Fear of flying usually begins with a bad flight. But it can also arrive as if "out of nowhere". Onset of this type usually happens in a person's twenties when they are on their own. As a person matures. they become more aware of their own vulnerability. This causes them to back off and avoid anxiety-provoking situations, or to seek more control when in such situations.
Why do we feel safer when we are in control? A part of the brain, the amygdala, monitors what is going on around you. It divides everything into "routine" or "non-routine". If routine, it takes no action. If non-routine, stress hormones are release to grab your attention. You then make an assessment to determine whether the non-routine situation is an opportunity, irrelevant, or a threat.
For example, when driving, if a car drifts into your lane, the amygdala recognizes this as non-routine. It releases stress hormones to grab your attention. The stress hormones shove thoughts aside and focus your attention on the car coming at you.
If you can decide what to do, at the moment you commit and begin carrying out a plan, a signal is sent to the amygdala telling it to stop releasing hormones. This control of the release of hormones controls the feelings of anxiety.
But when flying, control of anxiety isn't so easy. You can’t see what’s going on. When you feel the plane move or hear a noise, you don’t know what it means. You can’t make an assessment. If you can’t make an assessment, you can’t make a decision. If you can’t make a decision, you can’t make a commitment. Without commitment, the amygdala keeps releasing stress hormones. When a series of non-routine noises and motions in rapid succession, stress hormones build up rapidly and may cause high anxiety, panic, and the urge to escape.
Some anxious fliers turn to medication. Medication, however, interferes with desensitization. The more you medicate to fly, the more sensitive you become to flying. The more sensitive you become, the more medication you need. Finally, even slight movement of the plane, movements others don't even notice, can cause feelings of panic than medication cannot stop. Research by the Stanford University School of Medicine on this is at http://www.ncbi.nlm.nih.gov/pubmed/9299803
In some situations, systematic desensitization can train the amygdala not to produce stress hormones. At the beginning of the desensitization process, exposure must be so mild that the amygdala does not react. As treatment continues, intensity is slowly increased. Finally, the amygdala learns to regard full exposure to the situation as routine, and does not react to it.
Since there is no way to adjust the intensity of exposure, systematic desensitization cannot be used to treat flight phobia. Fortunately, the amygdala can be inhibited by linking each moments of flight to the memory of a situation it regards as benign. Then when flying, as the flight unfolds, things that previously triggered the amygdala do not trigger it. No stress hormones, no problem.
This method is explained in the book SOAR: The Breakthrough Treatment for Fear of Flying and at http://www.fearofflying.com/free-video/