May one fly during the three weeks?

It is permitted to fly on a plane during the three weeks and nine days. However, one is to abstain from traveling to dangerous areas, or for the sake of pleasure or a vacation.[1] Thus, one should only travel for necessary purposes, or for the sake of a Mitzvah. This especially applies during the nine days. On one occasion, the Rebbe advised that one who plans to travel during the three weeks, should at least begin his traveling preparations prior to the 17^th of Tammuz, such as through packing some of his belongings prior to the 17^th.[2]

[1] See Minchas Elazar 4:54 regarding why by Sefira it is allowed, implying that by Bein Hametzarim it is to be avoided

[2] Heichal Menachem 2:214, published in Shulchan Menachem 3:49