一般来说，交输引气启动发动机，供气一侧的发动机引气按钮是打开的，而受气一侧的发动机引气按钮是关闭的。然而，在2019年的A350手册中，受气一侧也被改成了 ON。
空客解释如下：
简单地说，根据当前 Bleed Air Monitoring (BAM) 的控制和监控逻辑，当 Common Remote Data Concentrators (CRDCs) A01出现故障时，会指令 Overpressure Shut-Off Valve (OPSOV) 关闭，从而无法交输引气。但是如果接通发动机引气按钮，则打开 OPSOV 的指令更优先，所以采取这个方法来暂时解决 CRDC 的问题。
ISI 36.00.00014 中有具体说明，这里大概解释一下。
Engine Bleed Aie System (EBAS) 中的 Bleed Air Monitoring (BAM)软件 安装在一个 BOMU上，并使用两个 CRDC：CRDC B05/02，以及CRDC A01/A02。
当前版本的 CRDC A01/A02 可能会出现故障，向 EIF 和 EEC 发出错误的发动机手柄关断指令，导致 OPSOV 关闭，从而无法交输启动发动机。而当发动机引气按钮接通后，BAS 会通过 CRDC B05/B02 给出指令要求打开OPSOV。CRDC B05/B02 的打开指令在地面优先于 CRDC A01/A02 的关闭指令，而在空中则相反。
所以，在 BAS 相关软件版本升级之前的解决方法就是，在交输引气程序中，要求两台发动机的引气按钮都接通。
具体请参考 Tech Request 原文，ISI 36.00.00014 就不贴出来了，有需要的可以在 AirbusWorld 上自行搜索，或者私信我。
A：
Airbus reviewed your query, and we would like to provide CSC with the following information:
The A350 FCOM Procedures-Normal Procedures-Supplementary Procedures-Engine-Crossbleed Engine Start procedure is updated from “ENG BLEED (ENG NOT STARTED)…OFF” to “ENG BLEED (ENG NOT STARTED)…ON” to make it complaint with the failure case of the Common Remote Data Concentrator (CRDC) A01 and A02.
As a background, one operator has reported an event related to a CRDC A01, leading to inability to start the engines on ground. With the current Bleed Air Monitoring (BAM) control and monitoring logic, when the aircraft is on ground and in case of CRDC A01/A02 misbehavior, the Overpressure Shut-Off Valve (OPSOV) is commanded CLOSED, resulting in the inability to start the associated side engine.
Airbus Flight Tests demonstrated that when both engine bleed pushbuttons are set to ON before the CROSSBLEED valve is opened, engine start in this failure mode is successful. Therefore, Airbus decided to update this procedure to ensure a successful start regardless of CRDCs availability. Please note that this change does not affect the other cases where this procedure is used.
For more information, please refer to the attached ISI article ref 36.00.00014 (Inability to start the engine on ground due to inoperative CRDC at dispatch), which is also available on AirbusWorld.