Making Calibre default to AZW3?

Hi

Just started using a Kindle Fire HD and found that books only show up in the books section if they are converted to AZW3 (most MOBI ones appear as personal documents).

However, Calibre always want to convert books to MOBI unless i change the setting manually to AZW3. Is there a way i can set it to default to AZW3 instead (using a plugin)?

Thanks.