Do 40% of illegal immigrants to the US from Mexico arrive by plane?
What are you gonna do with planes? He forgot planes. Oh, Trump, 40% of us come here by air. 40%
Politifact checked this:
Ramos said that about 40 percent of undocumented immigrants fly into the country and overstay their visas. While there is some evidence that overstays represent about 40 percent of the unauthorized population, and perhaps more, the best estimates of those flying in -- which are admittedly a bit shaky -- could be as low as one-quarter or somewhat more than one-third. The statement is partially accurate but leaves out important details, so we rate it Half True.
So 40% is all overstays, not just those who fly into the country on planes. So it's a real statistic but not properly described.
It has been noted that this is all undocumented immigrants, not just Mexican undocumented immigrants. It is conceivable that Mexicans could have a different rate than the group as a whole. And in fact, they do. In a previous question, it was answered that "9% of Mexicans arrested for being illegally present were found to be overstays." But that's lower, not higher.
The reason why overstays are important here is that the vast preponderance of people who enter the country by air do so legally at airports. Those who do not leave when scheduled are considered to have "overstayed" their visa. But while many overstays enter by plane, not all do. Some enter by ship or a ground vehicle (bus, car, etc.).
There is also an argument that the only "illegal immigrants" are those who arrive without legal sanction. That's not true of overstays, as they arrive legally and stay. I'm assuming that the term is being used in the broader form of everyone who is in the country without legal sanction.
The correct form of the statistic is that 40% of all illegal aliens in the United States arrived legally by normal transit, e.g. plane, ship, or ground. Limiting to Mexican-born, air travel, or those who transited illegally would each decrease that percentage.