the prototype ss.4 was built by sai ambrosini, passignano sul trasimeno, and then sent to eleuteri airport, castiglione del lago, also in the trasimeno area. there, the aircraft was successfully flown for the first time on 7 march 1939. the next day the ss.4 prototype was scheduled to be transported to aviano airbase by rail, but ambrosini's chief test pilot ambrogio colombo wanted a second test flight. after 45 minutes, an aileron malfunctioned just from eleuteri. colombo attempted to land but was unable to reach the runway and crashed near campagna, hitting a tree. colombo was killed when the engine pushed through the rear bulkhead and crushed him against the front of the cockpit. a memorial to him was erected near the site of the crash. the investigation into the crash concluded that the accident was due to imperfect construction which led to a faulty installation of the aileron which had failed. the study also pointed out the excessive vibrations transmitted by the engine to the wing.