A physical model of initiation of lightning flashes by aircraft in thunderstorms, based on the ''bi-directional, uncharged leader'' concept of Kasemir, is verified with airborne data from lightning strikes to instrumented airplanes (NASA F-106B and FAA CV-580). Characteristics of electromagnetic processes during lightning attachment to aircraft are identified with those in negative stepped leaders and positive leaders in natural lightning, in flashes triggered by wire-trailing rockets, and in laboratory discharges. It is shown that (1) a triggered flash starts on aircraft with either a negative corona or a positive leader that depends on the ambient electric field vector and the vehicle form factors; (2) the positive leader with continuous current that increases with time to the level of several hundred amperes is followed in a few milliseconds by the negative stepped leader with current pulses of a few kiloamperes; and (3) the two leaders develop in space simultaneously and bi-directionally from the oppositely charged extremities of the airplane. ¿ American Geophysical Union 1989