Northwest Airlines Flight 710

Northwest Airlines Flight 710 was a scheduled flight between Minneapolis, Minnesota and Miami, Florida, with a stop in Chicago. On March 17, 1960, the six-month-old Lockheed L-188 Electra operating the flight broke up in midair in southern Indiana, near Cannelton, killing all 63 people on board. After unexpectedly encountering clear-air turbulence at 18000 ft, the aircraft's right wing and part of the left wing separated, causing the fuselage to plummet and impact the ground at a nearly 90-degree angle, leaving a deep crater. Portions of the wings landed up to 4 mi away.

The in-flight breakup of the Electra closely resembled the September 1959 crash of Braniff International Airways Flight 542 near Buffalo, Texas, which killed all 34 occupants. That flight was also operated by a nearly new Electra. In that crash, the left wing separated and landed about one mile (two kilometers) from the main wreckage. Investigators were unable to determine the cause at the time, but the similarities between the two crashes led the Federal Aviation Agency to place flight restrictions on the Electra pending identification of the cause, and to order Lockheed to reevaluate the aircraft's structural integrity and demonstrate its airworthiness. The subsequent investigation, involving more than 250 engineers and technicians, determined that when an Electra with damage to the mounting structures of one of its outboard engines operated at high speeds or in turbulence, a destructive phenomenon called whirl mode wing flutter could occur, leading to wing failure.

After identifying the cause of the failures, Lockheed launched a program to design structural modifications to prevent whirl mode wing flutter and to retrofit all Electras already in service. These changes proved effective, and modifications to the fleet were completed on July 5, 1961.

Background

Northwest Airlines Flight 710 was a regularly scheduled flight between Minneapolis, Minnesota, and Miami, Florida, with a scheduled stop in Chicago.

On March 17, 1960, the flight was operated by a Lockheed L-188C Electra aircraft. At 12:51 p.m. Central Standard Time, the flight left Wold-Chamberlain Airport in Minneapolis with 69 passengers. It arrived at Midway Airport in Chicago after an uneventful flight, and 51 of the passengers departed the plane. Another 39 new passengers boarded while the crew prepared for the flight to Miami and the ground crew refueled the aircraft. Flight 710 took off from Midway Airport at 2:38 p.m. with 57 passengers and a crew of 6.

At 3:13 p.m., as the flight passed a navigational waypoint at Scotland, Indiana, the crew made a routine report to the Indianapolis Air Route Traffic Control (ARTC) Center with the aircraft's position and altitude at 18000 ft. The crew estimated that the flight would reach its next check point at Bowling Green, Kentucky, in twenty minutes, and reported that the flight was proceeding normally. Indianapolis controllers told them to contact the Memphis, Tennessee, ARTC in fifteen minutes on that center's radio frequency.

In the area, the weather at the plane's altitude was clear, with unrestricted visibility. Below it, there were two layers of broken to overcast clouds; cloud tops in the upper layer extended up to 5000 to 6000 ft altitude. An area of low pressure centered over the Lower Peninsula of Michigan and a low pressure trough extended southward along the Illinois-Indiana border. At the same time, a ridge of high pressure extended from the southern plains to the northeast, from Arkansas, through central Kentucky and into southern Ohio.

A different Electra flight that had passed through the same area at 2:30 p.m. that day reported that it had encountered severe airspeed fluctuations and turbulence in the area. Two U.S. Air Force B-57s operating near Louisville, Kentucky, reported moderate to severe clear-air turbulence in the area at high altitudes between 3:00 and 3:30, with one of them slowing down and descending from between 20,000 and 25,000 feet to 15,000 feet to escape the turbulence. A DC-7 passing through the area reported severe clear-air turbulence at 19,000 feet. Before leaving Chicago, the pilots of Flight 710 had been informed about the weather conditions in the area, but the weather reports did not mention anything about clear-air turbulence along the flight's path.

Accident

At about 3:20 p.m., Flight 710 was in level flight at 400 mph when the aircraft's right wing separated from the fuselage. The upper portion of the wing near the fuselage failed first, followed by the rest of the right wing and its engines over the next six to ten seconds. Portions of the outer left wing and its outboard engine also broke away. The loss of one wing and the partial loss of the other caused the aircraft to turn upside-down in the air.

Despite the aircraft losing its wings, its forward momentum carried it for more than 3 mi before it began to arc downward toward the ground. During the descent, at least one of the engines on the remaining left wing continued to run at full power, which accelerated the aircraft into the dive. The plane struck farmland about 8 mi east of Cannelton, Indiana, in a near-vertical, nose-first attitude at a speed of at least 600 mph. Upon impact, the pressurized fuselage ruptured, and the rapid decompression flung debris and human remains over several hundred yards with a thunderous blast. Most of the fuselage buried itself in the frozen, snow-covered ground, leaving a crater about 15 ft deep and 30 ft wide.

Eyewitnesses on the ground reported seeing the aircraft fly overhead and seeing two puffs of white smoke come from the aircraft, followed by a large cloud of dark smoke within a few seconds. They heard two loud explosions, then saw a large piece of the aircraft fall off and burst into flames, followed by additional flaming objects falling from the plane. They saw the fuselage arcing downward, and when that struck the ground they heard what sounded like a huge explosion that rattled windows for miles around. Pilots of six United States Air Force aircraft that were conducting refueling operations nearby at 31000 ft spotted a dark smoke trail that disappeared into the cloud layer below, with a second horizontal plume extending a considerable distance from the main smoke cloud.

The crash was the third involving a Lockheed L-188 Electra since the type entered commercial service just over a year earlier. It was the first accident for Northwest Airlines since the April 1956 crash of Northwest Orient Airlines Flight 2, in which a Boeing 377 Stratocruiser crashed into Puget Sound, killing five people.

Accident report

The CAB released the final accident report on April 28, 1961. The report concluded that the pilots of Flight 710 had been conducting the flight in accordance with company procedures, and that they had closely followed the filed flight plan. It stated that the weather conditions in the area of the accident that were reported to the pilots clearly indicated that there was a high likelihood that clear-air turbulence would be present, but that the United States Weather Bureau and Northwest Airlines did not mention anything about clear-air turbulence in the information they provided to the pilots. It mentioned that the wreckage of the aircraft was extremely damaged by fire and impact forces, but that in the systems and components that the investigators had been able to recover, they found no sign of distress or malfunction. The report extensively reviewed the dynamics of whirl mode wing flutter, and the effects that it would have had on the aircraft. It then outlined specific evidence recovered from components of the wings and engine mounts that showed how the wings had failed due to flutter causing rapidly reversing loads on the wings up to the point of failure. It mentioned that investigators had found damage to the rib and rib attachment structures in the left wing, and said that the damage could have been caused by wing flutter, but it more closely resembled damage that had been suffered by other Electras as a result of an abnormally hard landing. The report stated that there were several possibilities of what may have caused the weakening of the engine mounting structures that allowed whirl mode wing flutter to become self-sustaining, but that the complex interactions of the many components made it impossible to determine exactly what had happened. The report concluded that the probable cause of the crash was the failure of aircraft's right wing due to wing flutter that had been caused by oscillations of the outboard engine assemblies. Contributing to the crash was a reduced stiffness of the aircraft's wings and its flight into an area of clear-air turbulence.