Early Lightning models, the F.1, F.1A, and F.2, had a rated top speed of at in an ICAO standard atmosphere, and IAS at lower altitudes. Later models, the F.2A, F.3, F.3A, F.6, and F.53, had a rated top speed of at , and speeds up to indicated air speed for "operational necessity only". A Lightning fitted with Avon 200-series engines, a ventral tank and two Firestreak missiles typically ran out of excess thrust at on a Standard Day; while a Lightning powered by the Avon 300-series engines, a ventral tank and two Red Top missiles ran out of excess thrust at Mach 2.0. Directional stability decreased as speed increased, there were potentially hazardous consequences in the form of vertical fin failure if yaw was not correctly counteracted by rudder use. Imposed Mach limits during missile launches protected stability; later Lightning variants had a larger vertical fin, giving a greater stability margin at high speed.