PHILADELPHIA — A key piece of hardware that aviation experts and pilots said could solve the mystery in the fatal crash of the medical transport Learjet on Jan. 31 turned out to be useless.

Butler Aviation's STC for ADS-B Out applies to Learjet models 31, 31A, 35, 35A, 36, 36A and 60. The study assessed potential damage a drone impact could cause to critical flight surfaces on ...