Why is the second '-il' needed in the following sentence?

[Steven Wain]

Quand M. Sanderson a-t-il allé a Paris, et pourquoi son avion quittait-il en retard Londres?

Is it even right in the first place?

Thanks very much, 

Steven

[Neil Coffey]

The situation is slightly complicated, and you may wish to refer to the information on forming questions in French on the French Linguistics web site. However, basically the answer is that it would be usual to include the extra -il.

In general, when the subject of a question is a noun phrase (and not just a pronoun il/elle etc) and you use inversion, then you also insert the corresponding pronoun:

- always in yes/no questions (ones that can be answered by "yes" or "no")
- usually, with longer question words (of >1 syllable, like comment, pourquoi)
- usually, to avoid the subject and object being directly next to one another

That said, there is some speaker to speaker variation in such cases. I know, for example, that with one of the French-speaking colleagues that I regularly collaborate with on translation work, there have been a few cases where she has suggested removing the -il (or equivalent) with pourquoi where I would have expected many speakers to prefer it to be inserted.

[Steven Wain]

Ahh, thanks so much, that was such a helpful answer.

[Benoît]

There is one mistake in your sentence:
Quand M. Sanderson est-il allé à Paris (and no "a-t-il")

The second part of your sentence is correct, even if I would say:
"Pourquoi son avion a-t-il quitté Londres en retard ?"(instead of quittait-il).
I have no explanation for such preference, it's more a feeling than a true rule I guess...

[Alix Martin]

We would say :
"Quand M. Sanderson EST-il allé à Paris, et pourquoi son avion a-t-il quitté Londres en retard ?"
We can also say "Quand M. Sanderson est allé à Paris, et pourquoi son avion a quitté Londres en retard?" but it's not really correct.

Alix