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.  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.  Generally speaking, air carriers must have in place an  approved method of complying with Subpart I of 14  CFR Parts 121 and 135 (Airplane Performance Operating  Limitations), thereby proving the airplane’s performance  capability for every flight that it intends to make. Flight  crews must have an approved method of complying  with the approach and landing performance criteria in  the applicable regulations prior to departing for their  intended destination. The primary source of information for  performance calculations for all operators, including Part  91, is the approved Aircraft Flight Manual (AFM) or Pilot’s  Operating Handbook (POH) for the make and model of  aircraft that is being operated. It is required to contain the  manufacturer determined performance capabilities of the  aircraft at each weight, altitude, and ambient temperature  that are within the airplane’s listed limitations. Typically, the  AFM for a large turbine powered aircraft should contain  information that allows flight crews to determine that the  aircraft will be capable of performing the following actions,  considering the landing weight and other pertinent  environmental factor:  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 (14 CFR Part 121, § 121.613). This regulation  attempts to ensure that flight crews will always be able to  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 instrument flight rules (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. 14 CFR Part 135, § 135.225 states that pilots  may not begin an instrument approach unless the latest  weather report indicates that the weather conditions are  at or above the authorized IFR landing minimums for that  procedure. 14 CFR Part 135, § 135.225 provides relief from  this rule if the aircraft has already passed the final approach  fix (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. 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.  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  14 CFR Part 135, § 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, 14 CFR Part 135, § 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.  4-7  Aircraft Performance Operating Limitations  Weather Requirements and Part 121 Operators  Aircraft Performance Considerations  Weather Requirements and Part 135 Operators 