They had thought

[GoldfishLord]

Ukrainian officials have said that they urgently need F-16 warplanes to give them a chance of winning control of the air, which Nato military doctrine says is vital for attacking forces.

They had thought that the first Ukrainian pilots flying F-16s would not be ready for combat until the first quarter of 2024 but the US has now said that this timeline will be sped up.

Source: The Telegraph

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What's the reason the past perfect was used in the bold part instead of the simple past?

 

[Tarheel]

You could read it as: "They had thought that, but they don't anymore" or "They had thought that, but now they're not so sure". I think simple past doesn't work quite as well

 

[jutfrank]

The past perfect reinforces the main idea that it's earlier than expected.

They originally expected the pilots to be ready in 2024 before whatever happened to speed things up.

 

[GoldfishLord]

Is the past perfect used there to express the action was completed before a point in time in the past?

 

[jutfrank]

There's no action, just an expectation.

They expected X. Then they changed their mind, and now they expect Y. So X is before Y in terms of time sequence.

 

[GoldfishLord]

1. Is "flying" in "the first Ukrainian pilots flying F-16s" a noun or a participle?

2. Does "the first Ukrainian pilots flying F-16s" mean "the first Ukrainian pilots which would fly F-16s"?

 

[jutfrank]

1. That's not a good question. Think about the meaning instead. In this case flying is an action.
2. More or less, yes. I think you understand it correctly.

 

[Piscean]

2. Does "the first Ukrainian pilots flying F-16s" mean "the first Ukrainian pilots which would fly F-16s"?

I think it's time you stopped asking Does A mean B? questions. A means A. B might have a very similar meaning, but it is not the 'meaning' of A.

Which should be who or that.