In theory you don't. However, because of increased security in recent years, airlines insist on an acceptable form of photo ID before you are allowed to board an aircraft. A valid passport is the most acceptable form of ID. Check with the airline that you intend to travel with and ask if they will accept anything else if you haven't got a passport.
You do not need a passport to fly from London to Newcastle-upon-Tyne, as both locations are in the UK, however it is always best to take your passport with you just in case.
Yes, you need a passport to fly internationally.
Yes, you need a passport to fly internationally.
Yes, you need a passport to fly out of Tijuana.
Yes, babies need a passport to fly internationally.
Yes. You need a passport to fly anywhere.
Yes, children need a passport to fly internationally.
Yes, children need a passport to fly to Mexico.
Yes, minors need a passport to fly to Mexico.
Yes, you need a passport to fly from Tijuana to Guadalajara.
Yes you need a valid passport to fly from Glasgow to Spain.
Yes, you need a passport.