Verbs simple aspect Continuous(progressive) aspect Perfect and Perfect Continuous aspect

Contrast: be going to future versus present continuous as future

• 
  I’m going to visit my aunt in hospital tomorrow.
  
• 
  I’m visiting my aunt in hospital tomorrow.
  

Both these sentences are correct and the difference between them is slight. Sentence a) places more emphasis on the intention and sentence b) places more emphasis on the arrangement.
We do not generally use the present continuous with activities that do not involve the making of an arrangement or plan. We can say: I’m going to write a letter to my parents tonight. (the intention is important) but we do not say: I'm writing a letter to my parents tonight. (no arrangement is required).
We tend to avoid using verbs of motion, e.g. to go and to come in the be going to future and prefer instead to use the present continuous with these verbs:
We’re going to the theatre tonight.

• 
  Look at that helicopter coming down. It’s going to land in a minute.
  
• 
  b) Look at that helicopter coming down. It’s landing in a minute.
  

Sentence a) is correct but sentence b) is not possible. We do not use the present continuous to make a prediction based on present evidence.
We can only use the present continuous as future with a future time reference or where the future time reference is clear from the context. Where this is not the case, we use the be going to future:
He’s having a swim. (= now, if there is no time reference)
He’s going to have a swim, (in the future)
Verbs not normally used in the continuous are not used in the present continuous as future. We cannot say: I'm needing some help-next week.
We can sometimes use verbs not normally used in the continuous with the be going to future. This does not happen very often as these verbs do not describe events that we normally predict or intend. We can say:
I’m going to need some help next week.