Is it permissible for a traveler to combine and shorten the Maghrib and Isha prayers before the plane's scheduled departure at 6:00 PM, and must he perform the Fajr prayer on the plane or make it up later?

If the time for Maghrib prayer begins before the plane takes off, it is permissible to perform it at the airport. However, it is not permissible to shorten or combine prayers at the airport if the airport is within the city limits; rather, the prayer must be performed in full. But if the airport is outside the city limits, then you are permitted to avail yourself of the concessions of travel.

If the time for Fajr prayer begins while you are on the plane and you know that it will land at or after sunrise, then you must pray on the plane, facing the Qiblah as much as you can, and perform the bowing and prostrations by gesturing. You are not required to repeat the prayer upon landing. If you know that the plane will land before the prayer time expires, with enough time to perform the prayer, then delay it until you arrive.

If the time for a prayer that can be combined with the one after it (like Dhuhr with Asr, or Maghrib with Isha) begins while you are on the plane, then delay it until you arrive at the airport and combine it as a delayed combination (jam' ta'khir). If the prayer is one that cannot be combined with the one after it, such as Fajr, then if you can reach the airport before its time expires, pray it there. Otherwise, pray it on the plane.