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 ihraam from that point. If he does not pass it then he has to enter ihraam when he comes in line with it on land, on the sea or in the air. So you have to enter ihraam when the plane comes in line with it. As the plane will pass the meeqaat quickly, there is nothing wrong with entering ihraam 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 ihraam 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 ihraam 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 ihraam. Whoever enters ihraam after passing the meeqaat has to offer a sacrifice. and Allaah knows best.
Fataawa Islamiyyah, 2/198
From Fataawa 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 ihraam from their own meeqaat or when they come in line with it in the air or on the sea or on land.
Fataawa al-Lajnah al-Daa’imah. 11/130
The evidence for entering ihraam from a point that is in line with the meeqaat is the report narrated by al-Bukhaari (1458) from Ibn ‘Umar (may Allaah 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 Allaah (peace and blessings of Allaah 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 ihraam before reaching the meeqaat, because this is not what the Prophet (peace and blessings of Allaah be upon him) did, and the best of guidance is the guidance of Muhammad (peace and blessings of Allaah 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 ihraam.
Al-Haafiz Ibn Hajar (may Allaah have mercy on him) said:
It was not narrated from any of those who performed Hajj with the Messenger of Allaah (peace and blessings of Allaah be upon him) that they entered ihraam before Dhu’l-Hulayfah. If there was no meeqaat they then they would enter ihraam before it, so this would be more difficult and would bring a greater reward.
Fath al-Baari, 3/387.
And Allaah knows best.