When on the road is the prayer to be said?

On the road versus within the city:^[1] Tefilas Haderech is to be recited after leaving the city and its outskirts, which is defined as the area within 70 Amos from the city. One is not to say it while still within the city or its outskirts even if he has already began his travel. If one recited it while still in the city, after having decided to travel and preparing to leave, then he fulfills his obligation and is not to repeat it once he leaves the city that day. [If, however, he did not leave that day, or had not yet prepared himself to leave when he recited the prayer, then he must repeat the prayer upon traveling.]

Reciting the blessing within the first Parsa of the road:^[2] One is initially to say Tefilas Haderech while traveling within the first Parsa distance from the outskirts of the city. If he did not say the blessing within the first Parsa, he may do so until he reaches within a Parsa distance from his destination. However, once he reaches a Parsa distance from the city of destination, then he is to say it without a concluding blessing, as will be explained next.

If one already arrived to the city of his destination, may he still say Tefilas Haderech? If one forgot to say Tefilas Haderech during the outset of his journey, and is still in the midst of his travels he may still say it until he reaches a Parsa distance from his city of destination. Once he has reached within a Parsa distance from the city, he is to say Tefilas Haderech without a concluding blessing that day.^[3] However when he travels from that city, either on his return trip home, or if he is traveling to a different city, then he is to say Tefilas Haderech with a blessing if he is traveling a Parsa distance.^[4] Furthermore, in such a case, he may say Tefilas Haderech even while still in the city, and even if he is staying in the city for the night, and will be continuing his travels the next day.^[5] [If he is traveling from that city the same day as his arrival, then he may say Tefilas Haderech even within a Parsa distance from the city of destination, (and even within the city itself), being that he is currently in the midst of his travels and will still be traveling a Parsa that day.^[6]] 

When traveling on an airplane, when is Tefilas Haderech to be recited?^[7] If the airport from which you are flying from is not within the city and one must travel more than a Parsa distance [i.e. 3.8 km] from the city until one gets to the airport, then he is to recite the prayer of Tefilas Haderech during his travel to the airport after leaving the city, and this will include also the flight. [Thus, one who lives in northern or southern Israel and must travel to Ben-Gurion airport to catch his flight, is to recite Tefilas Haderech on his way to the airport.] If, however, the airport is within the same city, such as one who lives in New York City who is taking a flight from JFK, then the prayer of Tefilas Haderech is only to be recited on the plane. Ideally, it should be recited only after the plane takes off. Nonetheless, many Rabbanim are of the opinion that it should be recited while still on the runway directly prior to takeoff or within takeoff. Practically, one who did not say prior to takeoff should say it after taking off.

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[1] Admur 110:6

[2] Admur 110:8

[3] Admur 110:8; This applies even in accordance to the ruling of Admur in the Siddur that one says Tefilas Haderech one time for the entire trip, as since one will not be traveling a Parsa anymore that day it is not proper for him to say Tefilas Haderech now with a blessing having in mind his future travels of a Parsa which will only occur on later days.[Ketzos Hashulchan 110 footnote 5] If, however, he does plan to continue traveling that day, then obviously he may say Tefilas Haderech with a blessing even within the city. [Ketzos Hashulchan 110 footnote 3]

[4] So is implied from Admur and so rule Ketzos Hashulchan 67 footnote 3 regarding traveling to Kever Rachal and footnote 5.

[5] Admur 110:6; and so is implied from Ketzos Hashulchan 67 footnote 3 and 5 even according to the Sidddur

[6] Ketzos Hashulchan 67 footnote 3 regarding Kever Rachal

[7] See Sefer Haminhagim Chabad p. 14; Shulchan Menachem 1:135; Piskeiy Teshuvos 110:2; Halacha Berurah [Yosef] 110:10;