What to do in panic attack in an airplane?

Calm Breathing: Usually, panic attacks are associated with shortness of breath. Such rapid breathing may make the anxiety worse, as in hyperventilation. A good thing to do is to learn relaxation breathing, so one is able to do it at a moment's notice like in a car or a plane. A great book which teaches this easy technique is: "the relaxation response" by benson. You just practice 5 minutes once or twice a day.

Breath through it.: Breath in slowly ; deeply through your nostrils. Fill your lungs with air (but not to the point of discomfort). Hold for a count of five then slowly begin to exhale through an open mouth. Repeat at least 12 times, longer if you need to. If you haven't been assessed for your panic/ anxiety - that would be prudent. Anxiety is no fun but it is treatable.

It's best to prepare for this problem in advance with a Clinical Psychologist with expertise in multiple types of psychotherapy to learn how to overcome this phobia. Medication can help prescribed by a Psychiatrist.