Short answer
Before your plane crosses the miqat, not at Jeddah. Jeddah is inside the miqat line for flights from the US, UK and most of the world, so you must be in ihram in the air. Most people change at the departure airport or during a layover and make the intention when the crew announces the miqat.
Anyone heading for Makkah for Hajj or umrah must enter ihram at or before the miqat they pass, including when flying over it. Crossing it without ihram and then entering ihram in Jeddah requires a dam, unless the person goes back to a miqat and enters ihram there.
The four schools agree on this.
Explanation
The Prophet ﷺ set the miqats: Dhul Hulayfah for Madinah, al-Juhfah for Syria, Qarn al-Manazil for Najd and Yalamlam for Yemen, and said they are for their people and for anyone who passes them intending Hajj or umrah. Jeddah lies between these points and Makkah, so for almost every international flight it is inside the boundary. The four schools agree on the principle; a small number of modern scholars treat Jeddah as a miqat for air travellers, but the mainstream of all four schools, Hanafi included, does not.
- Put on the ihram clothes at home, at the departure airport or during your layover. Clothes alone do not start ihram.
- Pray two rakats of ihram before boarding if it is not a disliked time.
- On the plane, when the crew announces the miqat (usually 30 to 60 minutes before landing), make the intention and say the talbiyah. If you are unsure of the time, make the intention a little early.
- From then, the rules of ihram apply.
If you landed in Jeddah without ihram: travel back to a miqat such as Qarn al-Manazil (al-Sayl al-Kabir) or Juhfah before you start the umrah and enter ihram there. Then there is no penalty. If you enter ihram in Jeddah instead, the umrah is valid but a dam is due.
Someone who flies to Jeddah for real business or to visit family there, with Makkah as a later and separate plan, may enter ihram from Jeddah. This must be the real purpose, not a way around the miqat. If you are going to Madinah first, see ihram when going to Madinah first.
The evidence
Sahih al-Bukhari 1526
Sahih... فهن لهن ولمن أتى عليهن من غير أهلهن ...
"They are for their own people and for those who come to them from elsewhere, intending Hajj or umrah."
Anyone who passes a miqat intending Hajj or umrah must enter ihram there.
Where the ruling is recorded
- al-Hidaya, Kitab al-Hajj, section on the miqats. It is not allowed to pass the miqat towards Makkah without ihram.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Hajj, the miqats and passing them without ihram. The dam falls away if the person returns to a miqat and enters ihram there.
- Fatawa Mahmudiyya, Kitab al-Hajj, ihram for air travellers. Air travellers must enter ihram before the plane passes over the miqat.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.