Originally posted by Karnak:
The fastest dive that I am aware of by a WWII era aircraft was done by a Spitfire MkXVIII. It reached a speed of .92 mach.
The pilot was testing a high altitde preasurization system when it failed, he initiated an emergency dive in order to get to breathable air. Shortly after beginning the dive, he lost control of the aircraft. Control was regained at an altitude of less than 5,000ft. During the dive, the pilot observed the air going over the wing take on the "misty" look of supersonic speeds. It is probable that the top surface of the wing broke the sound barrier in relation to the air traveling over it. After regaining control, the pilot returned to base. After landing, the Spitfire was examined and judged flight ready.
You sure about it being flight ready?

Here's a quote from an eyewitness.
"To underline the remarkability of the Spitfire design, it is still on record as having achieved the fastest true airspeed ever reached by a piston-engined airplane. On April 27, 1944, a fully imstrumented PR. XI was dived by an RAE test pilot to a true Mach number of 0.91, but it lost its propeller and reduction gear in the process. The pilot still managed to execute a successful wheels-down landing on Farnborough airfield, in spite of the windscreen's and canopy's covering of black oil.
This hitoric flight was made by Squadron Leader AF Martindale, with whom I had previously served and whom I eventually succeeded as CO of high-speed flight. He was a tall, powerfully-built man, and this probably contributed to his survival, as he was pulling a stick force of well over 100 pounds when the propeller detached itself, thus subjecting the Spitfire to 11G.
I was on the airfield when the airplane touched down, and as I approached it, I could hardly believe my eyes, for there was a slight gap at the leading edge of each wing root, giving mini sweepback to the wings."
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When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things.
Chapter 13, verse 11