holding patterns typically
coincides with the flight course you fly after leaving the
holding fix. For level holding, a minimum of 1,000 feet
obstacle clearance is provided throughout the primary
area. In the secondary area 500 feet of obstacle clearance
is provided at the inner edge, tapering to zero feet at the
outer edge. Allowance for precipitous terrain is considered, and the altitudes selected for obstacle clearance
may be rounded to the nearest 100 feet. When criteria for
a climb in hold are applied, no obstacle penetrates the
holding surface. [Figure 3-27]
There are many factors that affect aircraft during holding maneuvers, including navigational aid ground and
airborne tolerance, effect of wind, flight procedures,
application of air traffic control, outbound leg length,
maximum holding airspeeds, fix to NAVAID distance,
DME slant range effect, holding airspace size, and
altitude holding levels. In order to allow for these factors when establishing holding patterns, procedure
specialists must apply complex criteria contained in
Order 7130.3, Holding Pattern Criteria.
ATC HOLDING INSTRUCTIONS
When controllers anticipate a delay at a clearance limit
or fix, pilots will usually be issued a holding clearance
at least five minutes before the ETA at the clearance
limit or fix. If the holding pattern assigned by ATC is
depicted on the appropriate aeronautical chart, pilots
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Instrument Procedures Handbook (IPH)仪表程序手册上(76)
