When should airplane passengers enter ihram?

Praise be to Allah.

Your meeqaat in this case is Qarn al-Manaazil, which nowadays is called al-Sayl al-Kabeer. 

The one who passes by the meeqaat has to enter ihram from that point. If he does not pass it then he has to enter ihram when he comes in line with it on land, on the sea or in the air. So you have to enter ihram when the plane comes in line with it. As the plane will pass the meeqaat quickly, there is nothing wrong with entering ihram a little before that, in order to be on the safe side. 

Shaykh Ibn Jibreen said: 

If there is no meeqaat on a person’s route, he should enter ihram when he comes in line with the nearest place to it, whether he is coming by land, by sea or by air. Airplane passengers should enter ihram when they come in line with the meeqaat or, to be on the safe side, before they reach it, so that they will not pass it before entering ihram. Whoever enters ihram after passing the meeqaat has to offer a sacrifice. and Allah knows best. 

Fatawa Islamiyyah, 2/198 

From Fatawa al-Lajnah al-Daa’imah: 

Jeddah is not a meeqaat for Hajj or ‘Umrah, except for its citizens and residents, and for those who come to it for a reason other than Hajj or ‘Umrah, then decide to go for Hajj or ‘Umrah. But those who have a meeqaat before Jeddah, such as Dhu’l-Hulayfah for the people of Madeenah and places beyond, or who come in line with it on land or in the air, or al-Jahfah for those who live there, or who come in line with it on land, on the sea or in the air, or such as Yalamlam as well, have to enter ihram from their own meeqaat or when they come in line with it in the air or on the sea or on land. 

Fatawa al-Lajnah al-Daa’imah. 11/130 

The evidence for entering ihram from a point that is in line with the meeqaat is the report narrated by al-Bukhaari (1458) from Ibn ‘Umar (may Allah be pleased with him) who said: When these two cities – meaning Kufa and Basrah – were conquered, they came to ‘Umar and said, “O Ameer al-Mu’mineen, the Messenger of Allah (peace and blessings of Allah be upon him) defined the meeqaat for the people of Najd as being Qarn, but it is out of our way, and if we want to go to Qarn it is too difficult for us.” He said, “Look for a place on your route that is in line with it,” and he defined Dhaat ‘Irq for them (as their meeqaat). 

Al-Haafiz ibn Hajar said in Fath al-Baari (3/389): 

“Look for a place that is in line with it” means find a place that is parallel with the meeqaat and make that your meeqaat. 

It should be noted that it is not Sunnah to enter ihram before reaching the meeqaat, because this is not what the Prophet (peace and blessings of Allah be upon him) did, and the best of guidance is the guidance of Muhammad (peace and blessings of Allah be upon him). But if a person is in a plane and cannot stop at the place that is in line with the meeqaat, then he may do what he thinks is more on the safe side so that he will not pass the meeqaat without being in ihram. 

Al-Haafiz Ibn Hajar (may Allah have mercy on him) said: 

It was not narrated from any of those who performed Hajj with the Messenger of Allah (peace and blessings of Allah be upon him) that they entered ihram before Dhu’l-Hulayfah. If there was no meeqaat they then they would enter ihram before it, so this would be more difficult and would bring a greater reward. 

Fath al-Baari, 3/387. 

And Allah knows best.