What is the ruling regarding someone for whom it was difficult to perform ablution for the Dhuhr, Asr, Maghrib, and Isha prayers while on an airplane, and their times expired, then they performed them after midnight, and is there any expiation upon them?

It is not permissible to delay the prayer beyond its necessary time, even if water or earth (for tayammum (definition)) is unavailable. You should have prayed Dhuhr and Asr before sunset, and Maghrib at the time of Isha if you intended to combine them, and there is no sin in that. The sin occurs in delaying Dhuhr and Asr until or after sunset, and its expiation is sincere repentance and a firm resolve not to repeat it.