Answer
(Fatwa: 1764/1127/L=1433)
The ruling in all the above mentioned cases is that if one does not know the direction of Qiblah due to cloudy sky or night or due to any other reason and there is none who could tell him the direction and neither there is any other way to know it then the person should decide himself and turn towards the direction where his heart tends and should offer salah. His salah shall be valid.
فإن لم يكن (الاستدلال على القبلة) لوجود غيم أو لعدم معرفته بها فبا السوال من العالم بها فإن لم يكن فيتحري ، وكذا يتحري لو سأله عنها فلم يخبره (الشامي 2/114)
The method of offering salah in plane is similar to train i.e. if the facility of facing Qiblah, qiyam, ruku and sujud is available and fardh salah is offered then the salah shall be valid. However, if Qiblah diverts during salah then the face should be turned towards that direction and if salah is offered while sitting in the seat (though the face was towards Qiblah) then salah shall not be valid. If someone does so he must offer that salah again.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband