The cover of Mortal Engines
Hot air balloons are a type of not mechanical flying transport. They fly when the flame inside heats up the air in the balloon, making it warmer than the air outside. As the hot air rises, it takes the balloon and the basket up with it. They are seen at various points throughout the story.
Hot air balloons are used around cities to easily get from one tier to another. They're used by the higher classes, as the poorer people mostly aren't able to obtain them. They are also used to visit flying towns like Airhaven and Kipperhawk. Hot air balloons are used as a means of escape if the town someones living in is about to be devoured by a bigger city. Balloons also serve similarly like air-taxis in Batmunkh Gompa.
In Fever Crumb, Fever Crumb and Kit Sodent escape from Godshawk's Head in a hot air balloon. While they are flying Kit dies and they land amongst the armoured fortresses and the balloon is carried awat.