The story begins on an Evergreen International Airlines Boeing 747-100, registered N475EV. The aircraft had recently been converted to a freighter for the use of charter operator Evergreen. On 12th December 1991, the plane was flying from Newark to Tokyo, with a stop in Anchorage, when the crew noticed a failure warning on the inertial navigation system (INS).

The flight was carrying six crew and cruising at 31,000ft over the province of Ontario. Upon noticing the INS warning, the crew rechecked their instruments and noticed a shocking change. The 747 had banked 90 degrees to the right and was descending at an angle of 30-35 degrees.

This uncontrolled descent caused the aircraft to accelerate rapidly and fall over 10,000 feet before the pilots could regain control. The 747 stabilized at 22,500ft before the crew landed safely in Duluth, Minnesota.

All eyes were on what caused the 747 to act this way. The answer was a 3x15ft hole in the forward section of the right-wing. The panels from the gaping hole caused damage to the flaps and the horizontal tail sections. However, it was unclear if the hole was a result of, or cause of, the 747s rapid descent.

The result of this rapid descent was the 747-100 exceeding its maximum speed limit of Mach 0.92. Official reports suggest the aircraft reached at least 0.98 Mach during its descent. However, some reports indicate the plane reached speeds as high as Mach 1.25, well beyond the supersonic limit.

While the NTSB initially did not confirm speeds higher than 0.92, Boeing has made statements to the effect in the past. In the Chicago Times in 1992, a spokesperson said,

“Original flight tests of 747s conducted in 1969 and 1970 took 747-100 models to speeds of Mach 0.99. In addition, Boeing knows one case in which a 747 operated by Evergreen International made an emergency descent at speeds that exceeded Mach 1.”