were designed to fly satisfactorily even at speeds greater than their critical Mach number. They did not possess sufficient engine thrust to break the sound barrier in level flight, but could exceed Mach 1.0 in a dive while remaining controllable. Modern jet airliners, such as Airbus and Boeing aircraft, have maximum operating Mach numbers slower than Mach 1.0. Supersonic aircraft, such as Concorde, Tu-144, the English Electric Lightning, Lockheed F-104, Dassault Mirage III, and MiG 21, are designed to exceed Mach 1.0 in level flight, and are therefore designed with very thin wings. Their critical Mach numbers are higher than those of subsonic and transonic aircraft, but are still less than Mach 1.0. The actual critical Mach number varies from wing to wing. In general, a thicker wing will have a lower critical Mach number, because a thicker wing deflects the airflow passing around it more than a thinner wing does, and thus accelerates the airflow to a faster speed. For instance, the fairly-thick wing on the P-38 Lightning has a critical Mach number of about .69. The aircraft could occasionally reach this speed in dives, leading to a number of crashes. The Supermarine Spitfire's much thinner wing gave it a considerably higher critical Mach number (about 0.89). See also Drag divergence Mach number References L. J. Clancy (1975) Aerodynamics, Pitman Publishing Limited, London Notes Aerodynamics