A350 在 HYD G (Y) SYS PRESS LO的程序中，并没有对侧风的限制，但是在补充程序 FCOM PRO-NOR-SUP-SUP-LG A FLIGHT WITH GEAR DOWN 中，却提到在起落架放下飞行的情况下，空中如果出现 HYD Y SYS PRESS LO 警告，则有最大侧风 20kt 的限制。
空客解释为，黄系统低压后，BACKUP STEERING FUNCTION 工作，原理是 PRIM 自动使用BACKUP STEERING FUNCTION，通过差动刹车来实现一定的转弯控制，此时差动刹车使用的是蓄压瓶的压力。
单独的 HYD Y SYS PRESS LO 警告后，蓄压瓶压力是没有问题的。但起落架放下飞行，长时间暴露在低温环境下，会导致蓄压瓶压力低，从而降低差动刹车性能。所以才有侧风 20kt 的限制。
TechRequest 原文：
Q:
In FCOM PRO-NOR-SUP-SUP-LG A FLIGHT WITH GEAR DOWN, the MAX XWIND FOR LANDING is 20KT in case of HYD Y SYS PRESS LO ECAM alert in flight. The explanation is :
As nose wheel steering is lost, and differential braking performance is slightly degraded due to lower accumulator pressure, the lateral control at low speed is ensured for a crosswind up to 20 kt only.
Why in PRO-ABN-HYD J HYD G(Y) SYS PRESS LO procedure, there's no any limitation about crosswind.
A:
In the specific case of flight with gear down, there is no automatic brake accumulator refill performed in flight and the landing gear is exposed longer than usual to cold outside air temperatures.
This may lead to a lower accumulator pressure, and thus to a slight degradation of the differential braking performance that may be required to control the thrust asymmetry at low speed in the case of combined failure of one engine plus yellow hydraulic system (loss of nose wheel steering).
As a consequence, a specific maximum crosswind at landing is applicable in these conditions. This limitation is not required in the general case of yellow hydraulic system low pressure, which explains why it is not part of the associated ECAM procedure.
