感谢刘赓教员提出的问题。
在其他所有需要重力放轮的地方都没有这一动作，很难理解其中关联。空客回复说在试飞中发现 RAT 的性能会受到主起落架和空调 RAM AIR OUTLET 气流的综合影响，所以要关闭两部 PACK。
注意这里的 RAM AIR OUTLET 不是指 EMERGENCY RAM AIR, 因为那个是只有 INLET 没有 OUTLET 的。指的是用于 组件内热交换空气的 OUTLET：
从下面两张图中可看出，RAT 放出后会处于主起落架和 RAM AIR OUTLETS （就是图中的 PACK AIR OUTLETS，经过机务证实）之间，所以可能影响了气流。
手册里 RAM AIR OUTLETS 的图看的不是很清楚，来几张自己拍的。就是那两个出风量最大，最热，噪音也最大的口子：
站在 RAT 下面的话就处于 RAM AIR OUTLETS 和起落架之间：
具体参考 TechRequest 原文：
Q:
In DEFERRED PROC of EMER CONFIG procedure, for L/G GRAVITY EXTN, PACK 1+2 need to be OFF. There're no this action in other L/G GRAVITY EXTN cases.
A:
Thank you for submitting your question about the need to switch both packs to off during the L/G GRAVITY EXTN DEFERRED PROC of the EMER CONFIG procedure.
Unlike other L/G GRAVITY EXTN cases, this DEFERRED PROC of the EMER CONFIG procedure occurs when the RAT is also extended. A350 Flight tests were conducted and an impact on the RAT performance was identified when the Main L/Gs are extended during the landing flight phase (speed lower than 220 kt) with the packs operative (Ram Air Outlet flow effect).
As such, and in order to limit the risk, PACK 1+2 must to be set to OFF during this DEFERRED PROC of the EMER CONFIG procedure.