感谢来自同事的问题。
问题 1
在 FCOM DSC-34-NAV-20-10-60 POSITION COMPUTATION 中，我们可以看到用来计算飞机位置的方式和数据源。其中 radio navaids 可以用来计算 IR/radio position (computed by the FMS)。在手册这部分提到的导航台只有 VOR, DME, LOC。
而在 FCOM DSC-22-FMS-20-20-20 AF，提到可用于位置计算的导航台还包括了 TACAN 和 VORTAC：
那么，TACAN 和 VORTAC 到底能能否用于位置计算？
问题 2
如果一个 DVOR 或者 VORTAC 的 VOR 部分或者 DME/TACAN 部分不工作，另一部分工作，是否需要拒选该导航台？
比如洛杉矶 LAX：
问题 3
关于拒选导航台，A320 和 A350 的描述稍有不同。A320 提到在不可靠和不工作时都需要拒选，而 A350 只提到在不可靠时需要拒选，那不工作时该如何操作呢？
比如莫斯科 UUWW 的一个 DVOR：
空客回复
问题 1:
空客飞机不具备完整的 TACAN 和/或 VORTAC 功能，因为这些属于军用功能。不过，当 TACAN 台站可用时，FMS 可以使用该台站的 DME 功能。在 VORTAC（VOR 与 TACAN 共址）的情况下，FMS 可以使用 VOR 功能获取方位信息，同时使用 TACAN 的 DME 功能获取斜距。
问题 2:
起飞时进行导航台拒选是为了在 GPS PRIMARY 丢失时避免 FMS 无线电更新出现错误。
当 FMS 仅依赖单一 VOR/DME 进行导航时，其位置可能变得不准确，这取决于所使用地面台站的质量。因此，在共址台站（例如 VOR/DME 或 VORTAC）中，如果仅 DME（或 VOR）部分不可用，则应按照 FCOM PRO-NOR-SOP-60 “MFD FMS Preparation” 中 “NAVAIDS” 章节的要求，将整个台站拒选。
此外，只有 VORTAC 才包含 VOR 部分，而 TACAN 本身不包含。在 LAX NOTAM A0395/25 N 的情况下，根据 FAA 网站信息，LAX 的 113.6 MHz 实际上是一个 VORTAC。
问题 3:
空客确认，在不可用和不可靠两种情况下，受影响的导航台都应被拒选。某些导航台可能在 AIP 中被宣布为不可用，但仍存在台站所有者未将其关闭的风险。
在这种情况下，FMS 仍会接收到发射的数据并基于这些数据进行计算，但这些数据很可能是错误的。因此，为了避免与不可用导航台仍可能发射数据相关的任何问题，空客建议将其拒选。
原文
Q
Q1
Are TACAN and VORTAC used for position computation?
In FCOM DSC-22-FMS-20-20-20 AF, it states that TACAN and VORTAC are included among the navigation aids that can be used for position computation.
![[Questions about NAVAIDs and position computation_image_1.png|600]]
However, in the entire subsection FCOM DSC-34-NAV-20-10-60 POSITION COMPUTATION, only DME is mentioned.
Therefore, are TACAN and VORTAC used for position computation?
Q2
If either the VOR part or the DME/TACAN part of a DVOR or VORTAC is inoperative while the another part is operative, is it necessary to deselect the navaid?
For example, Los Angeles (LAX):
![[Questions about NAVAIDs and position computation_image_2.jpg|400]]
![[Questions about NAVAIDs and position computation_image_3.jpg|300]]
Q3
Regarding the deselection of navaids, the descriptions for the A320 and A350 differ slightly. Is deselection required for the A350 when a navaid is unserviceable other than unreliable?
![[Questions about NAVAIDs and position computation_image_4.png|600]]
![[Questions about NAVAIDs and position computation_image_5.png|600]]
For example:
![[Questions about NAVAIDs and position computation_image_6.jpg|400]]
A
Answer 1:
Airbus Aircraft do not have full TACAN and/or VORTAC capability, as they are military functions.
However, the FMS is able to use the DME function of the TACAN station when available.
In the case of a VORTAC (VOR and TACAN collocated), FMS may use the VOR function for azimuth and the DME function of the TACAN for slant range.
Answer 2:
First, Airbus would like to kindly inform CSC that the navaid deselection at departure is performed in order to avoid erroneous FMS radio updating if GPS PRIMARY is lost.
When basing its navigation on a single VOR/DME, the FMS position may become inaccurate, depending on the quality of the ground station being used. As such, in the case of only the DME (or VOR) being unserviceable in a co-located station, such as VOR/DME or VORTAC, the entire station should be deselected as per FCOM PRO-NOR-SOP-60 “MFD FMS Preparation”, “NAVAIDS” chapter.
In addition, only a VORTAC has a VOR part, while a TACAN has not. In the case of LAX NOTAM A0395/25 N, according to the website of the FAA, LAX 113.6Mhz is, in fact, a VORTAC.
Answer 3:
Airbus confirms that in both unserviceable and unreliable cases, the affected navaid should be deselected.
A navaid may be declared unserviceable in the AIP, but there is still a risk that it is not turned off by the owner. In that case, the FMS will still receive emitted data and perform calculations based on these data which are, however, likely erroneous data.
Therefore, to avoid any issues linked to an unserviceable navaid which may or may not still emit data, Airbus recommendation is to deselect it.
