The British Air Registration Board refused to give the aircraft a certificate of airworthiness, citing insufficient lateral control, excessive rudder forces and the ability to over rotate on take off, stalling the wing on the ground (a fault of the de Havilland Comet 1). Boeing responded by adding 40 inches to the vertical tail, applying full instead of partial rudder boost and fitting an underfin to prevent over rotation. These modifications became standard on all 707 variants and were retrofitted to all earlier 707s. The 37 -420s were delivered to BOAC, Lufthansa, Air-India, El Al and Varig through November 1963; Lufthansa was the first to carry passengers, in March 1960.