Thirty years ago, a Boeing 747 freighter rolled into a steep dive without the pilots noticing. The aircraft would lose 10,000 feet of altitude and reach speeds of at least 0.98 Mach, flirting and causing structural damage. This is what happened over Canada on that day.

On December 12, 1991 a Boeing 747-100, registration N475EV departed New York’s John F. Kennedy International Airport for Tokyo, Japan. The aircraft was operated by Evergreen International Airlines and flying cargo under contract to Japan Airlines. A crew of five was onboard. At about 5:20 a.m. Central Standard Time, the 747 was cruising some 173 miles northeast of Canada’s Thunder Bay when the pilots noticed warning lights.

The crew noted that the inertial navigation system fail lights were illuminated and when they cross-checked their instruments, they figured out that the plane was in a more than 90-degree right bank and was entering a 30- to 35-degree dive.

According to the National Transportation Safety Board’s on the incident, the aircraft reached a maximum bank of 95 degrees and lost 10,000 feet of altitude before the pilots completed the aircraft’s recovery. Thankfully, the pilots were able to stabilize the aircraft before making a safe emergency landing in Duluth, Minnesota. There were no injuries.

The aircraft first flew in 1969 and Boeing used it for flight and certification testing before delivering it to Pan American Airways in 1970, This Day in Aviation.

It stayed with Pan Am for nearly 21 years before it was sold to Evergreen International and converted into a freighter.

Early reports mentioned that the plane hit 1.25 Mach during its dive, but they were never confirmed. A former 747 pilot in a Seattle Times said that it was plausible for the plane to disintegrate after exceeding its maximum speed of 0.92 Mach.

In a to the Chicago Tribune, Boeing says that it flew 747-100s even faster in test flights. It also said that N475EV exceeded Mach 1:

Tom Cole, a spokesman at Boeing Commercial Airplane Co., 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, he said.

The from the NTSB said that the aircraft hit 0.98 Mach.

N475EV didn’t make it through the ordeal entirely unscathed and the NTSB report notes damage:

A section of the forward inboard lower surface skin panel of the right wing had separated, and the leading edge of the right horizontal stabilizer was damaged by impact with departing wing debris. There was other damage involving the upper surface and trailing edge flaps of the right wing and minor left wing damage.

During the NTSB’s of the incident, it was found that other 747s suffered from uncommanded rolls caused by failures in the autopilot system.

Testing of the aircraft’s autopilot system indicated that the cause of N475EV’s uncommanded roll was likely a fault in the aircraft’s roll computer. An examination of the 747's autopilot system determined that a failure of the autopilot system could result in the aircraft slowly rolling into a bank. The roll could be so slow that without visual references, the crew wouldn’t even know. In this case, N475EV’s crew was flying at night.

The NTSB to , including identifying all possible autopilot failure modes that could lead to uncommanded rolls. It also recommended the FAA issue an airworthiness directive that required the installation of devices that gave aural and visual warnings for excessive bank.

As for N475EV, it was repaired and continued to fly with Evergreen until 1994, when it was sold to Tower Air. It was sold again to Kalitta Equipment LLC in 2000. Its registration expired in 2017 and most tracking sites note the airframe to be scrapped.