 The collapse was caused by the compromise of the tubular integrity, which was dependent on the keel beam that had been torn off on impact.The nose-gear light indicators were designed to be fail safe by having two separate light bulbs.This proved to be inadequate, as it was impossible to look through the cover to check whether one of the bulbs had been compromised, meaning that a failure of one bulb would not be detected until both bulbs malfunctioned.