http://revach.net/article.php?id=3969

Section: Halacha   Category: Tshuvos
The Rogatchover Gaon & Rav Shlomo Zalman Auerbach - Tefilas HaDerech On An Air Plane?
The Rogatchover Gaon, who lived through the days that air travel became possible, says that we see from the din of Shiluach HaKein that there are no roads in the air and it is not considered "Derech", therefore we do not say Tefilas HaDerech.  Rav Shlomo Zalman Auerbach finds this logic strange and says that if we look at the underlying reason for tefilas haderech, obviously we must say this tefila when traveling by plane.

The question is when?  The gemara says that tefilas haderech is said when you are two "Parsa" outside of the city limits.  The Tehila L'Dovid says that this doesn't make sense for train travel because if we are worried about, "Oyev V'orev BaDerech" bandits who for strategic reasons usually don't come to close the city, then we shouldn't say it at all on a train.  If we we are worried about a crash then there is no difference how far you have traveled, the danger starts as soon as the train starts moving.

Rav Shlomo Zalman Auerbach similarly says, that the time to say tefilas haderech on an air plane is as soon as the plane reaches an altitude that if it fell it would be dangerous.

Important Note: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation.  Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.