You can't really use them interchangeably because they are used to describe different phobias so basically its like.... Flying phobia = Scarred of flying Plane phobia = Scarred of Planes Hope this helps :D

Your question was about how you can use these words, but you haven't explained the context in which you are using them. As Bill points we use 'fear of...' rather than 'phobia' in everyday speech, or as Keeran says we can say someone 'is scared of...' Are you using 'phobia' because this is connected to some technical reason (such as studying Psychology)? It is possible that both 'plane phobia' and 'flying phobia' might exist in some technical usage. If 'plane phobia' exists as a term, then I think it might mean a person is not scared of being in a helicopter..? It would be an odd term. However, I have seen 'aerophobia' used also. Have a look at this website for examples of how 'aerophobia', 'fear of flying' and 'flying phobia' can be used in a more formal situation. https://www.anxietyuk.org.uk/get-help-now/anxiety-information/anxiety-disorders/fear-of-flying/ It is more normal to use the words "fear of..."

The distinction Keeran makes betwen "flying" and "planes" is valid. However, in the real world, you will probably never hear "flight phobia" (the common, vernacular phrase is "fear of flying") and never "plane phobia". The -phobia words are best left to medicine and psychiatry, so use "fear of ___" unless you are studying to be a doctor.