Supersonic Shock Waves on a Boeing 747-400!

From a seat in row 30 of a Virgin Atlantic Boeing 747-400, flying at around 12,000m at a cruising speed of around 850kph, a curious phenomenon was observed out of the window. A sharply-defined, crease-like shadow was playing on the engine cowlings, a couple of metres back from the leading edge of the air intake, and a clearly visible shock wave could be seen extending above the engine, roughly perpendicular to the direction of travel.

Most of the time, the shock wave remained stationary with respect to the plane, but during clear air turbulence, it could be seen wobbling about. As the plane started its descent, the shockwave gradually moved forwards relative to the plane, until it weakened and vanished as the 747-400's airspeed and altitude diminished.

The left hand photo below highlights the location of the shock wave, while the right hand photo shows it in detail, enhanced with PhotoShop's Unsharp Mask and Contrast controls to make it more clearly visible: see the wave's shadow on the engine cowling, and how the wave creates an apparent discontinuity in the distant clouds.

Location of Shock Wave Enhanced Detail of Shock Wave

It would be interesting to know who else has seen this effect: does it only occur on the 747-400 series, which are among the fastest passenger airliners in service (at least, since the much lamented demise of Concorde), or can it be seen on other commercial aircraft?