I feel terrible for you.
I'm guessing here, but whenever I see sudden death + airline flight I always think pulmonary embolism. This is what Tim Russert had, but they said heart attack on tv.
Its very difficult to detect and very difficult to identify the potential ahead of time. I found this article related to
embolisms and infants/children. It may point you in the right direction or line of questioning there.
Ask to speak with a hematologist there if you can. Most general doctors - even cardiologists are uninformed about Protein S, C, or Factor V Leiden. These gene deficiency disorders make the person highly susceptible to forming blood clots if they are stationery + at altitude for long periods of time. Many people have these deficiencies, but don't know it.
A simple blood test can be done to see if you or your spouse have either of these gene disorders and then if they are positive, its likely your child had it too, but not guaranteed. I have the Factor V Leiden, yet my brother is clear.
Hopefully you find out what the cause was as soon as possible, and hopefully this will give you some lead to an answer. Good luck and you have my condolences.