关于飞机位置的计算，A320/A330 与 A350 有一些不同，也存在一些疑问。本文会涉及三个疑问，并在与空客的沟通中，确认了 A330 和 A350 手册在两个地方描述不准确，后续手册会修改。
疑问 1：FM POSITION 的构成到底是什么
对于 FM POSITION，A320/A330 手册中不同部分的描述稍有些令人疑惑。
在 AUTO FLIGHT - FLIGHT MANAGEMENT - NAVIGATION - POSITION COMPUTATION - GENERAL 中是这样描述的：
从第一句话理解，FM POSITION 应该是 MIX IRS position + RADIO position 或者 MIX IRS position + GPS position。
需要注意的是，根据 GPS POSITION 一节中的描述，GPS position 就是 GPIRS position，而不是单纯的 GPS 位置。
然而，在 FM POSITION 一小节中关于 FM POSITION 的描述与 GPS POSITION 的描述非常相似，那个括号注释 “(GPIRS)” 很容易让人理解为在 FM POSITION 优先使用的就是 GPIRS。而不是在上述 GENERAL 中解释的 MIX IRS position + RADIO position 或者 MIX IRS position + GPIRS position：
尤其在 A330 手册中还用的是 “mixed IRS/GPS“，更是与 GPS POSITION 用词一模一样：
经过与空客多个回合的询问，首先确认了 FM POSITOIN 是 MIX IRS + GPIRS，而不只是 GPIRS。我一直认为 FM POSITION 部分的 “each FMGC displays an FM position that is a MIX IRS/GPS position (GPIRS)” 这句话仍然相当容易造成误解，因为那个括号是对 “GPS position”的注解，而不是对整个“MIX IRS/GPS position”的注解。但空客认为这个描述没有问题，并不打算更改。
不过，空客最后也承认了 A330 手册中 FM POSITION 部分的“mixed IRS/GPS position”用词不恰当，应该改成和 A320 手册相同的“MIX IRS/GPS position”。这也算是消除了一部分误解吧。
疑问 2：A350 是否存在 BIAS
A350 优先使用的是 consolidated GPIRS position ，这个的逻辑和 A320/330 的 FM position 不一样，那是否意味着 A350 不存在 MIX IRS position 和 GPIRS position 之间的 BIAS 呢？
空客确认了，A350 不存在 BIAS。
疑问 3：A350 的 3 IRS 是不是和 A320/A330 的 MIX IRS 一样？
A350 手册中对 3 IRS 的描述非常少，基本只出现在 DSC-34-NAV-20-10-60 C 中的 NAVIGATION MODES 表格中：
在沟通中，空客确认 A350 的 3 IRS 和 A320/A330 的 MIX IRS 是基本一样的。3 IRS 位置是 FMS 计算的，单 IRS 位置是 ADIRU 自己提供的。
另外也承认了在上表中，3 IRS 对应 Aircraft position 中“IR/radio” 用词不当，应该此时已经没有 radio 数据了。后续手册会修改。
原文
Q1:
Regarding the computation of aircraft position, there are some differences and questions between the A320/A330 and the A350.
Q1:
For FM POSITION, the descriptions in different parts of the A320/A330 manuals are somewhat confusing.
In the AUTO FLIGHT - FLIGHT MANAGEMENT - NAVIGATION - POSITION COMPUTATION - GENERAL section, it is described as follows:
From the first sentence, it is understood that FM POSITION should be MIX IRS position + RADIO position or MIX IRS position + GPS position.
According to the description in the GPS POSITION section, the GPS position is actually the GPIRS position, not simply the GPS position.
Therefore, it can be concluded that FM position should be MIX IRS position + RADIO position or MIX IRS position + GPIRS position.
However, the description in the FM POSITION subsection can easily cause confusion:
Firstly, should the wording be unified between A320 and A330, for example, both using MIX instead of one using MIX and the other using mixed?
Secondly, the descriptions of FM POSITION and GPS POSITION are very similar, especially for A330 (using mixed):
This can easily lead to the understanding that the priority for FM POSITION is GPIRS, rather than as explained in the GENERAL section as MIX IRS position + RADIO position or MIX IRS position + GPIRS position.
So, how exactly is FM POSITION constituted?
Q2:
The A350 prioritizes the consolidated GPIRS position. This logic is different from the FM position of A320/330. Does this mean that there is no BIAS between MIX IRS position and GPIRS position in the A350?
A1:
Airbus reviewed your queries, and we would like to provide you with the following information:
Q1:
CSC’s understanding of the priority for FM position is correct. The MIX IRS plus GPS position (GPIRS) is the first mode to be used for FM position. It is as described in the last paragraph of the GENERAL section:
“GPS/INERTIAL is the basic navigation mode, provided GPS data is valid and successfully tested. Otherwise, navaids plus inertial or inertial only are used. (Refer to FCOM/DSC-22_20-20-10 Navigation Modes).”
For the first paragraph of the GENERAL section that describes:
“Each FMGC computes its own aircraft position (called the "FM position") from a MIX IRS position and a computed radio position, or a GPS position.”
It describes what sources FMGC uses to compute the position but not in an order. In fact, this content was originally from the content of aircraft without GPS. On the aircraft without GPS, the FMS uses the MIX IRS position and a computed radio position for FM position. When the GPS Primary function was introduced on the aircraft, the phrase “or a GPS position” was added to this content.
Lastly, the difference in wording of ‘MIX IRS” in A320 and “mixed IRS” in A330, Airbus sees that there is no difference in the meaning of these two wordings. Therefore, we have no plan to change them. However, CSC can customize your own FCOM if CSC would like to unify these wordings.
Q2:
There is no “bias” on A350. The aircraft position is a consolidated position between the 3 ADIRU positions. Each ADIRU uses the GPS and the IR source to compute the GPIRS position.
When the GPIRS positions from the three ADIRUs are available, the ADIRS compares the three GPIRS positions, and selects the most accurate position to be the consolidated GPIRS position.
Q2:
Thanks for the explanation, and now the confusion of GENERAL section and GPS POSITOIN section are clear.
However, I still think the first paragraph of FM POSITION which is too similar to GPS POSITION should be revised to the wording like GENERAL. Because they are both "FM position", and the wording should be unified to avoid the misunderstanding to GPS position.
A2:
Airbus reviewed your query, and we would like to provide you with the following information:
About the FM position at flight initialization, please be clarified that:
On ground before takeoff thrust setting, the FM position is the following:
- If the GPS Primary is not available: FM position is the MIX IRS position
- If the GPS Primary is available: FM position is the “GPIRS position” along with the “MIX IRS position” (= MIX IRS/GPS position (GPIRS)).
About the GPS position, please be informed that on the aircraft, the GPS position is not actually a pure GPS position but a combination of GPS and IRS position called GPIRS.
For this reason, the term “mixed IRS/GPS position” is used in the GPS position topic.
We consider that the terms are appropriate and valid for their own topics, and we have no plan to update them.
We regret that we cannot respond to CSC’s request. We remain available for any additional information and assistance.
Q3:
According to Airbus' explanation:
- "MIX IRS/GPS position (GPIRS)" means the “GPIRS position” along with the "MIX IRS position".
- "mixed IRS/GPS position" means a combination of GPS and IRS position called GPIRS.
Although I still think this explanation is far-fetched, even if this explanation is valid, the wording in the A330 manual is problematic:
Because in the A330 FCOM, both the FM POSITION section and the GPS POSITION section use "mixed IRS/GPS position," and not one as "MIX IRS/GPS position" and the other as "mixed IRS/GPS position" in A320 FCOM.
So either the A330 manual needs to be revised, or both should be revised, as this indeed can cause misunderstandings. It's not just me, other company pilots have also expressed confusion and debated this issue. Therefore, I believe this is a common issue.
A3:
Airbus reviewed your query, and we would like to provide you with the following information:
After carefully reviewing the definitions of FM POSITION and GPS POSITION. We agree with CSC that the wording “mixed IRS/GPS position” used in A330 FM POSITION section should be revised to “MIX IRS/GPS position” to be the same as in A320 FM POSITION section.
We will update the A330/A340 FCOM accordingly. The update will be available in the next suitable A330/A340 FCOM revision.
Q4:
Additional questions.
In FCOM DSC-34-NAV-20-10-60 C NAVIGATION MODES:
Q1:
In the table above, should "IR/radio" be "3 IRS"? Because "no radio navaids available".
Q2:
Is "3 IR" for A350 the same with "MIX IRS" for A320?
Q3:
Confirm 3 IRS is from FMS, and 1 IRS only from ADIRU itself?
A4:
Airbus reviewed your query, and we would like to provide you with the following information:
Q1 and Q2:
Refer to the A350 FCOM-DSC-34-NAV-20-10-60-Position Computation, ADIRU provides the aircraft systems with the most accurate position that is available, using the following priority:
- The consolidated GPIRS position (computed inside ADIRU)
- Its own GPIRS position (computed inside ADIRU)
- The IR/radio position (computed inside FMS, then sent to ADIRU)
- Its own IR position (pure IR position, from ADIRU).
When the IR/radio position has no radio navaids available, The FMS uses the MIX IRS position for position computation. The MIX IRS is a center of gravity of the pure inertial position (IR position) elaborated by each IR (IR1, IR2 and IR3). It is similar to the MIX IRS position of A320.
We agree that the content of “3 IRS” mode should be improved to be clearer.
Please be informed that we are considering an update of the Aircraft Position section of A350 FCOM, we will take this opportunity to improve this content in the next suitable opportunity.
Q3:
As described above, the 3 IRS mode is a MIX IRS position computed inside the FMS, and the 1 IRS mode is a pure IR position from the ADIRU.
