When is the proper time to recite Tefillas Haderech when travelling in an airplane?
In Halichos Mordechai, The Traveler's Companion, by Rabbi Eliezer Wanger, he says (quoting the "אהלך באמיתך"):
If one is traveling by airplane and the airport is located outside the city — even if there is a distance less than a parsah between the city limit and the airport — one should say Tefilas Haderech in the car, taxi or bus that is taking him to the airport. This should be said once he is outside the city limits, and not after takeoff.
If the airport is located within the city limits, then one should wait to say Tefilas Haderech until after takeoff.
Earlier in the book, he describes a "parsah" as:
The distance of a parsah is equivalent to 8000 amos (4 mil) which in our terms range from 3.840 km. / 2.385 miles (According to RA"Ch Naeh) to 4.640 km. / 2.9 miles (According to the Chazon Ish) to 4.800 km./3 miles (A'Halech B'amitecha) .