Low Visibility Approaches
A working knowledge of approach lighting systems and regulations as they apply to the required visual references is essential to safe and successful approaches. Touchdown RVR is normally controlling for Category I, II, and III approaches. For Category I and II approaches, mid and rollout RVR are normally advisory. For Category III operations mid and rollout RVR may be controlling. In some countries, visibility is used instead of RVR. Approval from the regulatory agency is required to use visibility rather than RVR.
During Category I approaches, visual reference requirements typically specify that either the approach lights or other aids be clearly visible to continue below DA(H). During Category I and II approaches, descent below 100 ft. above touchdown zone elevation requires the red terminating bars or red side row bars (ALSF or Calvert lighting systems, or ICAO equivalent, if installed) to be distinctly visible. If actual touchdown RVR is at or above the RVR required for the approach, the runway environment (threshold, threshold lights and markings, touchdown zone, touchdown lights and markings) should become clearly visible resulting in a successful approach. After acquiring the red terminating bars or red side row bars, if the runway environment does not become distinctly visible execute an immediate missed approach.
Category III operations using fail passive autoland systems typically reach a DH of 50 ft. when approaching the threshold. In this instance, regulations require that the runway environment be clearly visible. If not, execute an immediate missed approach.
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5.20 FCT 737 (TM) October 31, 2006
737-600 - 737-900 Category III operations using fail operational autoland systems normally do not require specific visual references below AH.
A review of the approach and runway lighting systems available during the approach briefing is recommended as the pilot has only a few seconds to identify the lights required to continue the approach. For all low visibility approaches, a review of the airport diagram, expected runway exit, runway remaining lighting and expected taxi route during the approach briefing is recommended.
Regulatory agencies may require an additional 15% be added to the dry landing distance. Agencies may also require wind speed limitations less than maximum autoland wind speeds found in the FCOM.
AFDS System Configuration
The system configurations listed in this section may not include all of the systems and equipment required for each type of operation. The AFM or operating regulations may prescribe additional systems such as autobrakes, autothrottle or rain removal.
More detailed information concerning Category II and Category III operational requirements can be found in FAA advisory circulars or similar documents from other regulatory agencies.
Category II Operations
Category II approaches may be conducted using single or dual autopilots, or flight director only, with two engines. For single autopilot operation, the autopilot must be disengaged no lower than the minimum altitude listed in the Limitations Chapter of the FCOM. The autothrottles should be disconnected when the autopilot is disengaged.
Category II Approach Autopilot
The following equipment must be operative for an automatic approach requiring the use of Category II minima:
.
1 (or more) autopilots engaged
.
2 independent sources of electrical power. (The APU may be a substitute
source of power for the left or right electrical system.)
737-300 - 737-500
.
2 IRU's associated with the engaged autopilot in NAV mode
737-600 - 737-900
.
2 ADIRU's associated with the engaged autopilot in NAV mode
.
2 attitude indicators including attitude, radio altitude, ILS deviation,
DA(H) indication and AFDS status
.
both engines operating
.
FMA for each pilot.
Category II Approach Flight Director
The following equipment must be operative for a Flight Director (FD) approach requiring the use of Category II minima:
.
2 independent sources of electrical power. (The APU may be a substitute
source of power for the left or right electrical system.)
.
2 attitude indicators supplied by different symbol generators including
attitude, radio altitude, ILS deviation, DA(H) indication and AFDS status
.
2 separate flight directors, selected
737-300 - 737-500
.
2 IRU's in NAV mode
737-600 - 737-900
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