When is it obligatory to enter into a state of Ihram for Hajj when traveling by plane from Riyadh to Jeddah?

Your Miqat is Qarn al-Manazil (As-Sail al-Kabeer), and you must enter into a state of Ihram from there or from a point parallel to it, whether by air, land, or sea. If you are on an airplane, you must enter into Ihram when you are parallel to the Miqat. It is permissible to enter into Ihram a little before it as a precaution, due to the speed of the airplane. Whoever enters into Ihram after passing the Miqat must offer a compensatory sacrifice (dam jubran). Jeddah is not a Miqat except for its residents or for those who intend to perform Hajj or Umrah after arriving there. The ruling regarding entering into Ihram when parallel to the Miqat is based on the Hadith of Ibn Umar, may Allah be pleased with them both, when Umar designated Dhat Irq for the people of Najd because it was parallel to Qarn al-Manazil. It is not from the Sunnah to enter into Ihram before the Miqat, except for airplane passengers, for fear of passing the Miqat before entering into Ihram.