Rabbi Shlomo Zalman Auerbach was among those who strongly advised avoiding any activities during the entire Three Weeks which were even onlyÂ “possibly dangerous”. Included in these activities was air travel, specifically during the Nine Days, which Rabbi Auerbach looked uponÂ especially unfavorably. He argued that even though air travel is considered routine in our day, since one still recites Hagomel after a flight it is still to be categorized among things which are “dangerous” andÂ should be avoided during these days.
Nevertheless, there is one major exception to this rule. He permits one to fly to Israel during the Nine Days and even on Tisha B’av itself. This is true even according to the view (which he subscribed to) that merely visiting the land of Israel is not considered to be a component of the mitzva of yishuv eretz yisrael, settling the land of Israel. Even so, the benefits of being in Israel and the manyÂ mitzvot which one is able to fulfill while here make such travel justifiable.
It is also interesting to note that Rabbi Auerbach permittedÂ students from the Diaspora to fly home on Rosh Chodesh Av (the first of the Nine Days)rather than to leave Israel earlier and be forced to miss even a single day of the zman, the yeshiva’s study semester. It is reasonable to suggest that Rabbi Auerbach would also approve of any other air travel that had a mitzva component to it, as well.
On a related note, a number of halachic authorities permit one who has no other food with him to eat the kosher meat meal that airlines normally serve when flying during the Nine Days, arguing that the hunger one might be forced to endure is comparable to a choleh, one who is ill, for whom eating meat is permissible during the Nine Days. Of course, on short-haul flights one should not make use of this leniency.
 Shalmei Moed Chapter 90