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PART 135 OPERATORS
Unlike Part 91 operators, Part 135 operators may not
depart for a destination unless the forecast weather
there will allow an instrument approach and landing.
According to Part 135.219, flight crews and dispatchers
may only designate an airport as a destination if the latest weather reports or forecasts, or any combination of
them, indicate that the weather conditions will be at or
above IFR landing minimums at the estimated time of
arrival (ETA). This ensures that Part 135 flight crews
consider weather forecasts when determining the
suitability of destinations. Departures for airports
can be made when the forecast weather shows the
airport will be at or above IFR minimums at the
ETA, even if current conditions indicate the airport
to be below minimums. Conversely, Part 135.219
prevents departures when the first airport of intended
landing is currently above IFR landing minimums,
but the forecast weather is below those minimums at
the ETA.
Another very important difference between Part 91
and Part 135 operations is the Part 135 requirement
for airports of intended landing to meet specific
weather criteria once the flight has been initiated. For
Part 135, not only is the weather required to be forecast at or above IFR landing minimums for planning a
departure, but it also must be above minimums for initiation of an instrument approach and, once the
approach is initiated, to begin the final approach segment of an approach. Part 135.225 states that pilots
may not begin an instrument approach unless the
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latest weather report indicates that the weather conditions are at or above the authorized IFR landing
minimums for that procedure. Part 135.225 provides relief from this rule if the aircraft has already
passed the FAF when the weather report is received.
It should be noted that the controlling factor for
determining whether or not the aircraft can proceed
is reported visibility. Runway visual range (RVR),
if available, is the controlling visibility report for
determining that the requirements of this section
are met. The runway visibility value (RVV),
reported in statute miles (SM), takes precedent over
prevailing visibility. There is no required timeframe
for receiving current weather prior to initiating the
approach.
PART 121 OPERATORS
Like Part 135 operators, flight crews and dispatchers
operating under Part 121 must ensure that the appropriate weather reports or forecasts, or any combination
thereof, indicate that the weather will be at or above the
authorized minimums at the ETA at the airport to which
the flight is dispatched (Part 121.613). This regulation
attempts to ensure that flight crews will always be able
to execute an instrument approach at the destination
airport. Of course, weather forecasts are occasionally
inaccurate; therefore, a thorough review of current
weather is required prior to conducting an approach.
Like Part 135 operators, Part 121 operators are
restricted from proceeding past the FAF of an
instrument approach unless the appropriate IFR
landing minimums exist for the procedure. In addition, descent below the minimum descent altitude
(MDA), decision altitude (DA), or decision height
(DH) is governed, with one exception, by the same
rules that apply to Part 91 operators. The exception is
that during Part 121 and 135 operations, the airplane
is also required to land within the touchdown zone
(TDZ). Refer to the section titled Minimum Descent
Altitude, Decision Altitude, and Decision Height later
in this chapter for more information regarding MDA,
DA, and DH.
PERFORMANCE CONSIDERATIONS
All operators are required to comply with specific
airplane performance limitations that govern
approach and landing. Many of these requirements
must be considered prior to the origination of flight.
The primary goal of these performance considerations
is to ensure that the aircraft can remain clear of obstructions throughout the approach, landing, and go-around
phase of flight, as well as land within the distance
required by the FAA. Although the majority of in-depth
performance planning for an instrument flight is normally done prior to the aircraft’s departure, a general
review of performance considerations is usually
conducted prior to commencing an instrument
approach.
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AIRPLANE PERFORMANCE OPERATING
LIMITATIONS
Generally speaking, air carriers must have in place
an approved method of complying with Subpart I of
Parts 121 and 135 (Airplane Performance Operating
Limitations), thereby proving the airplane’s performance capability for every flight that it intends
　
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本文鏈接地址：Instrument Procedures Handbook (IPH)儀表程序手冊下(102)