He promised that he would/will phone us as soon as ...

[gar_001010]

He promised us that he (would phone/will phone) us as soon as the plane arrives
Please choose either "would phone" or "will phone"

 

[probus]

When is your assignment due @gar_001010?

 

[gar_001010]

Not an assignment, it was in a test I took and I'm trying to see if I chose correctly
I chose would phone by the way.

 

[probus]

You were correct.

 

[jutfrank]

I wonder here why the test writer has used 'arrives' in the present tense rather than 'arrived' and whether that is somehow meant to make you lean towards present tense 'will' rather than 'would' by placing the arrival of the plane more clearly in the future at the time of speaking.

If this is the only context, it's a very poor question.

 

[Tarheel]

I would have to have more context before deciding which to use.

 

[5jj]

@gar_001010: You give your current location as the United States, but your IP address suggests this is not the case. Please correct the information in your profile.

 

[Alex Case]

It's not a great question, but as mentioned above the present "arrives" can only go with "will", whereas "would" would need to go with "arrived". The former combination is reported speech without backshift, usually meaning that the event is still in the future when the conversation is reported. The latter combination is reported speech with backshift, which is always correct but is only necessary when the whole situation is in the past when the conversation is reported.