In this case, the problem domain is still very well understood and success is easily defined in concrete terms: human powered flight across a distance.
In startups, you often start with one problem in mind but stumble on another (and another, and another...). And even when you solve the technical problem, you still have to solve the business model problem.
Yes, you may solve the problem of finding the cheapest flight through search, but can it make money? And how long will it take to break even? etc etc
So redefine the problem as "Use my software skills to address an unmet need in a sufficiently large market" or "find a scalable business model" or something else along those lines.
In startups, you often start with one problem in mind but stumble on another (and another, and another...). And even when you solve the technical problem, you still have to solve the business model problem.
Yes, you may solve the problem of finding the cheapest flight through search, but can it make money? And how long will it take to break even? etc etc