Susannah has an intense fear of flying and endures flights with extreme distress. Although she knows her fear is irrational, it is overwhelming and uncontrollable. Susannah most likely suffers from:

Aerophobia

Explanation:

A phobia is an irrational fear of something. It can persist even when we know that we will not be harmed.

Aerophobia is the fear of flying or being on a flight (aeroplanes, helicopters, etc.). Experiencing this fear will often result in extreme anxiety and discomfort. Some doctors prescribe anti-anxiety medication for people who have aerophobia and need to take a flight.