Hux has always been scared of flying but found ways to cover it up. In school flight simulators were far too real for him but he passed off those nerves as scared of getting less than excellent grades. The Finalizer isn’t too terrifying as it’s a large ship that he forgets he’s flying unless he’s passing by windows or if the ship is under attack (the rocking terrorizes him). In mid-sized planes if he can’t find an inner room he finds himself with a purpose so he can think or do something to distract him from the fear. Small planes or planes with full windows around and above are the worst.

Pre-TFA On a secret mission (alone or with fellow operatives) he gets into a disaster where he needs to evacuate the immediate area. The only evacuation option is by plane. Refusal is not an option because his data needs to get back into FO hands and he can’t transfer the data from his remote location.

+ The pilot who rescues him could be Poe (making extra credits on the side. Not a lot of money in Resistance). With or without details concerning Poe's capture by Kylo later or post-Starkiller destruction.
++ Secret mission is to gain data for to research/design/building the Starkiller.
+++ “How to use renewable resources.” Imagine Hux as an excited designer speaking to one or more specialists. Renewable-energy/solar/star-powered resources. The specialists and him become best buddies, very passionate on the subject.