之前對于為什么RVR能夠替代的問題咨詢了FAA當局，近期有了回復，整體上和筆者查詢的資料內(nèi)容差不多，同樣是從RVR系統(tǒng)本身的作用范圍和航空器運行中的重要性進行了解釋，原文附在最后，其中的參考文件也歡迎感興趣的朋友自行學習，前文可以參見文章：

 關于中間端RVR能夠替代接地帶RVR的思考？

對于起飛和著陸運行都可以允許有RVR替代的存在，也就是跑道上的某一段的RVR數(shù)值替代另外一段的做法，使用中間端RVR進行替代首先要基于FAA6560 order的RVR傳感器設備的位置規(guī)定，在安裝位置的基礎上，可以發(fā)現(xiàn)在一個2000ft半徑的范圍內(nèi)，是可以共享RVR數(shù)值的，同時一個接地端的RVR傳感器是安裝在跑道入口0-2500ft的范圍內(nèi)，而中間端是安裝在跑道中間點1000ft的范圍內(nèi)，這就是一個RVR有效的范圍半徑，所以可以替代。

對于起飛滑跑來說，起飛階段是非常緩慢的，所以機組有大量的時間來評估能見度，讓航空器與跑道對正，同時基于上述的規(guī)定的RVR安裝位置，允許運營人使用中間端代替接地端，也就是說首先第一段解釋了RVR系統(tǒng)的作用范圍在設備上是允許的，其次第二段解釋了中間端的替代主要是因為機組可以有大量的時間來確定航空器的滑行狀態(tài)，同樣在著陸的時候，對于中間端和接地區(qū)之間也是可以替代。

末端的RVR不能作為替代RVR，這是因為航空器在接地區(qū)的RVR情況是最重要的，末端的RVR情況明顯不能有效覆蓋接地區(qū)的RVR情況，所以可以知道在國內(nèi)文件或者國際文件中，不管是起飛還是著陸，接地區(qū)的RVR不工作時，只有中點端的可以替代，但是千萬不要忘記一個前提，那就是RVR傳感器的位置設置嚴格符合FAA Order 6560.10的要求。

原文如下：

Thank you for your inquiry on the RVR readout values for TDZ and Midpoint RVR.

For both takeoff and landing operations there are authorizations to allow RVR substitutions as you mentioned. Using a MID RVR may be authorized (depending on the OpSPec and/or the procedure) based on the RVR siting requirements of FAA Order 6560.10. Within the RVR siting requirements we find that we can share RVR’s within a 2,000 foot radius. A Touchdown RVR sensor is located within 0 to 2,500 feet from the threshold and the midpoint is within 1,000 feet of the center point of the runway. This shows an effective radius of the RVR system.

During takeoff runs, the aircraft begins slowly so there is plenty of time for the pilot to evaluate visibility and aircraft alignment with regards to the runway centerline. This, combined with the siting requirements; allows operators to be authorized to substitute the Midpoint RVR for a Touchdown. The reverse is true for landing when substitution is authorized; the aircraft is slowing to taxi speeds at the end of the runway.

A rollout RVR VS is not authorized as an RVR substitute due to the fact that the RVR is most critical at the touchdown of the aircraft. The requirements of 91.175 Takeoff and Landing Under IFR place emphasis on what the pilot is required to see in order to proceed safely.

參考文獻：

FAA Order 6560.10 Runway Visual Range

FAA Order 6750.24 Instrument Landing System and Ancillary Electronic Component Configuration and Performance Requirements

Advisory Circular (AC) 120-118

FAA Order 8260.3, United States Standard for Terminal Instrument Procedures (TERPS)

The Instrument Procedures Handbook (IPH)

The Pilot’s Handbook of Aeronautical Knowledge

The Aeronautical Information Manual (AIM)