Grammar

Present continuous tense代替have arranged or planned to do in the future.

E.G. Mr. Wong is flying to Singapore for a meeting this weekend.
(Mr. Wong 已經plan定左在週末飛去新加坡for a meeting)

如果用future simple,
Mr. Wong will flying to Singapore for a meeting this weekend.
意思就會變成：
Mr. Wong 答應左(出於自願)在週末去新加坡for a meeting.

你記住：已經計劃了而將進行＞＞Present continuous tense

I think both answers are clear enough. I just want to add that you can still change "is flying" to "will fly" in this sentence, since we do not know whether this is a plan or something that is certain in your sentence.

This sentense is about the plan of Mr Wong so we need to use present continuous tense to talk about the things we planned and going to do.
圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_20.gif

we use future tense when we talk about or make some plans that are uncertain.
we use present continuous tense to talk about arrangements or plans that are certain.