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时间:2011-04-18 01:03来源:蓝天飞行翻译 作者:航空
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24.2.2 Circling Approach Procedure. An IAP with a circle.to.land maneuver or circle.to.land minimums does not meet criteria for straight.in landing minimums. While the regulations do not prohibit EFVS from being used during any phase of flight, they do prohibit it from being used for operational credit on anything but a straight.in IAP with straight.in landing minima. EFVS shall only be used during a circle.to.land maneuver provided the visual references required throughout the circling maneuver are distinctly visible using natural vision. An EFVS cannot be used to satisfy the requirement that an identifiable part of the airport be distinctly visible to the pilot during a circling maneuver at or above MDA or while descending below MDA from a circling maneuver.
24.2.3 Enhanced Flight Visibility. Flight visibility is determined by using natural vision, and enhanced flight visibility (EFV) is determined by using an EFVS. 14 CFR part 91.175 (l) requires that the EFV observed by using an EFVS cannot be less than the visibility prescribed in the IAP to be used in order to continue to descend below the DA or MDA.
24.3 EFVS Operations At or Below DA or MDA Down to 100 Feet Above the TDZE. The visual segment of an IAP begins at DA or MDA and continues to the runway. There are two means of operating in the visual segment..one is by using natural vision and the other is by using an EFVS. If the pilot determines that the EFV observed by using the EFVS is not less than the minimum visibility prescribed in the IAP being flown, and the pilot acquires the required visual references prescribed in 14 CFR part 91.175 (l)(3) using the EFVS, then the pilot can continue the approach to 100 feet above the TDZE. To continue the approach, the pilot uses the EFVS image to visually acquire the runway environment (the approach light system (ALS), if installed, or both the runway threshold and the TDZ), confirm lateral alignment, maneuver to the extended runway centerline earlier than would otherwise be possible, and continue a normal descent from the DA or MDA to 100 feet above the TDZE.
24.3.1 Required Visual References. In order to descend below DA or MDA, the following visual references (specified in 14 CFR part 91.175 (l)(3)) for the runway of intended landing must be distinctly visible and identifiable to the pilot using the EFVS:
24.3.1.1 The ALS (if installed), or
24.3.1.2 The following visual references in both (b)(1) and (b)(2) below:
 a) The runway threshold, identified by at least one of the following: the beginning of the runway landing surface, the threshold lights, or the runway end identifier lights (REIL).
 b) The TDZ, identified by at least one of the following: the runway TDZ landing surface, the TDZ lights, the TDZ markings, or the runway lights.
24.3.2 Comparison of Visual Reference Require-ments for EFVS and Natural Vision. The EFVS visual reference requirements of 14 CFR part 91.175 (l)(3) comprise a more stringent standard than the visual reference requirements prescribed under 14 CFR part 91.175 (c)(3) when using natural vision. The more stringent standard is needed because an EFVS might not display the color of the lights used to identify specific portions of the runway or might not be able to consistently display the runway markings. The main differences for EFVS operations are that the visual glide slope indicator (VGSI) lights cannot be used as a visual reference, and specific visual references from both the threshold and TDZ must be distinctly visible and identifiable. However, when using natural vision, only one of the specified visual references must be visible and identifiable.
24.3.3 Visual References and Offset Approaches. Pilots must be especially knowledge-able of the approach conditions and approach course alignment when considering whether to rely on EFVS during a non.precision approach with an offset final approach course. Depending upon the combination of crosswind correction and the lateral field of view provided by a particular EFVS, the required visual references may or may not be within the pilot’s view looking through the EFVS display. Pilots conducting any non.precision approach must verify lateral alignment with the runway centerline when determ-ining when to descend from MDA.

24.3.4 When to Go Around. Any pilot operating an aircraft with an EFVS installed should be aware that the requirements of 14 CFR part 91.175 (c) for using natural vision and the requirements of 14 CFR part 91.175 (l) for using an EFVS are different. A pilot would, therefore, first have to determine whether an approach will be commenced using natural vision or using an EFVS. While these two sets of requirements provide a parallel decisionmaking process, the requirements for when a missed approach must be executed differ. Using EFVS, a missed approach must be initiated at or below DA or MDA down to 100 feet above TDZE whenever the pilot determines that:
24.3.4.1 The EFV is less than the visibility minima prescribed for the IAP being used;
 
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