感谢陈文机长的问题。
在 A350 FCTM PR-NP-SOP-150 E ALTITUDE CONSIDERATIONS - OPT FL 中的最后一段，“If a FMS failure occurs” 容易让人产生疑惑，这个“a”是指“one”，表示一部 FMS 故障，还是一个普通不定冠词，表示 FMS 都故障了？
空客回复中表示这只是说 FMS 出了一个会导致 OPT FL 不显示的故障，可能只是一个小故障或者不知道什么的问题，并不特指一部还是两部 FMS 的完全失效。
原文：
Q1:
In A350 FCTM PR-NP-SOP-150 E ALTITUDE CONSIDERATIONS - OPT FL, "If a FMS failure occurs, the flight crew should refer to the CRZ module of the IN-FLT PERF application in order to determine the OPT FL."
Does "a FMS failure" means one FMS failure, or a failure of both FMS?
A1:
In fact the intent of the FCTM guidance is to remind flight crews that the OPT FL can be obtained from the CRZ module of the IN-FLT PERF application in the case of a failure causing an inability for the FMS to display the OPT FL.
Q2:
So the "failure" just means any problem which causing no display of the OPT FL, not means the totally FAULT of one FMS or both FMS. Is it correct?
A2:
Yes your understanding is correct.
