There's a bit of debate as to why vessels (planes, boats and seemingly buses) are called her. It's generally accepted for one of the following reasons
1) Ships used to be dedicated to the female in a man's life, going back further in history they were named after ancient goddesses. Whilst in ancient times women were considered to be bad luck to be on board a ship, it is not bad luck to name the ship
2) The less interesting theory is that the word 'ship' was a feminine grammatical term and its simply carried through history. Modern English doesn't have male/feminine terms but those of you who have studied French or German will be will aware of masculine and feminine grammar.
Calling buses 'her'