Aircraft speed:

Officially, the world fastest aircraft speed jet still remains the SR 71 Blackbird. Although the official record is about Mach 3.3, many believe the aircraft speed could truly fly much faster. However, it seems doubtful that the SR 71 aircraft speed could go much faster than about Mach 3.5, with a very slim chance of Mach 4. This is due to thrust and structural limitations. By Mach 3.6 or 3.8 aircraft speed, shock waves generated by the nose of the aircraft are produced at an angle such that they impact the engine nacelles and cause the engines to unstart.

In addition, heat generated by the aircraft speed increases to the point that certain portions of the plane structure begin to fail by about Mach 3.5 aircraft speed .
The SR71 aircraft speed record has been broken by NASA X 43 Hyper X experimental aircraft speed. This unmanned automobile was designed to test a new engine called a scramjet at aircraft speeds ranging from Mach 7 to Mach 10. At the X 43 cruising altitude of 100,000 ft, these Mach numbers correspond to aircraft speeds of approximately 4,750 mph (7,640 km/h) to 6,750 mph (10,855 km/h).

The X 43 was accelerated to these high Mach numbers by a booster soar, and the aircraft speed only flew under scramjet power for a few seconds before falling into the sea. From a purist point of view, the X 43 did not truly break the jet powered aircraft speed record since it was unable of cruising at and supporting these high speeds. Nonetheless, some believe the SR 71 aircraft speed record has already been bested by its supposed replacement dubbed the Aurora. If this mysterious new spy plane does really exist, it is believed to cruise at aircraft speeds ranging from Mach 5 to Mach 8.