"This accident, like so many before it, was caused by the pilot’s decision to undertake the flight in which the likelihood of encountering instrument conditions existed, in the mistaken belief that he could cope with en route instrument weather conditions, without having the necessary familiarization with the instruments in the aircraft and without being properly certificated to fly solely by instruments."
Civil Aeronautics Board Aircraft Accident Report on the "Buddy Holly" accident, released on September 23, 1959.
Useful site:
https://lessonslearned.faa.gov/index.cfm